You can measure it directly using an oscilloscope. Connecting an oscilloscope. After this, the voltage is measured according to the principle indicated above. Knowing the nominal resistance of the resistor and the total voltage in the circuit, it is easy to calculate using Ohm’s law
Since voltage is measured between two points, the oscilloscope input has two terminals. Moreover, they are not equivalent. One terminal, called "phase", is connected to the input of the vertical beam deflection amplifier. The second terminal is “ground” or “housing”. It is called so because it is electrically connected to the body of the device (this is the common point of all its electronic circuits). The oscilloscope shows the phase voltage relative to ground.
It is very important to know which of the input conductors is phase. In imported devices, specialized probes are usually used, the ground of which has an alligator clip, as it is often connected to the body of the device under test, and the phase ends either with a “needle”, which can be conveniently and reliably “stuck” even into a small-sized contact, or with a clamp ( Fig. 6). In this case, it is basically impossible to confuse the phase and the body.
Rice. 6. Imported oscilloscope probe, “needle” on the left, clamp on the right.
Domestic oscilloscopes are most often equipped with cords that have standard 4 mm plugs for Russia (the name “banana”, which comes from audio equipment, is sometimes applied to them), Fig. 7. In this case, both plugs are the same, and additional features are used to distinguish them. There are several of these signs, and they can occur in any combination:
The ground wire is longer;
The ground wire is brown (standard) or black;
The symbol “housing” is marked on the body of the ground wire plug
or "earth"
However, unfortunately, these rules are not always followed. This especially applies to cables that have undergone repairs: any available conductor and the first plug they come across can be installed there. Therefore, there is another way to determine the phase and housing, which gives a 100% guarantee.
To determine which of the conductors is a phase and which is the housing, you need to, with the oscilloscope not connected anywhere, grab the contact of one of the input conductors with your hand, while not touching anything with the other hand. If this conductor is a body, then there will be only a horizontal scan line on the screen. If this conductor is a phase, then quite significant interference will appear on the screen, representing a highly distorted sinusoid with a frequency of 50 Hz (Fig. 8).
Rice. 8. Noise on the oscilloscope screen when you touch the phase of the input cable with your hand.
This interference occurs due to the fact that there is capacitance between the human body and the network wires laid in the room. And a current arises flowing through the following circuit: phase of the lighting network AC 220 V 50 Hz – capacitance between the network wires and the human body – human hand – amplifier input (phase of the input cable) – electronic circuit of the amplifier – oscilloscope body – capacitance between the body and the Earth – neutral wire of the network (it is always grounded). The circuit is closed, current flows. The magnitude of this current is 10^-8...10^-6 amperes, but the oscilloscope input has a very high resistance (about 10^6 Ohms), so a fairly large voltage appears on it. The sine wave looks distorted because the capacitive reactance of the network - human body section depends on the frequency: the higher the frequency, the lower the resistance. Therefore, high-frequency components (mains harmonics and interference that penetrates into it) create greater current and greater voltage at the input of the oscilloscope.
Having determined the phase and housing of the input cable, you can connect the oscilloscope to the circuit under study. If there is no clearly defined common wire, then the housing is connected to any of the points, the voltage between which needs to be examined. If there is a common wire in the circuit - a point conventionally accepted as zero potential, connected to the device body or actually grounded, then it is better to connect the oscilloscope body to this point. Failure to follow this rule can lead to significant measurement errors (sometimes so large that the measurements cannot be trusted at all).
At its core, an oscilloscope is a voltmeter that displays a voltage graph. However, it can also be used to observe the shape of the current. To do this, a resistor Rt is connected in series with the circuit under study (here the index “t” means current), Fig. 9. The resistance of the resistor Rt is chosen much lower than the resistance of the circuit, then the resistor does not affect its operation and its inclusion does not lead to changes in the operating mode of the circuit. According to Ohm's law, a voltage appears across the resistor:
This voltage is measured by an oscilloscope. And knowing the value of Rt, you can convert the voltage shown by the oscilloscope into current.
Rice. 9. Measuring current with an oscilloscope.
A dual-channel (and dual-beam) oscilloscope can display waveforms of two signals simultaneously. To do this, it has two inputs (channels), usually designated I and II. It should be remembered that one of the input terminals of each channel is connected to the oscilloscope body, therefore The housing terminals of both channels are connected to each other. Therefore, these terminals must be connected to the same point in the circuit, otherwise a short circuit will occur in the circuit (Fig. 10).
Rice. 10. Connecting a two-channel oscilloscope. Input grounds can create a short in the circuit.
In Fig. 10a, circuit points B and D turned out to be closed to each other through the oscilloscope body (the closing conductor is shown with a dotted line). As a result, the circuit configuration changed.
The ability to observe not any two voltages, but only those having a common point, is a disadvantage, but a small one - in electronics, one of the poles of the power source is always a common wire, and all voltages are measured relative to it.
Using a two-channel oscilloscope, you can simultaneously observe both the voltage and current in a circuit. And thus measure the phase shift between current and voltage. The oscilloscope connection diagram in this case is shown in Fig. eleven.
Rice. 11. Connecting an oscilloscope to measure phase shift.
Channel I measures voltage and channel II measures current. This inclusion is most optimal, because the voltage dropped across the resistor Rt and supplied to channel II is 30...100 times less than in channel I, therefore, it is more susceptible to interference and synchronization from low voltage is not as good. In addition, the design of most oscilloscopes is somewhat “single-ended” - the synchronization from the channel I signal is usually better and more stable. Thus, connecting channel I to voltage provides a more stable waveform image.
Connection error in fig. 11b is that the housing terminals of both inputs are not connected at one point. As a result, the resistor Rt is short-circuited through the oscilloscope body. The most unpleasant thing is that the voltage on the resistor Rt is not equal to zero - due to the fact that the resistance of the input cable wires (through which this resistor is closed) is not zero. Therefore, with such a connection, you may not notice this error (after all, the oscilloscope shows something), and the result of measuring the current will be incorrect.
The inclusion shown in Fig. 11c is unsuccessful in that channel I of the oscilloscope does not measure the voltage in the circuit under study, but the sum of the voltages in the circuit and across the resistor Rt (the voltage is measured not at the load, but at the source). The voltage on Rt, although small in magnitude, still introduces an error in the voltage measurement.
The oscilloscope connection shown in Fig. 11a not only provides the greatest measurement accuracy, but also allows in some cases to use a resistor Rt with a fairly high resistance. This is important when measuring small currents: if both the current in the circuit and the resistance Rt are small, then the voltage arising at Rt may be so small that the sensitivity of the oscilloscope is not enough to display it.
A digital oscilloscope, of course, is much more advanced than a conventional electronic one; it allows you to store waveforms, can connect to a personal computer, has mathematical processing of results, on-screen markers, and much more. But with all the advantages, these new generation devices have one significant drawback - their high price.
It is this that makes a digital oscilloscope inaccessible for amateur purposes, although there are “pocket” oscilloscopes costing only a few thousand rubles that are sold on Aliexpress, but they are not particularly convenient to use. Well, just an interesting toy. Therefore, for now we will talk about measurements using an electronic oscilloscope.
You can find a sufficient number of forums on the Internet on the topic of choosing an oscilloscope for use in a home laboratory. Without denying the advantages of digital oscilloscopes, many forums advise choosing simple, small-sized and reliable domestically developed oscilloscopes S1-73 and S1-101 and the like, which we previously met in.
At a fairly affordable price, these devices will allow you to perform most amateur radio tasks. In the meantime, let's get acquainted with the general principles of measurements using an oscilloscope.
Figure 1. Oscilloscope S1-73
What does an oscilloscope measure?
The measured signal is fed to the input of the vertical deflection channel Y, which has a high input resistance, usually 1MΩ, and a small input capacitance, no more than 40pF, which allows minimal distortion to be introduced into the measured signal. These parameters are often indicated next to the vertical channel input.
Figure 2. Oscilloscope S1-101
High input resistance is typical for voltmeters, so we can confidently say that the oscilloscope measures voltage. The use of external input dividers allows you to reduce the input capacitance and increase the input impedance. This also reduces the influence of the oscilloscope on the signal being examined.
Y channel bandwidth
The oscilloscope measures voltages over a very wide range: from DC voltages to fairly high frequency voltages. The voltage range can be quite varied - from tens of millivolts to tens of volts, and when using external dividers, up to several hundred volts.
It should be borne in mind that the bandwidth of the vertical deviation channel Y d.b. no less than 5 times higher than the frequency of the signal that will be measured. That is, the vertical deflection amplifier must pass at least the fifth harmonic of the signal under study. This is especially required when studying rectangular pulses that contain many harmonics, as shown in Figure 3. Only in this case an image with minimal distortion is obtained on the screen.
Figure 3. Synthesis of a square wave signal from harmonic components
In addition to the fundamental frequency, Figure 3 shows the third and seventh harmonics. As the harmonic number increases, its frequency increases: the frequency of the third harmonic is three times higher than the fundamental one, the fifth harmonic is five times higher, the seventh harmonic is seven times higher, etc. Accordingly, the amplitude of higher harmonics decreases: the higher the harmonic number, the lower its amplitude. Only if the vertical channel amplifier can pass higher harmonics without much attenuation will the pulse image be rectangular.
Figure 4 shows a square wave waveform when the Y channel bandwidth is insufficient.
Figure 4.
This is approximately what a square wave with a frequency of 500 KHz looks like on the screen of an OMSH-3M oscilloscope with a bandwidth of 0...25 KHz. It is as if rectangular pulses were passed through an RC integrating circuit. Such an oscilloscope was produced by Soviet industry for laboratory work in physics lessons in schools. Even the supply voltage of this device, for safety reasons, was not 220, but only 42V. It is quite obvious that an oscilloscope with such a bandwidth will allow you to observe a signal with frequencies of no more than 5 KHz almost without distortion.
A typical general-purpose oscilloscope most often has a bandwidth of 5 MHz. Even with such a bandwidth, you can see a signal up to 10 MHz and higher, but the image obtained on the screen allows you to judge only the presence or absence of this signal. It will be difficult to say anything about its shape, but in some situations the shape is not so important: for example, there is a sine wave generator, and it is enough to simply make sure whether this sine wave exists or not. Just such a situation is shown in Figure 4.
Modern computing systems and communication lines operate at very high frequencies, on the order of hundreds of megahertz. To see such high-frequency signals, the oscilloscope's bandwidth must be at least 500 MHz. Such a wide band greatly “expands” the price of the oscilloscope.
An example is the U1610A digital oscilloscope shown in Figure 5. Its bandwidth is 100 MHz, and the price is almost 200,000 rubles. Agree, not everyone can afford to buy such an expensive device.
Figure 5.
Let the reader not consider this drawing to be an advertisement, since all the seller’s coordinates are not painted over: any similar screenshot could be in place of this drawing.
Types of signals under study and their parameters
The most common type of oscillation in nature and technology is the sinusoid. This is the same long-suffering function Y=sinX that was taught in trigonometry lessons at school. Quite a lot of electrical and mechanical processes have a sinusoidal shape, although quite often other signal forms are used in electronic technology. Some of them are shown in Figure 6.
Figure 6. Electrical waveforms
Periodic signals. Signal characteristics
A universal electronic oscilloscope allows you to accurately examine periodic signals. If a real sound signal, for example, a musical soundtrack, is applied to input Y, then chaotically flashing bursts will be visible on the screen. Naturally, it is impossible to study such a signal in detail. In this case, the use of a digital storage oscilloscope will help, which allows you to save the oscillogram.
The oscillations shown in Figure 6 are periodic, repeating after a certain period of time T. This can be considered in more detail in Figure 7.
Figure 7. Periodic oscillations
Oscillations are depicted in a two-dimensional coordinate system: voltage is measured along the ordinate axis, and time is measured along the abscissa axis. Voltage is measured in volts, time in seconds. For electrical vibrations, time is more often measured in milliseconds or microseconds.
In addition to the X and Y components, the oscillogram also contains a Z component - intensity, or simply (Figure 8). It is she who turns on the beam during the forward stroke of the beam and extinguishes it during the reverse stroke. Some oscilloscopes have an input for controlling brightness, which is called the Z input. If a pulse voltage from a reference generator is applied to this input, then frequency marks can be seen on the screen. This allows you to more accurately measure the duration of the signal along the X axis.
Figure 8. Three components of the signal under study
Modern oscilloscopes usually have time-calibrated sweeps that allow accurate time reading. Therefore, there is practically no need to use an external generator to create marks.
At the top of Figure 7 is a sinusoid. It is easy to see that it begins at the origin of the coordinate system. During the time T (period) one complete oscillation is performed. Then everything repeats, the next period begins. Such signals are called periodic.
Below the sine wave are rectangular signals: square wave and square wave. They are also periodic with period T. The pulse duration is designated as τ (tau). In the case of a square wave, the pulse duration τ is equal to the duration of the pause between pulses, exactly half the period T. Therefore, a square wave is a special case of a rectangular signal.
Duty factor and duty cycle
To characterize rectangular pulses, a parameter called duty cycle is used. This is the ratio of the pulse repetition period T to the pulse duration τ. For a meander, the duty cycle is equal to two, a dimensionless quantity: S= T/τ.
In English terminology, it's just the opposite. There, pulses are characterized by a duty cycle, the ratio of the pulse duration to the repetition period Duty cycle: D=τ/T. The fill factor is expressed in %%. Thus, for a meander D=50%. It turns out that D=1/S, the duty cycle and the duty cycle are mutually inverse, although they characterize the same pulse parameter. The square wave oscillogram is shown in Figure 9.
Figure 9. Waveform of D=50% square wave
Here the oscilloscope input is connected to the output of the function generator, shown right there in the lower corner of the figure. And here an attentive reader may ask a question: “The amplitude of the output signal from the generator is 1V, the sensitivity of the oscilloscope input is 1V/div, and on the screen there are rectangular pulses with a peak-to-peak range of 2V. Why?"
The fact is that the function generator produces bipolar rectangular pulses relative to the 0V level, approximately the same as a sine wave, with positive and negative amplitude. Therefore, pulses with a peak-to-peak range of ±1V are observed on the oscilloscope screen. In the following figure, let's change the Duty cycle fill factor, for example, to 10%.
Figure 10. Square pulse D=10%
It is easy to see that the pulse repetition period is 10 cells, while the pulse duration is only one cell. Therefore, D=1/10=0.1 or 10%, as can be seen from the generator settings. If you use the formula for calculating the duty cycle, you get S = T / τ = 10 / 1 = 1 - a dimensionless quantity. Here we can conclude that Duty cycle characterizes the impulse much more clearly than the duty cycle.
Actually, the signal itself remained the same as in Figure 9: a rectangular pulse with an amplitude of 1V and a frequency of 100Hz. Only the duty cycle or duty cycle changes, whichever is more familiar and convenient. But for ease of observation, in Figure 10 the sweep duration is halved compared to Figure 9 and is 1 ms/div. Therefore, the signal period occupies 10 cells on the screen, which makes it quite easy to verify that the Duty cycle is 10%. When using a real oscilloscope, the sweep duration is selected approximately the same.
Square Pulse Voltage Measurement
As was said at the beginning of the article, an oscilloscope measures voltage, i.e. potential difference between two points. Usually measurements are made relative to the common wire, ground (zero volts), although this is not necessary. In principle, it is possible to measure from the minimum to maximum signal value (peak value, peak-to-peak). In any case, the measurement steps are quite simple.
Rectangular pulses are most often unipolar, which is typical for digital technology. How to measure square wave voltage is shown in Figure 11.
Figure 11. Measuring the amplitude of a square wave pulse
If the sensitivity of the vertical deflection channel is selected as 1V/div, then it turns out that the figure shows a pulse with a voltage of 5.5V. With a sensitivity of 0.1V/div. The voltage will be only 0.5V, although both pulses look exactly the same on the screen.
What else can you see in a rectangular pulse?
The rectangular pulses shown in Figures 9, 10 are simply ideal, since they were synthesized by the Electronics WorkBench program. And the pulse frequency is only 100Hz, so there can be no problems with the “rectangularity” of the image. In a real device, at a high repetition rate, the pulses are somewhat distorted; first of all, various surges and bursts appear due to the inductance of the installation, as shown in Figure 12.
Figure 12. Real square pulse
If you do not pay attention to such “little things”, then the rectangular pulse looks as shown in Figure 13.
Figure 13. Rectangular pulse parameters
The figure shows that the leading and trailing edges of the pulse do not appear immediately, but have some rise and fall times and are slightly inclined relative to the vertical line. This slope is due to the frequency properties of microcircuits and transistors: the higher the frequency transistor, the less “filled up” the pulse fronts are. Therefore, the pulse duration is determined at 50% of the full swing.
For the same reason, the pulse amplitude is determined at the level of 10...90%. The pulse duration, like the voltage, is determined by multiplying the number of divisions of the horizontal scale by the division value, as shown in Figure 14.
Figure 14.
The figure shows one period of a rectangular pulse, slightly different from a meander: the duration of the positive pulse is 3.5 divisions of the horizontal scale, and the duration of the pause is 3.8 divisions. The pulse repetition period is 7.3 divisions. Such a picture can belong to several different pulses with different frequencies. Everything will depend on the duration of the sweep.
Let's assume the sweep duration is 1ms/div. Then the pulse repetition period is 7.3*1=7.3ms, which corresponds to the frequency F=1/T=1/7.3= 0.1428KHz or 143Hz. If the sweep duration is 1 μs/div, then the frequency will be a thousand times higher, namely 143 KHz.
Using the data in Figure 14, it is not difficult to calculate the duty cycle of the pulse: S=T/τ=7.3/3.5=2.0857, it turns out almost like a meander. Duty cycle fill factor D=τ/T=3.5/7.3=0.479 or 47.9%. It should be noted that these parameters in no way depend on frequency: the duty cycle and duty cycle were calculated simply from the divisions on the oscillogram.
With rectangular pulses, everything seems clear and simple. But we completely forgot about the sine wave. In essence, it’s the same thing: you can measure voltages and time parameters. One period of a sinusoid is shown in Figure 15.
Figure 15. Sine wave parameters
Obviously, for the sinusoid shown in the figure, the sensitivity of the vertical deflection channel is 0.5V/div. The remaining parameters can be easily determined by multiplying the number of divisions by 0.5V/div.
There may be another sine wave, which will have to be measured at a sensitivity of, for example, 5V/div. Then instead of 1V you get 10V. However, on the screen the image of both sinusoids looks exactly the same.
The timing of the sine wave shown is unknown. If we assume that the sweep duration is 5ms/div, the period will be 20ms, which corresponds to a frequency of 50Hz. The numbers in degrees on the time axis show the phase of the sine wave, although for a single sine wave this is not particularly important. More often it is necessary to determine the phase shift (directly in milliseconds or microseconds) between at least two signals. This is best done with a dual-beam oscilloscope. How this is done will be shown below.
How to measure current with an oscilloscope
In some cases, it is necessary to measure the magnitude and shape of the current. For example, alternating current flowing through a capacitor leads the voltage by ¼ cycle. Then a resistor with a small resistance (tenths of an ohm) is connected to the open circuit. Such resistance does not affect the operation of the circuit. The voltage drop across this resistor will indicate the shape and magnitude of the current flowing through the capacitor.
An ordinary dial ammeter is designed in approximately the same way, which is connected to an open circuit. In this case, the measuring resistor is located inside the ammeter itself.
The circuit for measuring current through a capacitor is shown in Figure 16.
Figure 16. Measuring current through a capacitor
A sinusoidal voltage with a frequency of 50 Hz and an amplitude of 220 V from the generator XFG1 (red beam on the oscilloscope screen) is supplied to a series circuit from capacitor C1 and measuring resistor R1. The voltage drop across this resistor will show the shape, phase and magnitude of the current through the capacitor (blue beam). How this will look on the oscilloscope screen is shown in Figure 17.
Figure 17. Current through a capacitor leads the voltage by ¼ cycle.
With a sine wave frequency of 50 Hz and a sweep of 5 ms/Div, one period of the sine wave occupies 4 divisions along the X axis, which is very convenient for observation. It is easy to see that the blue beam is ahead of the red one by exactly 1 division along the X axis, which corresponds to ¼ of the period. In other words, the current through the capacitor is ahead of the voltage in phase, which is fully consistent with the theory.
To calculate the current through a capacitor, it is enough to use Ohm’s law: I = U/R. If the resistance of the measuring resistor is 0.1 Ohm, the voltage drop across it is 7 mV. This is the amplitude value. Then the maximum current through the capacitor will be 7/0.1=70mA.
Measuring the shape of the current through a capacitor is not some very urgent task; everything is clear here without measurements. Instead of a capacitor, there can be any load: an electric motor winding, a transistor amplifier stage, and much more. It is important that this method can be used to study current, which in some cases differs significantly in shape from voltage.
Oscilloscope – multi-purpose a device that is used to study the shape and measure the parameters of signals, when studying the characteristics of various electronic devices.
Voltage measurement. Voltage measurements using an oscilloscope can be carried out using both the direct conversion method and the comparison method.
Method direct conversion(method calibrated deviation) provides for preliminary calibration of the Y channel using an amplitude calibrator. In this case, the required value of the deviation coefficient K d is set. The measured voltage is applied to the Y channel input, and the vertical size of the image on the CRT screen is determined l B (in divisions or units of length). Knowing the deviation coefficient K d or the sensitivity S u , with a symmetrical (or constant) voltage, you can find its amplitude
When measuring the amplitudes of an asymmetrical voltage, it is necessary to fix, using a scale grid in the absence of the measured voltage, the initial position of the horizontal line (or light spot) on the oscilloscope screen. Then, by applying the measured voltage to input Y and setting a still image, measure the amplitudes of each half-wave separately.
Method comparisons can be implemented using a dual-beam (dual-channel) oscilloscope. To do this, the signal under study is supplied to one input, for example Y 1, and a reference voltage, which can be either constant or alternating, is supplied to input Y 2. Then, by changing the value of the reference voltage, it is necessary to ensure that the calibration line created by the reference voltage aligns with the boundaries of the measured section of the oscillogram. The value of the desired voltage is determined by the value of the reference voltage.
Measuring Time Intervals can be carried out using the method direct conversion(method calibrated sweep factor) similar to the case of voltage measurement. Before measurement using a time calibrator, the required value of the sweep factor is set, which is the value of the horizontal scale division. In this case
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Where l x – dimensions of the studied section of the oscillogram.
Frequency measurement an alternating signal can be produced by measuring the period. Frequency is found as the reciprocal of the period.
When using a dual-beam (dual-channel) oscilloscope, frequency measurements can be made by comparisons oscillations under study with oscillations of a known frequency. In this case it is carried out simultaneous fixation two oscillations on the oscilloscope screen. The disadvantage of this method is its low accuracy.
More accurate modifications of the comparison method are: method Lissajous figures(method interference figures) and method circular scan. When implementing these methods, the oscilloscope functions as an indicator of the equality or multiplicity of the measured f X and the reference frequency f 0 and practically does not introduce error into the measurement result f X.
To obtain Lissajous figures, a signal of unknown frequency is applied to the Y input of the oscilloscope. The internal sweep of the oscilloscope is turned off and a sinusoidal voltage is supplied to the horizontal deflection plates from a high-precision measuring generator. In this case, the beam on the CRT screen makes a complex movement. The frequency of the measuring generator is selected so that a stationary image appears on the oscilloscope screen ( Lissajous figure). This occurs when there is an integer ratio between the frequencies of the two input signals, and the appearance of the Lissajous figure depends on the multiplicity f X / f 0, the ratio of voltage amplitudes and the phase shift between them. The frequency ratio is found as the ratio of the number of intersection points of the figure on the screen with the horizontal n X and vertical m Y reference lines (the ratio of the number of touches of the figure with the horizontal and vertical axes superimposed on the screen).
In Fig. examples of Lissajous figures are shown for various values of the frequency ratio f X /f 0 .
If the voltage of the measured frequency f X is applied to the Y input of the oscilloscope, and the voltage of the known frequency f 0 is applied to the X input, we obtain the relation
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from which the frequency value f X can be determined.
Usually they try to select the frequency of the reference generator equal to the measured frequency, since in this case the figure has the simplest form - a straight line, a circle, an ellipse.
The method, characterized by high accuracy, is simple, convenient and economical. Its disadvantage is the difficulty of deciphering figures with a frequency ratio of more than 10 and, consequently, the measurement error increases due to the establishment of the true frequency ratio. This method is advisable to use only with a relatively small multiple of the measured and known frequency, usually not exceeding 6–8.
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Measuring phase shifts
For a harmonic signal U(t) = Uo sin(t + 0), the phase is called the expression (t + 0) - the argument of the sine, where 0 is the initial phase of oscillations. The phase value depends on the selected time reference, so the physical meaning is the phase shift or the phase difference 1 – 2 of two signals with the same frequencies (Fig. Fig. 5.15a). The phase is measured in angular units - radians or degrees. The method for measuring phase shift using a two-channel oscilloscope is overlay method, which consists of receiving on the oscilloscope screen and combining oscillograms of the voltages U 1 and U 2 supplied to input A and output B (Fig. 5.9). From Fig. Rice. 5.15a it is clear that in this case
The phase difference between two signals can be determined by the time shift. A stationary picture of two oscillograms is obtained on the screen (Fig. Fig. 5.15b). Since the entire period T corresponds to an angle of 360, the phase difference is determined from the ratio = 360T/T . In this case, the important question is which of the signals is ahead “in phase” of the other signal. In Fig. 5.15b the voltage U 1 leads the voltage U 2 in phase by > 0, since the signal U 1 reaches its maximum earlier than the signal U 2 (the signal U 1 also reaches its minimum earlier than the signal U 2).
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The phase shift can also be determined by the interval T 1, but if during measurements one signal, for example U 2, on the oscilloscope screen will be slightly shifted vertically downward, as shown in Fig. Rice. 5.15b, then the measurement of the phase shift by the time shift T 1 turns out to be incorrect. This becomes obvious if we take into account that T 1 is not equal to the time shift between the same signals, cut off by the horizontal straight line to the right of T 1.
Phase shift measurements can also be carried out on a single-beam oscilloscope ellipse method. The ellipse is a special case of the Lissajous figure with f 1 = f 2 . Let voltages U x = U 0 sint and U y = U 0 sin(t + φ) be applied to the horizontal and vertical deflecting plates. With equal amplitudes and frequencies of signals at the Y and X inputs of the oscilloscope, a change in the phase shift leads to a change in the shape of the Lissajous figure from a straight line (φ = 0) through an ellipse to a circle (φ = 90 o), as shown in Fig. Rice. 5 .16.
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In general, the phase shift can be determined from an ellipse as follows. The gain factors for vertical and horizontal deflection are selected so that the ellipse fits into a square (Fig.). The phase shift value is found as the ratio of the ellipse parameters using the formula
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The disadvantage of this method is its ambiguity. Measurement results φ are unambiguous only within the range of 0–180 o, then (within the range of 180–360 o), the figures will be repeated, but the direction of movement of the beam will change.
To measure the phase difference, a circular scan created by voltage U 1 as a reference can also be used. In this case, the angular position of the luminous semicircle created by voltage U 2 is measured when it is applied to the input of channel Z of the CRT.
LABORATORY WORK 10TH GRADE.
Introduction to the digital oscilloscope interface.
Measuring current using an oscilloscope
1. Remember that before removing a flash memory device from a USB port, you always turn off the power to that port using the “Safely Remove” option.
Be careful with the USB port of your computer; a short circuit of its contacts can lead to failure of not only the port, but the entire computer!!!
The source of direct current in work on electrodynamics will be one of the USB ports of the computer. Connect the USB port switching unit to the electrical circuit (hereinafter current source) to one of the USB ports. Connect an oscilloscope voltage sensor to the second USB port with a cable (hereinafter oscilloscope).Connect the oscilloscope probes to the output terminals of the DC source.
If you have problems setting up an oscilloscope or other sensor, perhaps you ran the program before installing the sensor driver, poll the sensor again
(button) or restart the program.
2. Launch the Digital Lab program. In the window that opens with a list of jobs, select job scenario 3.1 “Introduction to the oscilloscope interface.” The window with a list of jobs can also be called up by clicking the button in the top menu of the program.
3. Oscilloscope - a device that allows you to measure DC and
time-varying electrical signal. Using the button, open the computer settings window (Fig. 1)
Fig.1 Review the contents of the nested lists of setting parameters in each of the
parameters settings windows. An oscilloscope can simultaneously measure voltage in two sections of a circuit using two channels. Check the box for selecting the “red” channel (Channel No. 1). Operating mode “auto” and sweep “5 ms/div”, sensitivity of Channel No. 1 “1 V/div”, zero line position “0”, signal type “Constant” *, check the “Signal display” and
* The “Variable” option in the “Signal Type” window when setting up the registration parameters of an oscilloscope sensor allows you to cut off a constant or slowly changing (with a characteristic time of about 0.1 s) voltage component and show only a rapidly changing signal (with a characteristic time of 0.05 s or less ). In the set of works “Digital Laboratory. Basic level" this option is not used anywhere.
"Displaying the zero line." You can leave the settings in the remaining windows unchanged for now. Lock the selected parameters (button)
4. Start measurements in the Digital Laboratory program (button) and after marking the zero line with a red line, connect the oscilloscope leads in the “red” braid to the terminals of the current source. Notice in which direction the signal shifts when you connect the cable with the blue tip to the source terminal
“+”, and with a red tip - to the “minus” terminal. Stop measurements (button)
and with the left mouse button place a yellow vertical marker on the working field on the first horizontal division. Pay attention to the voltage numbers
and time in the upper left corner (or at the bottom of the window) of the registration window. Time
is counted from the green vertical marker located on the left border of the working field. You can move the green marker with the right mouse button. Right-clicking outside the left border of the registration window returns the green marker to the left edge of the field.
5. Return to the oscilloscope parameter settings window, change the voltage sensitivity of Channel No. 1 and time base. Enable registration via Channel No. 2 by setting the signal type window (Fig. 1) to “Constant”. Having accepted the parameters, check how the oscilloscope readings on the working field have changed. Having replaced the Channel No. 1 (red) probes with the Channel No. 2 probes, check how Channel No. 2 works, then remove the signal from the source with both channels, connecting the channel terminals so that the signal from them is of different polarity.
6. Assemble an electrical circuit consisting of a series-connected resistor with a resistance of 200 Ohms, a variable resistance (its resistance varies from 0 to 100 Ohms), an LED, a switch and a current source. Connect the terminals of Channel No. 1 of the oscilloscope to the output terminals of the current source, and the terminals of Channel No. 2 to the ends of the 200 Ohm resistor (Fig. 2). By closing the key and rotating the variable resistance knob, make sure that the readings at the terminals of the current source do not change, and the voltage on the 200 Ohm resistor changes synchronously with the change in the brightness of the LED (the LED will light only if the correct polarity of the supplied voltage is maintained). Stop recording at maximum LED brightness and measure the voltage across the 200 ohm resistor.
resistance Rsh = 10 Ohm (Fig. 3), leaving the oscilloscope probes on a 200 Ohm resistor. Close the circuit, start recording, and after stopping recording, make sure that the voltage across the 200 ohm resistor and the brightness of the LED do not change. A 10 ohm resistor with a resistance small compared to the total resistance of the circuit will be called shunt. The shunt in this circuit reduces the current by about 5%, that is
does not affect the voltage on the elements in the circuit or the brightness of the LED. By including it in the section of the circuit through which the current needs to be measured, by measuring the voltage across it, the current is measured, since Ohm’s law I=U/R is satisfied for the resistor.
8. Remove the LED from the circuit (Fig. 3). Switch the oscilloscope Channel 1 probes from
current source to shunt. Open the “Source Data” tab (button) and enter
shunt resistance value table Rsh= 10 Ohm (Fig. 4).
Fig.4 Select the polarity of the oscilloscope sensor connection so that
A positive signal was recorded for each channel. Start recording and, having received a signal from both channels of the oscilloscope, stop recording. By placing a yellow marker on the screen. Go to the “Table” tab of the “Processing” window and select a cell in the “U, B” column (Fig. 5).
(blue braid of the oscilloscope cable and blue signal color on the screen) of the oscilloscope into the selected cell of the Table. To fill in the column with the voltage on the shunt, select a cell in the “Ush, V” column (Fig. 5) and press the red button - the voltage value measured on Channel No. 1 (red braid and red signal color on the screen) will be sent to the corresponding cell of the Table. Calculate the current through the shunt Ish and enter it in the cell at the bottom of the table (Fig. 5). After entering the “Initial Data”, this “gray” cell turns “yellow” when the correct value is entered Ish– “green”, if an erroneous value is entered – “red”. When the cell is “green”, further calculations of the value Ish and the corresponding cells in the Table are filled in automatically (Fig. 6).
9. Start recording and, by changing the position of the variable voltage resistor knob, achieve a change in the voltage on the 200 Ohm resistor and the current (and, accordingly, the voltage on the shunt) in the circuit. While stopping recording, record several voltage values across the resistor and shunt. Without filling out several lines in the Table, the construction of the Graphics (see clause 10) will not be carried out.
ATTENTION! We remind you that the number of rows in the Table is increased by pressing the button on the keyboard when at least one cell in the previous row is filled.
10. Go to the tab “Graph U(Ish) of the dependence of the voltage on a 200 Ohm resistor on the current through the resistor (it is equal to the current through the shunt) and analyze the resulting graph. Having selected the function Y=AX in the function selection window to describe the experimental graph (the best straight line is selected by clicking on the button next to the function type selection window, Fig. 7), make sure that Ohm’s law U=RI is satisfied, and the proportionality coefficient A corresponds
resistor value R 200 Ohm.
11. Enter in the Report (button) one of the screens with the oscilloscope signal, the contents of the “Initial data” and “Table” tabs, the resulting U(I) graph, as well as a photo of the last electrical circuit on which measurements were taken, made using WEB - cameras, and a screenshot of the oscilloscope settings window (key combination Alt-PrtScr), at which measurements were taken.
ATTENTION! Copying into the Report the contents of any tab of the “Processing” window and the video frame with the installation recorded by the WEB camera is carried out to the place indicated not by the keyboard cursor, but by the MOUSE CURSOR. The contents of the tab are NOT INSERTED INTO THE REPORT IF YOU HAVE NOT OPENED THE TAB.
“GRAPH” MEANS “I DRAW”
DEVICES FOR STUDYING FORM 3 RADIO SIGNALS
We live in a technological civilization. People have created a second nature - a world of mechanisms, complex machines, radio-electronic devices that use almost the entire known range of electromagnetic radiation. But the human organs of vision can only perceive visible light. We cannot see electric current, radio waves, we cannot measure even the simplest parameters of an electrical signal without the help of instruments. When working with complex electronic equipment, the task of reproducing signal shapes often arises, i.e. dependence of the instantaneous voltage value on time. Its solution allows you to immediately evaluate many parameters of oscillations, for example, distortion of their shape, the presence of interference, and much more. Waveform reproduction plays an important role when checking and configuring audio and video circuits of equipment.
To visualize signals, instruments called oscilloscopes are used, but determining the shape of signals is possible not only in the time domain, but also in the frequency domain. The task of reproducing a signal in the frequency domain is solved by spectrum analyzers and amplitude-frequency characteristics meters, which will be discussed in the final part of this brochure.
ELECTRONIC OSCILLOSCOPES
Currently, one of the most common radio measuring instruments is the electronic oscilloscope, and this is not surprising, because it has exceptional clarity of presentation of the signals under study, convenience and versatility. An oscilloscope allows you to examine any electrical processes, even if the signal appears at a random moment in time and lasts billionths of a second. From the image on the oscilloscope screen, you can determine the amplitude of the signal in question and the duration of any of its sections. An oscilloscope can measure frequency, phase, modulation ratio, and other complex measurements.
Oscillographic measurements are distinguished by a wide range of frequencies under study (from direct current to microwave), the ability to store and subsequently reproduce signals, high sensitivity and the ability to separate signals from noise.
CLASSIFICATION OF OSCILLOSCOPES
By purpose and principle of operation
Oscilloscopes are divided into:
Universal, high-speed, stroboscopic, memory and special.
By the number of simultaneously observed signals they are divided into one-, two- and multi-channel oscilloscopes.
By display device Oscilloscopes are divided into electron beam and matrix (gas-discharge, plasma, liquid crystal, etc.).
Based on the principle of information processing Oscilloscopes are divided into analog and digital.
Universal oscilloscopes are general-purpose instruments designed for observing harmonic and pulsed signals. With their help, you can examine single pulses and bursts of pulses, simultaneously obtain images of two signals on one scan, examine in detail any part of a complex signal, and much more. They make it possible to study signals with durations from a few nanoseconds to several seconds in the amplitude range from fractions of millivolts to hundreds of volts, as well as measure the parameters of such signals with an error of 5-7% acceptable for practice. The bandwidth of universal oscilloscopes is 300... 500 MHz and more.
Universal oscilloscopes are divided into two groups: devices of monoblock design and devices with replaceable blocks.
All-in-one general purpose oscilloscopes are the most common type of oscilloscope.
Oscilloscopes with replaceable blocks are distinguished by their versatility, achieved through the use of replaceable blocks for various purposes.
High-speed and sampling oscilloscopes are used to study transient processes in high-speed semiconductor devices, integrated circuits and switching elements.
Storage oscilloscopes can save and reproduce an image of a signal for a long time after it disappears at the input. The main purpose of these devices is to study one-time and rarely repeated processes.
Special-purpose oscilloscopes are designed to study television signals; they allow not only to study any part of the television signal with high temporal stability, but also to transmit it digitally to a computer for further processing.
BASIC UNITS OF A UNIVERSAL OSCILLOSCOPE
Rice. 1. Oscilloscope S1-107 General view
In Fig. Figure 1 shows the appearance of the S1-107 universal analog oscilloscope, and Fig. 2 shows its functional diagram. Despite the variety of universal oscilloscopes, their functional circuits are generally the same.
The oscilloscope consists of:
- Cathode ray tube (CRT);
- Vertical channel Y;
- Horizontal channel X;
- Channel Z;
- Multimeter;
- Power supply.
Vertical channel amplifies or weakens the signal being studied to a value convenient for studying on the indicator. Control knob position V/div sets channel gain Y. A channel consists of an input divider, which includes connectors, attenuators, and switches; an amplifier that amplifies the signal and splits the polarity of the signal for symmetrical supply to the CRT plates, delay lines and output amplifier. The delay line delays the signal for the time required to trigger the horizontal deflection channel, i.e., the sweep generator and the on-axis amplifier X so that the horizontal movement of the beam begins before the amplified signal arrives at the CRT plates. This allows you to observe the leading edge of the signal.
Rice. 2. Functional diagram of the S1-107 oscilloscope
Horizontal channel generates a sawtooth voltage synchronous with the signal being studied to create a time axis on the CRT screen. The trigger pulse generator produces short trigger pulses. The sweep generator creates a linearly increasing voltage. Slew rate adjustable by knob Time/div. This voltage is fed to the output amplifier X) which splits the polarity of the signal and amplifies the scanning voltage to the value required for the required image scale. A positively increasing sawtooth voltage is applied to the right deflector plate of the CRT, and a negative one is applied to the left one. As a result, the beam passes across the tube screen from left to right a set number of scale divisions per unit of time. When the synchronizer is switched to the continuous oscillation mode, a self-oscillating mode of sweep operation is ensured.
The internal synchronization amplifier amplifies part of the signal under study and transmits it to trigger the sweep.
Oscilloscopes have calibrated scans and are equipped with mesh scales for easy reading, which are applied on the inside of the tube screen. This eliminates operator error due to parallax phenomena.
The oscilloscope also includes amplitude and time calibrators, designed to calibrate the scales of vertical and horizontal deflection channels, and power supplies with stabilization.
Many modern oscilloscopes have built-in multimeters that allow you to accurately measure DC and AC voltages, currents and resistances. The S1-107 oscilloscope multimeter works as follows. The measured alternating currents and resistances are converted into alternating voltage. Then the alternating voltages are converted into a direct voltage proportional to the magnitude of the measured parameters. Then the analog signal is converted to digital using an ADC and enters a character generator designed to generate and write characters on the CRT screen.
The oscilloscope can operate in either oscilloscope mode or multimeter mode. Combining these modes is impossible in this model.
DIGITAL OSCILLOSCOPES
Rice. 3. Digital oscilloscope
A digital oscilloscope allows you to simultaneously observe a signal on the screen and obtain numerical values for a number of its parameters with greater accuracy than is possible by reading quantitative values directly from the screen of a conventional oscilloscope. This is possible because the signal parameters are measured directly at the input of a digital oscilloscope, while the signal passed through the vertical deflection channel can be measured with significant errors. These errors can reach 10%.
The parameters measured by modern digital oscilloscopes are: signal amplitude, frequency or duration. On the oscilloscope screen, in addition to the oscillograms themselves, the state of the controls (sensitivity, sweep duration, etc.) is displayed. Provision is made for outputting information from the oscilloscope to printing and other functionality. However, this does not limit the capabilities of digital oscilloscopes. Interfacing digital oscilloscopes with microprocessors allows you to determine the effective value of the signal voltage and even calculate and display Fourier transforms for any type of signal.
Digital oscilloscope devices perform full digital signal processing, so they typically use the latest panel displays.
Modern digital oscilloscopes automatically set the optimal image size on the handset screen.
The functional diagram of a digital oscilloscope (Fig. 4) contains an input signal attenuator; vertical and horizontal deflection amplifiers; amplitude and time interval meters; signal and meter interfaces; microprocessor controller; sweep generator; synchronization circuit and cathode ray tube.
Digital oscilloscopes provide automatic setting of image sizes, automatic synchronization, difference measurements between two marks, automatic measurement of peak-to-peak, maximum and minimum amplitude of signals, period, duration, pause, rise and fall of pulses, etc.
The amplitude and time parameters of the signal under study are determined using meters built into the device. Based on the measurement data, the microprocessor controller calculates the required deflection and sweep coefficients and, through the interface, sets these coefficients in the hardware of the vertical and horizontal deflection channels. This ensures constant image dimensions vertically and horizontally, as well as automatic signal synchronization.
The microprocessor controller also polls the position of the front panel controls, and the polling data, after encoding, is again sent to the controller, which, through the interface, turns on the appropriate automatic measurement mode. The measurement results are displayed on the handset screen, and the amplitude and time parameters of the signal are displayed simultaneously.
Rice. 4. Functional diagram of a digital oscilloscope
PORTABLE MULTIMETERS-OSCILOSCOPES
Recently, a new and rather original variety has appeared on the market of control and measuring instruments: portable digital multimeters-oscilloscopes.
These small-sized and relatively inexpensive devices combine the function of a multimeter, which allows you to measure voltages, currents and resistances, measure capacitances, inductances, parameters of transistors and diodes, and a simple oscilloscope.
The most common multimeter-oscilloscopes on the Russian market are BEETECH (Fig. 5), Velleman, METEX and Tektronix.
Rice. 5. Multimeter-oscilloscope BEETECH 70
Rice. 6. Velleman HPS10 Portable Personal Oscilloscope
The Velleman HPS10 oscilloscope (Fig. 6) does not have the functions of a multimeter, but it is a full-fledged oscilloscope with a bandwidth of 2 MHz and an ADC quantization frequency of 10 MHz. The device has high sensitivity - from 5 mV per 12 divisions, and the sweep range is from 200 ns to 1 hour (!) per 32 divisions. The device can operate from the mains via an adapter or from built-in batteries, which last for 20 hours. The device has an LCD display with a resolution of 128 x 64 pixels. Such an oscilloscope allows you to even view a television signal (albeit rather crudely).
Portable oscilloscopes are often supplied in plastic cases, which, in addition to the device itself, contain adapters, probes, a power adapter and an instruction manual.
In most cases, such a device is quite sufficient for measuring signals during installations.
WORKING WITH AN OSCILLOSCOPE
Modern oscilloscopes provide a rich set of tools for studying waveforms and measuring their parameters.
It is easiest to work with low-frequency signals, for example, with signals in the audio frequency range (Fig. 7); studying high-frequency signals and signals of complex shapes (Fig. 8) requires additional skills.
Rice. 7. Audio frequency signal on the screen of a digital oscilloscope
Specialized television oscilloscopes have scanning circuits that allow you to select any frame and any line from a television signal, but when working with general-purpose oscilloscopes, you need to choose which synchronization pulses to trigger the scanning - frame or line. Some oscilloscopes have TV-V and TV-H positions on the sweep mode switch (triggering with vertical and horizontal sync pulses, respectively). If there are no such modes, then to view one frame you need to set the scan speed to 2 ms/div, and to view one line - 10 μs/div. Typically, the sweep is triggered by a television signal with a negative polarity of the trigger pulses.
When working with an oscilloscope, it is important to select the correct sweep trigger mode. Most often, the trigger mode is chosen by the signal being studied, the so-called. internal triggering (in two-channel oscilloscopes these modes are called CH1 and CH2). If the equipment under test uses external clock signals, then it is logical to use them to trigger the oscilloscope sweep. This type of synchronization is called external and is usually denoted EXT. If electrical devices are being studied, then synchronization from the network - LINE - may be useful.
A convenient image scale is set with the V/div switch.
Rice. 8. TV signals on the screen of a digital oscilloscope
A two-channel oscilloscope allows, as shown in Fig. 8, Simultaneously view various components of the television signal.
Rice. 9. Damping pulse
Rice. 10. Color burst
By changing the sweep speed and the V/div value, you can examine the general appearance of a complex signal or “stretch” its individual fragment. In Fig. 9 shows one line of a television signal, and Fig. 10 – “stretched” color burst signal.
Rice. 11. Duration measurement
Very often, when working with oscilloscopes, there is a need to measure the parameters of the signals being studied. Analog oscilloscopes are less convenient. In order to determine the amplitude or duration of a signal, you need to calculate how many vertical or horizontal cells the signal under study occupies, and then multiply the number of cells by the value of the V/div or Time/div switch division. For example, if the vertical signal occupies 3.5 cells and the V/div switch is set to 100 mV, then the signal amplitude will be 350 mV. The accuracy of this method is low.
Digital oscilloscopes are much more convenient. For example, in order to measure the pulse amplitude on the oscillogram in Fig. 9, you need to turn on the voltage measurement mode, then move cursor 1 to the top of the pulse, and cursor 2 to its base. The oscilloscope will automatically measure the voltage, and the following message will appear on the right side of the screen: Delta – 296 mV.
Durations are measured in the same way, only in this mode the cursors look like vertical lines (Fig. 11).
On the periphery of the screens of digital oscilloscopes (Fig. 7-11) a variety of service information is displayed, which allows, without looking at the device controls, to determine in what position the V/div, Time/div switches, synchronization modes are located, and to become familiar with voltage and duration readings etc.
The interfaces of modern digital oscilloscopes vary from manufacturer to manufacturer, so you should carefully read the User's Guide before you begin.
- The main measurement mode should be the closed input mode (see Fig. 2). This will protect the device circuits from damage by unexpectedly high voltage;
- Before starting measurements, set the V/div switch to the “coarsest” limit, successively increasing the gain, achieve the desired image size on the screen;
- Use standard probes and probes of the oscilloscope, this increases the accuracy of measurements and reduces the risk of damage to the device;
- If the image on the oscilloscope screen has sufficient amplitude, but you cannot see it, most likely the position of the Time/div switch is selected incorrectly. By changing its position, achieve the most stable image, then select the signal element on which the synchronization will be carried out using the Sync Amplitude knob. If necessary, change the polarity of the synchronization signal and the type of synchronization.
HOW TO CHOOSE AN OSCILLOSCOPE?
An oscilloscope is a complex and expensive device; there are hundreds of models on the market - from the simplest and most budget-friendly to extremely expensive, specialized and precision instruments. How to make the right choice and purchase exactly the oscilloscope that will be useful to you when setting up audio-video equipment? In this chapter we will give you some tips.
Before you start choosing an oscilloscope, you need to clearly understand what tasks will be solved with its help. At the same time, it is necessary to remember about the prospects, since an oscilloscope is not purchased for one year and not to perform one single job.
1. Which oscilloscope should I choose: analog or digital?
Analog oscilloscopes provide the ability to continuously monitor an analog signal in real time, have simple, intuitive controls and are low in cost. At the same time, analog oscilloscopes have low accuracy compared to digital ones; at low scan speeds they are characterized by flickering.
Digital oscilloscopes allow you to “freeze” the image on the screen, have high measurement accuracy, a bright, well-focused image of the signal at any sweep speed, but they are much more expensive, difficult to operate and in some cases display the signal incorrectly.
The undeniable advantages of digital oscilloscopes are also the ability to measure voltages and signal durations “on the fly,” as well as the ability to connect to external recording devices, and the availability of autodiagnostics and autocalibration tools.
2. Determine the required bandwidth
One of the main characteristics of an oscilloscope that influences the choice of device is the bandwidth, which depends on what signals need to be measured and with what accuracy.
Keep in mind that digital oscilloscopes have two fundamentally different bandwidths: repeating (or analog) bandwidth and one-shot bandwidth. Most real-world signals contain many high-frequency harmonics, so wideband oscilloscopes display such signals more accurately.
When making accurate timing measurements, the oscilloscope's bandwidth must be at least three times the first harmonic of the highest-frequency signal being measured. And for accurate amplitude measurements, it is desirable that the oscilloscope bandwidth be ten times greater than the frequency of the measured signal.
Analog oscilloscopes rarely have bandwidths greater than 400 MHz, while digital oscilloscopes can have bandwidths up to 50 GHz.
3. Determine the required number of channels
Two-channel oscilloscopes are the most popular, but recently four-channel models have become increasingly widespread, since their per-channel cost is lower than that of two-channel models, and the capabilities are significantly wider. However, operating such a device can be difficult.
Some oscilloscopes have 2 full channels and 2 additional channels with a limited sensitivity range. In this case, the oscilloscope has only 2 analog-to-digital converters (ADCs), the inputs of which are switched into 4 channels.
4. Determine the required sampling rate (for digital oscilloscopes)
For problems involving single-shot or transient variations, sampling rate is of paramount importance. The sample rate parameter indicates the speed at which the oscilloscope can sample the input signal. Higher sampling rates result in higher bandwidth for single-shot signals and greater temporal resolution.
Most digital oscilloscope manufacturers use a sample rate to bandwidth ratio of 4:1 (with interpolation) or 10:1 (without built-in interpolation) for single-shot signals to prevent signal distortion.
5. Determine the required amount of memory (for digital oscilloscopes)
The amount of memory required depends on the total duration of the signal being analyzed and the desired time resolution. If signals are studied over a long period of time with high resolution, then more memory will be required. Larger memory will allow higher sampling rates to be used at slow scan speeds, reducing the likelihood of receiving a corrupted signal and providing more information about the signal.
Keep in mind that increasing the amount of memory can cause the oscilloscope to slow down significantly as it needs to process more data.
6. Determine the required capabilities to start the device
For most general purpose oscilloscope users, simply edge triggering is often not enough. For many tasks, it is also useful to have additional triggering capabilities that can detect events that are otherwise very difficult to track. The ability to trigger on a television signal allows you to set the device to a specific field or line.
7. Determine the required transient detection capabilities
In principle, any analog oscilloscope is always capable of displaying glitches and jitter. The only question is whether the rise rate in the vertical deviation channel (ultimately the bandwidth) and the brightness of the oscillogram are sufficient to study these processes. Oscilloscopes with pulse noise triggering capabilities allow you to isolate hard-to-detect pulse noise and trigger the oscilloscope on it. This additional feature is very useful when searching for the cause of abnormal operation of the circuit under study.
8. Additional features
Many modern digital oscilloscopes can perform the function of a spectrum analyzer, but in the audio domain it is usually poorly implemented.
Most digital and analog-to-digital oscilloscopes can interface with a personal computer, printer, or plotter via GPIB, RS-232, or Centronics interfaces. In recent years, the USB interface has been increasingly used.
Many modern digital oscilloscopes are equipped with disk drives or flash memory connectors that allow you to save screen images of waveforms (in various formats) and measurement results in numerical form, and then quickly transfer them to a computer for further processing. These capabilities can save time when, for example, you want to paste an oscilloscope screen into a report or copy waveform data into a spreadsheet.
Try to work with the device, evaluate how easy it is to operate, is it possible to intuitively control the device while the main attention is on the circuit under study? Evaluate the screen's response speed and the time it takes the oscilloscope to execute commands. Does the device have a command memory?
MEASUREMENT OF AMPLITUDE-FREQUENCY CHARACTERISTICS
When monitoring the technical condition of radio-electronic equipment, an important place is occupied by measuring the amplitude-frequency characteristics of its various components.
When measuring the amplitude-frequency characteristics (AFC) of devices or their components, it is convenient to represent them in the form of a four-terminal network. Then the frequency response is the dependence of the modulus (absolute value) of the transmission coefficient of the quadrupole on the signal frequency.
The transfer coefficient is the ratio of the power or voltage at the output of a quadrupole to the power or voltage at its input.
If the output voltage is less than the input voltage, the signal is weakened when the signal passes through the quadrupole. Such a four-terminal network is called passive (an example is a passive electrical filter), and the transmission coefficient is the attenuation coefficient.
When the output voltage is greater than the input voltage, the signal is amplified, and the transmission coefficient is the gain. The four-terminal network in this case is called active (an example is an audio frequency signal amplifier).
The value of the transmission coefficient of the quadripole and the value of the signal frequency at which it was determined form a point in the coordinate system, and the collection of such points form the frequency response curve in the required frequency range. In Fig. Figure 12 shows as an example the frequency response of an antenna amplifier operating in the television broadcast range.
Rice. 12. Frequency response of the antenna amplifier
METHODS FOR MEASURING PARAMETERS OF AMPLITUDE-FREQUENCY CHARACTERISTICS
Measurement of the parameters of the amplitude-frequency characteristics of quadrupoles is carried out using a sweep frequency generator (SWG) and an indicator device.
The frequency of the generator smoothly changes according to a certain law in the required frequency band, and the frequency response curve is reproduced on an oscilloscope-type indicator.
The block diagram of the simplest automatic frequency response meter is shown in Fig. 13.
Rice. 13. Block diagram of an automatic frequency response meter
The signal from the frequency converter is fed to the input of the quadripole under study. Due to the presence of this four-port network depending on the transmission coefficient module on the signal frequency, the signal at its output is amplitude modulated. The envelope of this signal, isolated on the detector head, which is part of the indicator device, controls the vertical deflection of the indicator beam, drawing the frequency response curve.
The control of the frequency of the main frequency and the horizontal deflection of the indicator beam is carried out by a modulating voltage block, which simultaneously synchronizes the operation of these two nodes.
In an frequency response meter built according to such a structural diagram, the horizontal position of the beam on the indicator screen corresponds to the frequency at the input of the quadripole under study, and the vertical position corresponds to the value of the modulus of the transmission coefficient at this frequency. Thus, the frequency response curve of the quadripole under study is automatically drawn on the screen.
The automatic amplitude control unit is used to ensure a constant output signal level throughout the entire frequency swing range.
Part of the signal from the MFC is fed to a frequency tag block, in which a whole spectrum of calibration frequencies is generated within the operating range of the MFC. At the moment the frequency of the MCG coincides with any of these frequencies, signals are generated that are fed to the indicator block and observed on the screen in the form of amplitude marks.
An attenuator is used to calibrate the change in the output voltage of the MCG.
Depending on the sweep bandwidth, devices are divided into narrow-band, mid-band, broadband and combined. Narrowband frequency response meters provide a sweep band that is a fraction or a few percent of the central frequency, while broadband meters provide a sweep band that is the full frequency range of the device. Combined devices combine the functions of both narrowband and broadband devices.
Frequency response meters can have a linear or logarithmic amplitude scale.
The most widely used are universal frequency response meters, which allow solving a wide range of measurement tasks. In Fig. Figure 14 shows the domestically produced frequency response meter X1-50, which is used when setting up and testing television equipment. The presence of a built-in grid field generator allows you to check the linearity of the television image, and with the help of an external measuring bridge, check the matching of the antenna leads.
Rice. 14. Frequency response meter X1-50
- An important role is played by matching the device output with the load resistance. If at frequencies up to tens of megahertz the mismatch only leads to a decrease in the level of the output signal, then at higher frequencies it leads to an increase in the unevenness of the output signal in the swing band. Matching the input of the device under study is possible by connecting at the end of the cable connecting them to the output of the frequency response meter, a resistance close to the wave one. If the four-port network under study has a low-impedance input with a characteristic impedance different from the output impedance of the frequency response meter, then it must be connected to the device through a matching device.
- If the output of the device under study is low-impedance, for example, a filter, a television antenna amplifier, or a coaxial transmission line, it should be connected to the input of the indicator device through a matched detector head, and if the output impedance of the quadripole differs from the load resistance of the detector head, a matching device must be installed between them.
- When studying the frequency response of amplifiers, distortions caused by their overload are possible, as a result of which the top of the frequency response curve will appear flatter than it actually is. In this case, a signal with a minimum level must be supplied to the amplifier input.
- When setting up multi-stage devices, such as intermediate frequency amplifiers, video amplifiers, when you need to view the frequency response of each stage separately, use the high-impedance detector head included with the device.
- If your frequency response meter has a two-channel indicator, you can adjust the frequency response of devices by comparing them with reference ones. To do this, the signal from the output of the frequency response meter is fed simultaneously to the inputs of the tuned and reference devices, and their outputs are connected to separate channels of the indicator, the gain of which is set to the same. By changing the device settings, we achieve alignment of its frequency response with the reference one.
- Along with the study of the frequency response of quadripoles, frequency response meters make it possible to solve a number of other measurement tasks, such as measuring the quality factor of an oscillating circuit, the slope of the frequency response, impedances and SWR of the load, and the study of cables.
MEASUREMENT OF RADIO SIGNAL SPECTRUM PARAMETERS
In the practice of working with complex modern electronic equipment, visual observation of the signal shape using an oscilloscope is sometimes insufficient. More sensitive and informative is analysis of signal spectral characteristics . It is especially important to know the spectral composition of signals at the present time, when the problem of electromagnetic compatibility of radio-electronic equipment is acute, when it is necessary to determine the parameters of the signal at the input and output of its transmission line.
Currently, there are two main methods for measuring the characteristics of the signal spectrum: calculating Fourier transforms and using digital filters.
The Fourier transform allows you to represent a complex signal as a set of harmonic sinusoidal oscillations with different frequencies and amplitudes.
In practice, this means that almost any signal can be decomposed into a finite number of harmonics with frequencies 
, amplitude 
and phase – 
, Where:
k=1, 2, 3…;
f 0
– frequency of the first harmonic;
T- time;
a k and b k– conversion coefficients.
Graph of values depending on k called the Fourier line spectrum. An example of such a spectrum obtained analytically is shown in Fig. 15, and the photo of the spectrum analyzer screen is in Fig. 16.
Rice. 15. Fourier line spectrum
Rice. 16. Spectrum of the signal emitted by the speaker
Thus, the signal spectrum is characterized by the frequency, amplitude and phase of its components, which are measured during the creation and operation of electronic equipment and electronic components.
In addition to these basic characteristics, the signal spectrum is characterized by shape and width.
The rapid development of computer technology now makes it possible to create spectrum analyzers using a digital filter that operate effectively in the low-frequency (sound) range, which was an almost impossible task for older types of analyzers. Digital filters are universal, stable, do not require adjustment, and have a wide operating range. It is safe to assume that spectrum analyzers of this type will dominate this segment of the instrumentation market in the near future.