Circuits of simple pulse generators. Powerful laboratory pulse generator Capacitor pulse generator
Pulse generators are an important component of many radio-electronic devices. The simplest pulse generator (multivibrator) can be obtained from a two-stage ULF (Fig. 6.1). To do this, simply connect the amplifier's input to its output. The operating frequency of such a generator is determined by the values of R1C1, R3C2 and the supply voltage. In Fig. 6.2, 6.3 show multivibrator circuits obtained by simply rearranging the elements (parts) of the circuit shown in Fig. 6.1. It follows that the same simple diagram can be depicted in different ways.
Practical examples of using a multivibrator are shown in Fig. 6.4, 6.5.
In Fig. Figure 6.4 shows a generator circuit that allows you to smoothly redistribute the duration or brightness of the LEDs connected as a load in the collector circuit. By rotating the R3 potentiometer knob, you can control the ratio of the durations of the LEDs of the left and right branches. If you increase the capacitance of capacitors C1 and C2, the generation frequency will decrease and the LEDs will begin to blink. As the capacitance of these capacitors decreases, the generation frequency increases, the flickering of the LEDs will merge into a continuous glow, the brightness of which will depend on the position of the potentiometer R3 knob. Based on such a circuit design, various useful structures can be assembled, for example, a brightness control for an LED flashlight; toy with blinking eyes; a device for smoothly changing the spectral composition of the radiation source (multi-colored LEDs or miniature light bulbs and a light-summing screen).
The variable frequency generator (Fig. 6.5) designed by V. Tsibulsky allows you to obtain sound that smoothly changes over time in frequency [P 5/85-54]. When the generator is turned on, its frequency increases from 300 to 3000 Hz in 6 seconds (with a capacitor capacity of SZ 500 μF). Changing the capacitance of this capacitor in one direction or another accelerates or, conversely, slows down the rate of change in frequency. You can smoothly change this speed with variable resistance R6. In order for this generator to act as a siren, or to be used as a sweeping frequency generator, it is possible to provide a circuit for forced periodic discharge of the SZ capacitor. Such experiments can be recommended for independent expansion of knowledge in the field of pulse technology.
A controlled square pulse generator is shown in Fig. 6.6 [R 10/76-60]. The generator is also a two-stage amplifier covered by positive feedback. To simplify the generator circuit, it is enough to connect the emitters of the transistors with a capacitor. The capacitance of this capacitor determines the operating frequency of generation. In this circuit, a varicap is used as a voltage-controlled capacitance to control the generation frequency. An increase in the blocking voltage on the varicap leads to a decrease in its capacity. Accordingly, as shown in Fig. 6.7, the operating frequency of generation increases.
The varicap, as an experiment and to study the operating principle of this semiconductor device, can be replaced with a simple diode. It should be taken into account that germanium point diodes (for example, D9) have a very small initial capacitance (of the order of several pF), and, accordingly, provide a small change in this capacitance depending on the applied voltage. Silicon diodes, especially power diodes designed for high current, as well as zener diodes, have an initial capacity of 100... 1000 pF, so they can often be used instead of varicaps. Pn junctions of transistors can also be used as varicaps, see also Chapter 2.
To control the operation, the signal from the generator (Fig. 6.6) can be applied to the input of the frequency meter and the tuning limits of the generator can be checked when the control voltage changes, as well as when changing a varicap or its analogue. It is recommended that the results obtained (control voltage values and generation frequency) when using different types of varicaps be entered into a table and displayed on a graph (see, for example, Fig. 6.7). Note that the stability of generators based on RC elements is low.
In Fig. 6.8, 6.9 show typical circuits of light and sound pulse generators made on transistors of various conductivity types. Generators are operational in a wide range of supply voltages. The first of them produces short flashes of light with a frequency of one Hz, the second produces pulses of sound frequency. Accordingly, the first generator can be used as a beacon, a light metronome, the second - as a sound generator, the oscillation frequency of which depends on the position of the potentiometer R1. These generators can be combined into a single unit. To do this, it is enough to turn on one of the generators as a load of the other, or in parallel with it. For example, instead of a chain of LEDs HL1, R2 or in parallel with it (Fig. 6.8), you can turn on the generator according to the circuit in Fig. 6.9. The result will be a periodic sound or light and sound signaling device.
The pulse generator (Fig. 6.10), made on a composite transistor (p-p-p and p-p-p), does not contain capacitors (a piezoceramic emitter BF1 is used as a frequency-setting capacitor). The generator operates at a voltage from 1 to 10 B and consumes a current from 0.4 to 5 mA. To increase the sound volume of a piezoceramic emitter, it is tuned to the resonant frequency by selecting resistor R1.
In Fig. Figure 6.11 shows a rather original generator of relaxation oscillations, made on a bipolar avalanche transistor.
The generator contains as an active element a transistor of the K101KT1A microcircuit with inverse switching in the mode with a “broken” base. The avalanche transistor can be replaced with its analogue (see Fig. 2.1).
Devices (Fig. 6.11) are often used to convert the measured parameter (light intensity, temperature, pressure, humidity, etc.) into frequency using resistive or capacitive sensors.
When the generator is operating, a capacitor connected in parallel to the active element is charged from the power source through a resistor. When the voltage on the capacitor reaches the breakdown voltage of the active element (avalanche transistor, dinistor, or similar element), the capacitor is discharged into the load resistance, after which the process is repeated with a frequency determined by the constant of the RC circuit. Resistor R1 limits the maximum current through the transistor, preventing its thermal breakdown. The timing circuit of the generator (R1C1) determines the operating range of generation frequencies. Headphones are used as an indicator of sound vibrations for quality control of generator operation. To quantify the frequency, a frequency meter or pulse counter can be connected to the generator output.
The device is operational in a wide range of parameters: R1 from 10 to 100 kOhm (and even up to 10 MOhm), C1 - from 100 pF to 1000 μF, supply voltage from 8 to 300 V. The current consumed by the device usually does not exceed one mA. It is possible for the generator to operate in standby mode: when the base of the transistor is shorted to ground (common bus), generation is interrupted. The converter-generator (Fig. 6.11) can also be used in the mode of a touch key, a simple Rx and Cx meter, a tunable wide-range pulse generator, etc.
Pulse generators (Fig. 6.12, 6.13) are also made on avalanche transistors of the K101KT1 microcircuit of the p-p-p type or K162KT1 of the p-p-p type, dinistors, or their analogues (see Fig. 2.1). The generators operate at a supply voltage above 9 B and produce a triangular voltage. The output signal is taken from one of the terminals of the capacitor. The input resistance of the cascade following the generator (load resistance) must be tens of times greater than the value of resistance R1 (or R2). A low-resistance load (up to 1 kOhm) can be connected to the collector circuit of one of the generator transistors.
Quite simple and often encountered in practice pulse generators (blocking generators) using inductive feedback are shown in Fig. 6.14 [A. With. USSR 728214], 6.15 and 6.16. Such generators are usually operational over a wide range of supply voltage variations. When assembling blocking generators, it is necessary to observe the phasing of the terminals: if the “polarity” of the winding is connected incorrectly, the generator will not work.
Such generators can be used when testing transformers for the presence of interturn short circuits (see Chapter 32): such defects cannot be detected by any other method.
Literature: Shustov M.A. Practical circuit design (Book 1), 2003
Pulse generators are used in many radio devices (electronic meters, time relays) and are used when setting up digital equipment. The frequency range of such generators can be from a few hertz to many megahertz. Here are simple generator circuits, including those based on digital “logic” elements, which are widely used in more complex circuits as frequency-setting units, switches, sources of reference signals and sounds.
In Fig. Figure 1 shows a diagram of a generator that generates single rectangular pulses when the S1 button is pressed (that is, it is not a self-oscillator, the diagrams of which are given below). An RS trigger is assembled on the logical elements DD1.1 and DD1.2, which prevents the penetration of bounce pulses from the button contacts to the recalculating device. In the position of the contacts of button S1, shown in the diagram, output 1 will have a high level voltage, output 2 will have a low level voltage; when the button is pressed - vice versa. This generator is convenient to use when checking the performance of various meters.
In Fig. Figure 2 shows a diagram of a simple pulse generator based on an electromagnetic relay. When power is applied, capacitor C1 is charged through resistor R1 and the relay is activated, turning off the power source with contacts K 1.1. But the relay does not release immediately, since for some time current will flow through its winding due to the energy accumulated by capacitor C1. When contacts K 1.1 close again, the capacitor begins to charge again - the cycle repeats.
The switching frequency of the electromagnetic relay depends on its parameters, as well as the values of capacitor C1 and resistor R1. When using the RES-15 relay (passport RS4.591.004), switching occurs approximately once per second. Such a generator can be used, for example, to switch garlands on a New Year tree or to obtain other lighting effects. Its disadvantage is the need to use a capacitor of significant capacity.
In Fig. Figure 3 shows a diagram of another generator based on an electromagnetic relay, the operating principle of which is similar to the previous generator, but provides a pulse frequency of 1 Hz with a capacitor capacity 10 times smaller. When power is applied, capacitor C1 is charged through resistor R1. After some time, the zener diode VD1 will open and relay K1 will operate. The capacitor will begin to discharge through resistor R2 and the input resistance of the composite transistor VT1VT2. Soon the relay will release and a new cycle of generator operation will begin. Switching on transistors VT1 and VT2 according to a composite transistor circuit increases the input impedance of the cascade. Relay K 1 can be the same as in the previous device. But you can use RES-9 (passport RS4.524.201) or any other relay that operates at a voltage of 15...17 V and a current of 20...50 mA.
In the pulse generator, the diagram of which is shown in Fig. 4, the logic elements of the DD1 microcircuit and the field-effect transistor VT1 are used. When changing the values of capacitor C1 and resistors R2 and R3, pulses with a frequency from 0.1 Hz to 1 MHz are generated. Such a wide range was obtained through the use of a field-effect transistor, which made it possible to use resistors R2 and R3 with a resistance of several megaohms. Using these resistors, you can change the duty cycle of the pulses: resistor R2 sets the duration of the high level voltage at the output of the generator, and resistor R3 sets the duration of the low level voltage. The maximum capacitance of capacitor C1 depends on its own leakage current. In this case it is 1...2 µF. The resistance of resistors R2, R3 is 10...15 MOhm. Transistor VT1 can be any of the KP302, KP303 series. The microcircuit is K155LA3, its power supply is 5V stabilized voltage. You can use CMOS microcircuits of the K561, K564, K176 series, the power supply of which lies within the range of 3 ... 12 V, the pinout of such microcircuits is different and is shown at the end of the article.
If you have a CMOS chip (K176, K561 series), you can assemble a wide-range pulse generator without using a field-effect transistor. The diagram is shown in Fig. 5. For the convenience of setting the frequency, the capacitance of the timing circuit capacitor is changed with switch S1. The frequency range generated by the generator is 1...10,000 Hz. Microcircuit - K561LN2.
If you need high stability of the generated frequency, then such a generator can be made “quartzized” - turn on the quartz resonator at the desired frequency. Below is an example of a quartz oscillator at a frequency of 4.3 MHz:
In Fig. Figure 6 shows a diagram of a pulse generator with adjustable duty cycle.
Duty cycle is the ratio of the pulse repetition period (T) to their duration (t):
The duty cycle of high-level pulses at the output of logic element DD1.3, resistor R1, can vary from 1 to several thousand. In this case, the pulse frequency also changes slightly. Transistor VT1, operating in key mode, amplifies the power pulses.
The generator, the diagram of which is shown in the figure below, produces pulses of both rectangular and sawtooth shapes. The master oscillator is made on logical elements DD 1.1-DD1.3. A differentiating circuit is assembled on capacitor C2 and resistor R2, thanks to which short positive pulses (about 1 μs in duration) are formed at the output of the logical element DD1.5. An adjustable current stabilizer is made on field-effect transistor VT2 and variable resistor R4. This current charges the capacitor C3, and the voltage across it increases linearly. When a short positive pulse arrives at the base of transistor VT1, transistor VT1 opens, discharging capacitor S3. A sawtooth voltage is thus formed on its plates. Resistor R4 regulates the charging current of the capacitor and, consequently, the steepness of the increase in the sawtooth voltage and its amplitude. Capacitors C1 and SZ are selected based on the required pulse frequency. Microcircuit - K561LN2.
Digital microcircuits in generators are interchangeable in most cases and can be used in the same circuit as microcircuits with “NAND” and “NOR” elements, or simply inverters. A variant of such replacements is shown in the example of Figure 5, where a microcircuit with K561LN2 inverters was used. Exactly such a circuit, preserving all parameters, can be assembled on both K561LA7 and K561LE5 (or K176, K564, K164 series), as shown below. You just need to observe the pinout of the microcircuits, which in many cases even coincides.
The pulse current generator (PGG) is designed for the primary conversion of electrical energy. Includes an AC electrical network with a frequency of 50 Hz, a high-voltage transformer, a rectifier, a current-limiting device, and protection equipment. In the GIT, charging and discharging circuits are distinguished, which are interconnected by a bank of capacitors. The GIT, which is a power source, is connected to the technological unit through a discharge circuit.
Pulse generators are characterized by the following main parameters: voltage across the capacitor bank U, electric capacity of the battery C, energy accumulated in capacitors W n, energy in impulse W 0 pulse repetition rate υ.
The purpose of the charging circuit is to charge a bank of capacitors to a given voltage. The circuit includes a current-limiting device, a step-up transformer and a high-voltage rectifier. Selenium or silicon pillars are used to rectify the charging current. Using a high-voltage transformer, the initial voltage of the 380/220 V supply network is increased to (2-70) 10 3 V.
In the scheme L - C – D we have ή 3 > 50%.
When using pulsed current generators, energy losses are significant at the stage of discharge formation. The common system that combines pulse current and voltage generators does not have this drawback (Fig. 30). In this system, the breakdown of the forming gap is produced by the energy of the capacitor bank of the voltage generator, which creates a current-carrying channel in the main working gap and ensures the release of the main discharge energy in the discharge gap of the pulse current generator.
The characteristic ratio of electrical voltages and capacitances for such a system is: » where index 1 corresponds to the voltage generator, and index 2 to the current generator. So, for example
The energy and weight-size parameters of the generator significantly depend on the high-voltage transformer and rectifier. The efficiency of the charging-rectifying device increases when using high-voltage silicon pillars. Rectifiers have high characteristic values - specific
volume from 0.03 to 0.28 m 3 /kW and specific gravity 25-151 kg/kW.
In electric pulse installations, single units are also used, including a transformer and a rectifier, which reduces the main dimensions and simplifies the switching network.
Pulse capacitors are designed to store electrical energy. High-voltage pulse capacitors must have increased specific energy capacity, low internal inductance and low resistance at high discharge currents, and the ability to withstand multiple charge-discharge cycles. The main technical data of pulse capacitors are given below.
Voltage (nominal), kV................................5-50
Capacitance (nominal), µF. . ....................................0.5-800
Discharge frequency, number of pulses/min.................................1-780
Discharge current, kA................................................... .............0.5-300
Energy intensity, J/kg.................................................... .......4.3-30
Resource, number of pulses................................................... .10 e - 3 10 7
One of the main characteristics of pulse capacitors, which affects the size of the battery and the electric pulse installation as a whole, is the indicator of specific volumetric energy intensity
(3.23)
Where E n- accumulated energy; V to- capacitor volume.
For existing capacitors ω s= 20 -g 70 kJ/m 3, which determines the increased dimensions of the storage devices. So the battery capacity for E n= 100 kJ is 1.5-5.0 m 3. In storage devices, capacitors are connected into batteries, which ensures the summation of their electrical capacity, which is equal to 100-8000 μF.
High-voltage switches are used to instantly release electrical energy accumulated in a capacitor bank in a process unit. High-voltage switches (discharge arresters) perform two functions: they disconnect the discharge circuit
from the storage device when charging it; instantly connect the drive to the load circuit.
Various design schemes of arresters and types of switches corresponding to these schemes are possible: air, vacuum, gas-filled, contact disc, ignitron and trigatron, with a solid dielectric.
The basic requirements for switches are as follows: withstand high-voltage operating voltage without breakdown, have low inductance and low resistance, and provide a given current pulse repetition rate.
In laboratory electric pulse installations, mainly air-type spark gaps are used, which provide switching of high energies over a long service life and have a relatively simple design (Fig. 31).
Dischargers of this type have a number of significant disadvantages that limit their use: the influence of the surface condition and the state of the atmospheric air (dust, humidity, pressure) on the stability of the reproduced pulse; nitrogen oxides are formed, which have an effect on humans; powerful high-frequency sound pressure is generated.
In industrial mobile installations, mechanical disc switches have become widespread (see Fig. 31, A). Dischargers of this type are simple in electrical circuit and design, reliable during transportation and operation in areas with rough terrain, but require regular cleaning of the surface of the disc elements. I
The electric pulse installation also includes control units for the pulse generator and the technological process, protection and interlock systems, and auxiliary systems that provide mechanization and automation of processes in the technological unit.
The control unit includes electrical circuits for starting, blocking and a synchronization pulse generation circuit.
The interlock system serves to “instantly switch off the high voltage voltage. The control system consists of a voltmeter and a kipovoltmeter, indicating the mains voltage and the capacitor bank voltage, respectively, indicator lamps, sound signals, and a frequency meter.
Technological node
The technological unit is designed to convert electrical energy into other types of energy and to transfer the converted energy to the processing object.
In relation to the specifics of discharge-pulse technology for rock destruction, the technological unit includes: a working discharge chamber, a working element in the form of an electrode system or an electrohydraulic fuse, a device for inlet and outlet of working fluid and a device for moving electrodes or an exploding conductor (Fig. 32). The working discharge chamber is filled with a working liquid or a special dielectric compound.
Discharge (working) chambers are divided into open and closed, buried and surface, stationary, mixed and remote. Cameras can be disposable or reusable; vertical, horizontal and inclined. The type and shape of the working chamber must ensure maximum release of accumulated electrical energy, maximum hp. converting this energy into mechanical energy, transferring this energy to the processing object or to its specified zone.
The working technological element is designed to directly convert electrical energy into mechanical energy and to input this energy into the working environment, and through it to the processing object. The type of working element depends on the type of electrical discharge in the liquid used in a given technological process - with free formation of the discharge, electrode systems are rational (Fig. 33, A); with an initiated discharge - an electro-hydraulic fuse with an exploding conductor (Fig. 33.6).
The working body experiences dynamic loads, the action of an electromagnetic field and ultraviolet radiation, as well as the influence of the working fluid.
The electrode system is used with free discharge formation. According to the design factor, rod linear and coaxial systems are distinguished. The simplest in design are linear (opposing or parallel) systems with combinations of electrode shapes: tip - tip and tip - plane. The disadvantages of linear systems are their significant inductance (1-10 µH) and non-directional action.
Coaxial systems are more advanced, having low self-inductance and high efficiency. converting accumulated electrical energy into plasma energy. The disadvantage of coaxial systems is their low reliability and fragility. The electrode system is technologically advanced and highly productive due to the high frequency of the process of creating mechanical loading forces.
Based on the number of repeated discharges, single-acting and multiple-acting systems are distinguished. Reusable systems are more economical and productive. The amount of energy converted by the electrode system also affects design and durability.
In the mining industry, electrode systems designed for pulse repetition rates of 1-12 per minute have become more widely used. During an electrical discharge, due to thermal processes, erosion of the electrodes occurs, the intensity of which depends on the material of the electrodes and the working fluid, as well as on the amount of energy released in
discharge channel. The working part of the electrodes is made of steel St3 or St45; the diameter of the protruding part must be more than 8 mm with a length of at least 12 mm. In the electrode zone, the melting temperature of iron is reached in 10 -6 s, and the boiling point in 5 10 -6 s.
The resulting intense destruction of the electrode is accompanied by the formation of plasma jets (vapors and liquid drops of metal). The weakened zone of the electrode is the insulating layer at the boundary between the output of the rod - the current conductor and water.
The main requirements for the electrode system are: high electrical energy conversion coefficient, high
operational and technological indicators, economically feasible durability. Electrodes made of an alloy of copper, tungsten carbide and nickel have the greatest erosion resistance.
The surface area of the cathode should exceed the area of the anode by 60-100 times, which, combined with the application of a positive voltage pulse to the anode, will reduce energy losses at the stage of discharge formation and increase efficiency. systems. Rational insulation materials are fiberglass, vacuum rubber, polyethylene.
An electrohydraulic fuse is used in an initiated discharge; it absorbs dynamic loads, the effects of high-current fields and working fluid, which leads to the destruction of the housing, insulation and electrode.
In an electrohydraulic fuse, the positive electrode is isolated from the body; an exploding conductor is installed between the electrode and a grounded body, which acts as a negative electrode.
Depending on the technological problems being solved, conductors made of copper, aluminum, and tungsten are used; Conductor dimensions range from diameter 0.25-2 mm, length 60-300 mm. The design of the electrohydraulic fuse must ensure the concentration of energy in the required direction and the formation of a cylindrical shock wave front, as well as the manufacturability of operations for installing and replacing the exploding conductor.
To fulfill part of these requirements, it is necessary that the body of the electrohydraulic fuse serves as a rigid barrier for the propagating wave front.
This is ensured by the use of special cumulative recesses in the fuse body and a certain combination of linear dimensions of the body and conductor. Thus, the diameter of the fuse body should be 60 times or more the diameter of the exploding conductor.
In recent years, new design schemes and special devices have been developed that increase the efficiency of the working bodies, ensuring that the action is directed towards the processing object of the generated waves and hydraulic flow.
Such devices include passive reflective surfaces, electrodes with complex geometries, and divergent wave generators. There are also devices for drawing the exploding conductor, which complicates the design of the fuse, but increases the manufacturability of the process.
To directly convert the energy of an electric discharge into the energy of a compression pulse, special electric explosive cartridges are used (Fig. 34).
The working fluid filling the technological unit plays a very significant role in the process of electrical discharge. It is in the liquid that the discharge is reproduced with the direct conversion of electrical energy into mechanical energy.
Ionization is observed in the liquid, as well as gas release of unreacted oxygen and hydrogen (up to 0.5 10 -6 m 3 / kJ), the liquid is drawn into motion by the propagating wave front, which forms a hydraulic flow in the technological unit, capable of performing mechanical work.
Water (technical, sea, distilled) and aqueous electrolytes are used as the working fluid; hydrocarbon (kerosene, glycerin, transformer oil) and silicone (polymethylsiloxane) liquids, as well as special dielectric, liquid and solid compositions. Process water, whose specific electrical conductivity is (1-10) S/m, has become more widely used.
The electrical conductivity of the liquid significantly affects the amount of energy required to form a discharge, since it determines the magnitude of the breakdown voltage and the speed of movement of the streamers. The minimum voltage at which streamers appear is estimated at 3.6 10 3 V/mm.
The specific electrical conductivity values (S/m) of some liquids used to fill the technological unit are given below.
Process water (tap)................................................... ............(1-10) 10 -2
Sea water................................................ ........................................1-10
Distilled water................................................ ........................4.3 -10 -4
Glycerol................................................. ........................................................ ..6.4 10 -6
It can be seen that dielectric liquids have low ionic conductivity. The specific electrical resistance of the liquid (r l) also determines the value of the electrical efficiency. and depends on the amount of energy introduced per unit volume of working fluid. Thus, for water, the parameter rj decreases with increase to values of 500-1000 kJ/; with a further increase in W 0, the parameter rz stabilizes within the range of 10-25 Ohm-m.
The electric discharge in a liquid also depends on the density of the working liquid - with increasing density, the peak of overvoltages and the steepness of the current decline decrease. To increase the voltage of the discharge circuit, and, accordingly, the value of the breakdown voltage, working fluids with low specific conductivity (for example, industrial water) should be used.
The use of liquids with higher conductivity facilitates the formation of sliding discharges; increases energy losses at the stage of channel formation and reduces the amplitude of the shock wave.
Viscous compositions are also used as a working fluid (spindle oil - 70%, aluminum powder - 20%, chalk - 10%), which increases the amplitude of the shock wave by 20-25% and reduces energy losses.
Metallized dielectric thread and paper tapes impregnated with electrolyte are also used as a dielectric. The introduction of a solid dielectric reduces the total energy consumption for breakdown (4-5 times), reduces the required number of streamers (4-6 times), reduces thermal radiation and ultraviolet radiation. The introduction of solid particles of conductive additives into the working fluid flow is used instead of exploding conductors.
Mitchell Lee
LT Journal of Analog Innovation
Steep pulse sources that simulate a step function are often useful in some laboratory measurements. For example, if the slope of the fronts is on the order of 1...2 ns, you can estimate the rise time of the signal in the RG-58/U cable or any other, taking a segment only 3...6 m long. The workhorse of many laboratories - the ubiquitous HP8012B pulse generator - does not reach 5 ns, which is not fast enough to solve such a problem. Meanwhile, the rise and fall times of the gate driver outputs of some switching controllers can be less than 2 ns, making these devices potentially ideal pulse sources.
Figure 1 shows a simple implementation of this idea, based on the use of a flyback converter controller operating at a fixed switching frequency. The controller's own operating frequency is 200 kHz. Applying part of the output signal to the SENSE pin causes the device to operate at a minimum duty cycle, generating output pulses with a duration of 300 ns. Power decoupling is of no small importance for this circuit, since the output current supplied to a 50 Ohm load exceeds 180 mA. 10 µF and 200 ohm decoupling elements minimize peak distortion without sacrificing edge steepness.
The output of the circuit is connected directly to the 50 ohm terminated load, providing a signal swing of about 9 V across it. In cases where pulse quality is of paramount importance, it is recommended to suppress the triple pass signal by absorbing reflections from the cable and remote load using the series termination shown in the circuit. Series matching, that is, matching on the transmitting side, also turns out to be useful when the circuit operates on passive filters and other attenuators designed for a certain impedance of the signal source. The output impedance of the LTC3803 is approximately 1.5 ohms, which should be taken into account when choosing the value of the series terminating resistor. Series matching works well up to impedances of at least 2 kΩ, above which it becomes difficult to provide the necessary bandwidth at the resistor-to-circuit junction, resulting in degraded pulse quality.
In a series-matched system, the output signal has the following characteristics:
- pulse amplitude - 4.5 V;
- rise and fall times are the same and equal to 1.5 ns;
- pulse flat top distortion - less than 10%;
- the decline in the peak of the impulse is less than 5%.
When connecting a 50 ohm load directly, the rise and fall times are not affected. To get the best pulse shape, connect a 10uF capacitor as close as possible to the V CC and GND pins of the LTC3803, and connect the output directly to the terminating resistor using stripline technology. The characteristic impedance of approximately 50 ohms has a 2.5 mm wide printed conductor on a 1.6 mm thick double-sided printed circuit board.
Related materials
PMIC; DC/DC converter; Uin:5.7÷75V; Uout:5.7÷75V; TSOT23-6
| Provider | Manufacturer | Name | Price |
|---|---|---|---|
| EIC | Linear Technology | LTC3803ES6-5#TRMPBF | 85 rub. |
| Triema | Linear Technology | LTC3803ES6#PBF | 93 rub. |
| LifeElectronics | LTC3803ES6-3 | on request | |
| ElektroPlast-Ekaterinburg | Linear Technology | LTC3803HS6#PBF | on request |
- Linear Technology is generally a top company! It’s a very, very pity that they were gobbled up by consumer goods Analog Devices. Don't expect anything good from this. I previously came across an article by an English-speaking radio amateur. He assembled a generator of very short pulses with a width of a few nanoseconds and rise/fall times of picoseconds. On a very high-speed comparator. Sorry I didn't save the article. And now I can’t find it. It was called something like “...real ultrafast comparator...”, but somehow it’s not right, I can’t Google it. I forgot the name of the comparator, and I don’t remember its company. Then I found a comparator on ebay, it cost about 500 rubles, in principle, budgetary for a really worthy device. Linear Technology has very interesting microcircuits. For example LTC6957: rise/fall time 180/160 ps. Awesome! But I’m unlikely to be able to build a measuring device myself using such a device.
- Is this not the case on the LT1721? Tunable 0-10ns.