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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Sweep frequency generator. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Measuring technology In order to have an idea about the band of frequencies passed by the AF amplifier, the depth of tone adjustments or other frequency properties of a sound reproducing device, it is necessary to take an amplitude-frequency characteristic (AFC). The technique is known - armed with an AF generator and an AC voltmeter or an output meter, they control the output signal level of the device when the input frequency changes. And then, according to the data obtained, a curve is built, according to which the bandwidth of the transmitted frequencies, the unevenness of the frequency response, and the attenuation of the signal at a certain frequency, and other necessary parameters are determined. It is worth making some improvements to one or another stage of the amplifier, changing the ratings of the parts of the feedback circuit - and again, all over again. The procedure for such tests is, of course, tedious. That is why radio amateurs have been looking for ways to visually observe the frequency response for a long time. One of them is the use of a sweeping frequency generator, which makes it possible to "draw" the frequency response envelope on the oscilloscope screen. In the simplest sense, a swept frequency generator (GCh) is an AF generator with a device that allows you to smoothly change ("pump") the frequency of the output sinusoidal oscillations in a given frequency range. The supply of such oscillations to the input of a controlled amplifier will be equivalent to manual tuning of the generator frequency. Therefore, the amplitude of the output signal AF will vary depending on the frequency of the input at the moment. This means that on the screen of an oscilloscope connected to the load of the output stage, one can observe the frequency response envelope, composed of the peaks of sinusoidal oscillations of different frequencies. It is not so easy to "pump" the frequency of the AF generator in a wide range, therefore, the GKCH based on the AF generator is overgrown with many stages and becomes a very complicated device for a novice radio amateur. As practice shows, it is somewhat easier to get a prefix-GKCh, in which the oscillations of the AF are formed as a result of the beating of the signals of two generators operating at frequencies of hundreds of kilohertz. Moreover, one of the generators in this case is tunable, say, by the sawtooth voltage of the oscilloscope sweep generator, and the other operates at a fixed frequency. The Kursk radio amateur I. Nechaev went down this path, having developed the proposed GKCh specifically for our cycle. The generator turned out to be combined, because in addition to the AF, it also allows you to explore the IF amplifiers of superheterodyne radio receivers. The scheme of the sweep frequency generator is shown in fig. 1. Its main nodes, as you probably guessed, are non-tunable and tunable generators. The first of them is made on the VT4 transistor according to the capacitive three-point scheme. The oscillation frequency (about 470 kHz) depends on the inductance of the coil L3 and the capacitance of the capacitor C11. Oscillation occurs due to positive feedback between the emitter and base circuits of the transistor. The feedback depth depends on the capacitance of the capacitors SI and C12, which form a voltage divider, and is chosen so that the oscillation shape is as close as possible to a sinusoidal one.
The oscillations of this generator, taken from the emitter resistor R18, are fed to the decoupling stage, made on the transistor VT5, and from its collector load (resistor R15) to the mixer, assembled on the transistor VT3. The vibrations of another oscillator, tunable, made on the transistor VT1, also according to the capacitive three-point scheme, are sent to the mixer in a similar way. The oscillation frequency of this generator depends on the inductance of the coil L1 and the capacitance of the circuit connected between the collector and emitter terminals of the transistor. And it, in turn, is made up of capacitor C3 connected in parallel, varicaps VD1, VD2 and capacitor C4 connected in series with these parts. So that the frequency of the generator can be changed, a constant voltage of positive polarity is applied to the anodes of the varicaps. When, for example, set the mode "Gen." (just frequency generation) and press the SA1 switch button, then the resistor R5 connected to the varicaps is connected through the contacts of the SA1.1 section to the variable resistor R2 engine, and the supply voltage is supplied to the upper output of the variable resistor according to the circuit through the SA1.2 section. By moving the variable resistor slider, it is now possible to change the oscillation frequency of the generator from about 455 to 475 kHz (the average frequency of 465 kHz is the intermediate frequency of superheterodyne receivers). From the coupling coil L2, oscillations of this frequency are fed to the voltage divider R9R14.1, and from the variable resistor engine R14.1 - to the output connector XS2. From this connector, the signal is fed to the input of the IF amplifier (or its stages) of the radio receiver. At the load of the mixer (resistors R13, R14.2), difference frequency oscillations are distinguished within the range of approximately 500 Hz ... 20 kHz, depending on the frequency of the tunable generator. It is not possible to receive a signal with a frequency of less than 500 Hz due to the phenomenon of frequency synchronization of both generators with small differences in tuning. Details C6, R13, C8 is a low-pass filter that attenuates the oscillations of the generators that have passed through the mixer. From the engine of the variable resistor R14.2, the AF signal is fed to the XS3 connector, which, when the set-top box is operating, is connected to the input of the AF amplifier under test. To ensure that the frequency of the tunable oscillator changes within the specified limits, it is necessary to supply a constant voltage from 2 to 0 V from the variable resistor R9 engine. With a smaller range of voltage changes, the frequency range of the signal taken from the XS2 and XS3 connectors will be correspondingly reduced. To obtain a swinging oscillation frequency of the AF, press the button SA3 "GKCH AF" (at the same time, the SA1 button is released and the SA1.2 section connects through the resistor R1 the upper output of the resistor R2 according to the circuit with the XS1 connector - it is supplied with a sawtooth sweep voltage from the oscilloscope. Resistor R1 limits the amplitude of this voltage across the resistor R2 is up to 9 V, so that the maximum frequency changes of the tunable generator are 20 kHz (as in the case of a constant voltage generator). it is higher in the circuit, the greater the range of frequency change. When checking the IF paths of the receivers, press the SA2 "GKCH IF" button. In this case, a fixed constant voltage is supplied to the varicaps, taken from the divider R3R4, as well as a sawtooth voltage supplied through the capacitor C1 from the engine of the variable resistor R2. A fixed voltage sets the generator frequency to 465 kHz, and a sawtooth changes it in both directions by a maximum of 10 kHz (when the variable resistor slider is set to the upper position according to the diagram). As already mentioned, when the tunable oscillator is operating in the frequency swing mode, it is necessary to apply a sawtooth voltage with an amplitude of 2 V to the resistor R9. Moreover, the voltage must be increasing so that the frequency response corresponds to the generally accepted outline - lower frequencies on the left, and medium and higher - on the right. The owners of oscilloscopes, in which just such a sweep voltage is output to a special socket, completely repeat the prefix according to the above diagram and select the desired amplitude of the saw at the terminals of the resistor R2 by changing the value of the resistor R1. Owners of oscilloscopes with a sawtooth voltage of sufficient amplitude, but falling, can be recommended to replace transistors with structures similar in power, but opposite to those indicated in the diagram, change the polarity of switching on the varicaps and the oxide capacitor C10, as well as the polarity of the supply voltage. The owners of the OML-2M (OML-3M) oscilloscope already know that the sawtooth voltage output to the socket on the rear wall of the oscilloscope reaches a maximum amplitude of 3,5 V, which is less than required. Therefore, two options are possible. In the first case, you can generally remove the resistor R1 and feed the saw to the XS1 connector, connected to the top output of the variable resistor R2 according to the diagram. In this case, the maximum frequency in the swing mode will decrease from 20 to 15 kHz, which is quite acceptable for testing and adjusting many low-end mono and stereo amplifiers. If it is necessary to investigate better amplifiers with a bandwidth of up to 20 kHz, you will have to supplement the set-top box with a two-stage amplifier based on transistors VT6, VT7 and turn it on instead of the limiting resistor R1. The amplitude of the saw on the resistor R2 will increase to 8 ... 8,5 V. You may be wondering if it's worth using two stages to get just less than triple the gain (from 3,5V to 8,5V). Indeed, for such an amplification, one cascade would be enough. But at the output it will turn out a falling sawtooth voltage. To achieve not only the desired gain, but also a given signal polarity, the amplifier had to be made on two transistors. Let's move on to the story about the details of the prefix-GKCH. Transistors VT3 and VT7 can be, in addition to those indicated in the diagram, KT361D, GT309A - GT309G, KT326A, KT326B, P401 - P403, P416, the remaining transistors - KT315A - KT315I, KT301G - KT301Zh, KT312A - KT312V. Varicaps VD1, VD2 - KV109A - KV109G. Capacitors C1, C2, C7, C9 - BM, MBM, KLS; C10 - K50-12; the rest - CT, KD, PM, KLS. The variable resistor R2 can be SPO-0,5, SDR-9a, SDR-12, the dual resistor R14 is SDR-4aM, but it can also be replaced with single ones (R14.1 and R14.2) of the same type as R2. Fixed resistors - MLT-0,125. Switches - P2K with dependent fixation, when one of the keys is pressed, the rest are in the depressed position. Inductors can be wound on IF frames from the Alpinist-405 radio receiver or other similar frames with a ferrite trimmer. Coils L1 and L2 are wound on one such frame, and L3 on the other. The coil details are: L1 - 500 turns, and L2 (it is placed on top of L1) - 50 turns of wire PEV-2 0,09; L3 - 170 turns of wire PEV-2 0,1 ... 0,12. Connectors - high-frequency, from television receivers. The power supply must be with a stabilized voltage (the frequency stability of the generators depends on this) and is designed for a load current of at least 10 mA. Some parts of the console are mounted on one side boards (Fig. 2) from double-sided foil fiberglass. The conclusions of the parts are soldered directly to the conductors - foil strips. The board simultaneously serves as the front wall of the case (Fig. 3), switches and variable resistors are fixed on it (resistor R2 is equipped with a scale).
On one side wall of the housing there is an input connector XS1, on the other - output connectors XS2 and XS3. Between the terminals of the switches, variable resistors and connectors, parts are mounted that are not shown on the printed circuit board drawing. Power conductors with plugs at the ends are brought out through the holes in the side wall - they are inserted into the sockets of the power supply (or connected to the outputs of a source, for example, composed of two 3336 batteries connected in series). The lower case cover is removable. If the set-top box is mounted without errors and serviceable parts are used in it, both generators will start working immediately. To verify this, you need to press the SA1 button, apply power to the set-top box, set the variable resistor sliders to the upper position according to the diagram and connect the oscilloscope input probes to the XS2 connector - it must work in automatic mode with internal synchronization and a closed (or open) input . Having chosen the sensitivity of the input attenuator of the oscilloscope so that the image span on the screen is at least two divisions, you can turn on the standby mode on the oscilloscope and "stop" the image with the corresponding knobs. The oscillation form should be close to sinusoidal, and the frequency should be in the range of 400...600 kHz. Next, you can check the operation of the second generator by connecting the oscilloscope to the output of the emitter of the transistor VT4 (the input of the oscilloscope is closed). There should also be sinusoidal oscillations with a frequency within the limits specified for the first generator. Now you can start setting up the generators and calibrating the scales (there are two of them - for the oscillations of the IF and AF) of the variable resistor R2. You will need a frequency meter, which is connected to the XS2 connector. The slider of the variable resistor R14.1 is left in the position of the maximum output signal, and the slider of the resistor R2 is moved to the bottom according to the scheme, i.e., no DC voltage is applied to the varicaps. By controlling the frequency of the generator, set it equal to 475 kHz with a trimmer for coils L1, L2. Then the slider of the resistor R2 is moved to the upper position according to the scheme and the generator frequency is measured - it should be equal to 455 ... 450 kHz. If it is larger, a capacitor C3 of a smaller capacity is selected or excluded altogether. At a lower frequency, a larger capacitor is selected, after which the generator is again tuned to a frequency of 475 kHz with the lower position of the resistor R2 slider. Leaving the resistor slider in this position, switch the frequency meter to the XS3 connector and measure the difference frequency. Reduce it with the trimmer of the L3 coil to the minimum possible, trying to get "zero beats". Coil trimmers can then be countered with nitro paint or a drop of glue. By connecting an oscilloscope to the XS3 connector and setting the variable resistor R2 slider, for example, to the middle position, they control the shape of the oscillations. If necessary, improve it pick up the resistor R15. Reconnect the frequency meter to the XS2 connector and, smoothly moving the slider of the variable resistor R2 from the lower position to the upper position, measure the generator frequency at various points. On the scale of the resistor put down the frequency values. Similarly calibrate the second scale by connecting the frequency meter to the XS3 connector. The next step is to check and establish a two-stage sawtooth voltage amplifier (if you decide to assemble it). First, a signal is fed to the XS1 connector from the socket on the rear wall of the OML-2M (OML-3M) oscilloscope, and the input probe is connected to the lower output of the resistor R21 according to the circuit (i.e., they practically control the input signal). The sensitivity of the oscilloscope is set equal to 1 V / div., And the beginning of the sweep line is shifted to the lower left corner of the scale. The oscilloscope operates in automatic mode with a closed input, the sweep duration is 5 ms / div. On the screen you will see an increasing sawtooth voltage, the top of the saw may go beyond the extreme vertical line of the scale. With the sweep length adjustment knob, set such a sawtooth voltage so that it fits exactly between the extreme vertical lines of the scale (Fig. 4, a), and measure the amplitude of the saw - it can be about 3 V.
Then switch the input probe of the oscilloscope to the output of the collector of the transistor VT6, and set the sensitivity of the oscilloscope to 0,5 V / div. On the screen you will see an image of a falling saw. Bring the beginning of the sweep line to the middle line of the scale and measure the signal amplitude - it should be about 0,8 V (Fig. 4b). If the nature of the saw is greatly distorted (a “step” appears at the end of it), you will have to select a resistor R21. Set the sensitivity on the oscilloscope to 1 V / div, and connect its input probe to the output of the collector of the transistor VT7, and on the console, press the SA1 button so that the resistor R2 is connected to R24. The image shown in Fig. 4, c, may appear on the oscilloscope screen - a distorted saw. You can get rid of distortion by a more accurate selection of the resistor R23, and sometimes also the resistor R21, so that the image shown in Fig. 4d is obtained on the screen. A slight non-linearity of the saw first appears due to some "delay" in the opening of the transistor VT6 as the sawtooth voltage increases. This non-linearity will practically not affect the operation of the GKCh. As for the maximum amplitude of the saw, it does not differ much from 9 V. Of course, it can be increased, but in this case it will be necessary to power the two-stage amplifier with a slightly higher voltage - 10 ... 12 V. At the time of establishing the amplifier, instead of resistors R21 and R23, it is desirable to solder variables with a resistance of 1,5 ... 2,2 MΩ and 1 MΩ, respectively. How to work with our GKCh? You already know that, depending on the device being tested (IF or AF amplifier), one or another output connector of the generator is used - it is connected to the input of the device. The input probe of the oscilloscope is connected to the output of the device under test. When you turn on the GKCh on the oscilloscope screen, you can see the envelope of the amplitude-frequency characteristic of the device. More specifically, the following can be said. When checking the superheterodyne IF amplifier, the XS2 connector is connected with a high-frequency cable (or shielded wire) through a 0,05 ... 0,1 μF capacitor to the base of the frequency converter transistor, and the oscilloscope input probe is connected to the receiver detector. Variable resistor R14.1 set such an output signal of the GKCH so that the observed image is not distorted (there was no limitation of the characteristic from above), and the variable resistor R2 selects such an oscillator frequency so that the U-shaped envelope of the IF amplifier characteristic is located in the middle of the oscilloscope screen. If the signal from the MCC turns out to be excessive even in the almost lower position of the resistor R14.1 slider, it can be reduced by connecting an additional voltage divider between the MCC and the receiver. We will tell you more about the use of the GKCh to test the IF path later, when we touch on the methodology for testing and establishing a superheterodyne radio receiver. And today we will carry out some practical work on checking the AF amplifier. It is best to focus on an amplifier with tone controls for low and high frequencies. For example, let's use the amplifier described in B. Ivanov's article "Electrophone from EPU" in "Radio", 1984, No. 8, p. 49-51. If you remember, in our cycle we already met a part of this construction - node A2. Now you need to add node A1 to it with two tone controls, connect to the amplifier instead of a dynamic head an equivalent load with a resistance of 8 ... 3 Ohms and connect the amplifier input to the XS5 connector of our set-top box (Fig. 1) through an oxide capacitor with a capacity of 10 ... XNUMX uF (since there is no decoupling capacitor either at the output of the set-top box or at the input of the amplifier).
On the oscilloscope, the sweep duration is 5 ms / div., The sensitivity is 2 V / div., The input is closed, the sweep is automatic with internal synchronization (the synchronization control must be in the middle position to prevent image twitching at the beginning of the sweep), the sweep line is in the middle scales. Author: B. Ivanov, Moscow; Publication: N. Bolshakov, rf.atnn.ru
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