ZCH generator. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Measuring technology

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In amateur radio practice, it is difficult to do without an audio frequency generator (AF). With it, you can not only set up a low-frequency amplifier well, but also take the amplitude-frequency characteristic of the amplifier, measure its gain. The AF generator is also necessary for modulating high-frequency electrical oscillations, with the help of which the high-frequency paths of radio receivers are tuned.

The most interesting for mass repetition is the so-called RC generator, the master node of which is made according to the Wien bridge scheme. Such a generator is easy to manufacture, reliable in operation.

For the convenience of working with the generator, the frequency range of the oscillations generated by it is divided into several subranges. The frequency of oscillations within the subband is regulated using dual variable resistors of a special design. However, it is not easy to purchase such a resistor, and it is rather difficult to make a similar one from two variables, since their characteristics must be identical.

Instead of dual resistors, you can use a block of dual capacitors of variable capacitance, which are used in radio receivers to tune into a station. The technical characteristics of the generator will not become worse from such a replacement, and the number of capacitors in the device will decrease due to the fact that the RC feedback circuits of the master node will consist of two variable capacitors and constant resistors connected to them when switching subranges.

Fig. 1

Schematic diagram of the generator is shown in the figure. The generator generates electrical oscillations of a sinusoidal form with a frequency of 25 Hz to 25 kHz. The entire range is divided into three subranges: 25...250 Hz; 0,25...2,5 kHz; 2,5...25 kHz. The maximum signal voltage at the output of the device is 1,5 V. The coefficient of non-linear distortion of the signal shape is about 0,3%.

The master node of the generator is made on the operational amplifier DA1, from the output of which the signal is fed to the input of the emitter follower on the transistor VT2.

In the generator, the Wien bridge is used in the feedback circuit of the operational amplifier. The shoulders of the bridge form a positive feedback circuit, which consists of serial (C3.2, R9) and parallel (C3.1, R3) RC circuits, and a negative feedback circuit (NFB) - parts VT1, R7, R12.

The oscillation amplitude at the output of the amplifier is regulated by a tuned resistor R7. Transistor VT1, connected according to the circuit of an electrically adjustable resistor, stabilizes the voltage at the output of the generator. It happens in the following way.

When the amplitude of the output signal changes, the voltage from the output of the emitter follower through the VD1R8 chain is supplied to the gate of the field-effect transistor VT1 and regulates the resistance of the source-drain channel of the transistor. A change in the channel resistance leads to a change in the depth of the OOS and, as a result, to a change in the gain of the amplifier DA1. For example, as the signal amplitude increases, the gate voltage will increase. Transistor VT1 will start to close, the resistance of its channel will increase and the OOS coefficient will increase: the voltage at the output of the generator will decrease.

With a decrease in the signal amplitude, the voltage at the transistor gate will also decrease, which will lead to a decrease in the FOS value and an increase in the signal amplitude.

The voltage at the output of the generator is regulated smoothly by a variable resistor R14. The maximum voltage is removed from the chain R15R16 ("Output 1:1"), and reduced by 10 times - from the resistor R16 ("Output 1:10").

When connected to a load generator, its resistance must be at least 1 kOhm.

Generator parts, except for a block of variable capacitors, a switch SA1 and a variable resistor R14, are mounted on a printed circuit board made of foil textolite.

The device can use transistors KP303V (VT1), KT603A, KT603V, KT603G, KT608A, KT608B, KT815A - KT815G (VT2), diodes D220, D223, KD521A -KD521D, KD522A, KD522B.

The block of variable capacitors can be from any radio receiver, in particular from Selga. If the minimum capacitance value of the variable capacitor is less than 15 pF, it is necessary to install additional capacitors with a capacity of 10 ... 15 pF. They are connected in parallel to each section of the capacitor C3. The handle that is put on the capacitor bank must be of insulating material.

Resistors R3, R9 (MLT) are made up of several smaller resistors connected in series.

The adjustment of the generator begins with the fact that the capacitor C3 is set to the middle position, and the variable resistor R14 is set to the upper position according to the diagram.

Adjusting the tuned resistor R7 ensures that the signal voltage at the output of the generator (jack "Output 1: 1") is approximately 1 ... 1.5 V. The voltage is controlled using an oscilloscope, which is connected to the "Output 1: 1" jack. When adjusting the voltage, it is necessary to ensure that the nonlinear distortion of the signal observed on the oscilloscope screen is minimal.

When switching from one subrange to another, the voltage at the generator output must be stable.

After that, proceed to the calibration of the generator scale. To do this, switch SA1 is switched to the first subrange and a frequency meter or an oscilloscope is connected to the "Output 1: 1" jack. With the help of these devices, the frequency of oscillations is controlled.

The variable capacitor is transferred to the position in which the value of its capacitance is maximum (it is desirable that this be the extreme left position). In this case, the frequency of the generated oscillations should be equal to 25 Hz.

If the actual value of the frequency controlled by the frequency meter or oscilloscope is not equal to 25 Hz, it is necessary either by adjusting the variable capacitor C3 (if the oscillation frequency is less than 25 Hz) or by selecting the resistor R3 (if the oscillation frequency is more than 25 Hz) to ensure that the value of the generated fluctuations corresponded to the given one.

The position of the variable capacitor knob, at which the frequency is 25 Hz, is marked on the scale of the device.

Then reduce the capacitance of the capacitor C3 to a value at which the oscillation frequency is 35 Hz. This point is also marked on the scale of the instrument. Again, change the capacitance of the capacitor C3 to a value at which the frequency is 45 Hz. And mark this point. And so - up to a frequency value of 250 Hz.

When the scale of the first subrange is calibrated, switch SA1 is switched to the second subrange and the scale of the second subrange is calibrated. To do this, the pointer of the capacitor C3 is combined with the leftmost mark of the scale and the selection of the resistor R4 is achieved so that the frequency value at this point is equal to 250 Hz. Then the capacitor pointer is combined with the extreme right mark of the scale and the selection of the resistor R10 is achieved so that at this point the oscillation frequency is 2,5 kHz.

Similarly, a selection of resistors R5, R11 calibrate the scale of the third subrange.

The generator is fed from a stabilized DC voltage source of 12...15 V, designed for a load current of 20...30 mA.

Author: I.Nechaev

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