ZCH generator on the K174UN7 chip. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Measuring technology

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In the laboratory of a beginner radio amateur, the 3Ch generator occupies not the last place; with its help, various electro-acoustic devices and their components are tuned and tested. But not every generator allows you to connect a low-impedance load to the output, for example, an acoustic system or a dynamic head. Readers are offered a description of a generator that allows you to do this. It is assembled on the UMZCH K174UN7 chip, and its circuit is shown in fig. 1.

(click to enlarge)

The generator generates sinusoidal electrical signals in the frequency range of 20 Hz...20 kHz, which is divided into three subranges: 20...200 Hz, 0,2...2 kHz and 2...20 kHz. The microcircuit is included according to the standard scheme. The frequency-setting circuit of the generator is formed by a Wien bridge, through which a positive feedback (POS) is carried out from the output of the amplifier to its input. Wien's bridge consists of resistors R1-R3 and two capacitors C3 and C7, to which capacitors C1, C1, C2 and Sat are connected by switch SA5 at the lower frequency subranges. The depth of the PIC is regulated by a trimmer resistor R6. A smooth change in frequency within each subband is made by a double variable resistor R1.

In order for the amplitude of the generated signal to remain constant when the frequency changes, an output voltage stabilization device is introduced into the generator. It is made on the elements VT1, C9, C13, R5, VD1, R8 and R7. The channel of the field-effect transistor VT1 is included in the negative feedback circuit (OOS) of the microcircuit and determines its overall gain, and hence the amplitude of the output voltage.

The device works as follows. The output signal from the engine of the resistor R7 through the resistor R8 is fed to the diode VD1, rectified, smoothed by the capacitor C13 and fed to the gate of the transistor. With an increase in the amplitude of the output voltage, the closing voltage at the gate of the transistor also increases. The channel resistance grows, which leads to an increase in the depth of the OOS, a decrease in the gain of the microcircuit, and, consequently, the amplitude of the output voltage. Thus, its stabilization is ensured.

A high-resistance load is connected to the XS1 sockets, for example, a frequency meter or an oscilloscope. Low-resistance load - dynamic heads, acoustic systems, etc. - are connected to the XS2 sockets. Sockets XS3 (Output 1:1) and XS4 (Output 1:10) are used to connect the devices under study, the voltage at these outputs is smoothly regulated by resistor R11. The generator is powered by a stabilized power supply with a voltage of 12 ... 15 V and a maximum current of up to 1 A.

Most of the generator parts are placed on a printed circuit board made of one-sided foil fiberglass, a sketch of which is shown in Fig. 2.

All sockets, as well as elements C1, C2, C5, C6, R1, R11, R12, R13 are placed on the front panel of the generator. The body of the device can be plastic or metal. If you control the frequency of the generator with a frequency meter, for example, a multimeter with a built-in frequency meter, then the axis of the resistor R1 does not need to be provided with a pointer, and you can do without a scale on the front panel, which will simplify the design and reduce the dimensions of the generator.

The following parts can be used in the device: diode VD1 - KD522, KD521 with any letter index, oxide capacitors - K50-6, K50-35 or similar imported ones, the rest - K10-17, K73, and capacitors C1 and C6, C2 and C5 , as well as C3 and C7, it is desirable to select so that their capacities differ from each other by no more than 5%. Trimmer resistors - SPZ-19a, variables: dual R1 - SP-Ill, R11 - SPO, SP4, fixed resistors - MLT, S2-33. Switch - any small-sized. The microcircuit must be equipped with a radiator with an area of ​​at least 10 cm², which can be made from an aluminum plate. To turn on the generator in the power circuit, it is useful to install a switch, and to indicate this mode, a circuit of series-connected LEDs (AL307, AL341 with any letter index) and a resistor with a resistance of 0,75 ... 1 kOhm must be inserted between the power bus and the common wire.

Setting up the generator comes down to adjusting the boundaries of the subranges by selecting the capacitances of the capacitors C1 - C3, C5 - C7 and setting the required output signal amplitude. The last operation is carried out using resistors R6 and R7. Resistor R7 sets the amplitude - with the transistor indicated on the circuit, it can be changed in the range from 1 to 5 V, with a larger amplitude, noticeable distortions occur. In this case, the slider of the resistor R6 should be installed as close as possible to the upper position according to the diagram. At the beginning of the setting, the resistor R6 slider is set to the upper position according to the diagram, and R7 to the lower position, the rest of the generator controls are approximately in the middle position. In this case, there should be no output signal, but if it is present, then this means that the amplifier has been excited at a high frequency. In this case, a capacitor with a capacity of 5 ... 500 pF must be installed between pin 2000 and the common wire.

Then, smoothly rotating the slider of the resistor R6, generation is achieved, and the required amplitude (from 7 to 1 V) of the output signal is set with the resistor R5 and its stability is checked over the entire frequency range. If necessary, repeat the setting. If the amplitude of the output voltage should lie within 0,5 ... 1 V, then the transistor KP303A or B should be installed in the generator. The device will be configured correctly if the amplitude of the output signal changes by no more than 10% over the entire operating frequency range. If necessary, calibrate the scale using a frequency meter. Having spent some time selecting the capacitances of capacitors C1, C2, C5 and C6, it is possible to ensure that the scales on all three subranges will coincide, differing only in a multiplier, then only one scale can be dispensed with.

The coefficient of non-linear distortion of the output signal is determined to a large extent by the parameters of the microcircuit. It also depends on the accuracy of the selection of capacitors and resistors in the Wien bridge and can be several percent. In addition, when a low-impedance load is connected to sockets XS2, XS3, a slight change in the generated frequency is possible.

Author: I. Nechaev, Kursk

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