Wide range function generator. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Wide range function generator. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Measuring technology

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Having decided to build a measuring generator for their home laboratory, radio amateurs have recently increasingly opted for a closed relaxation system consisting of an integrator and a comparator. This is explained by the fact that such generators, as a rule, are easier to manufacture than sinusoidal signal generators, and their capabilities are much wider. However, when using op-amps of widely used series (K140, K153, K553, etc.), it is not possible to obtain a high slew rate of the integrator output voltage and a short comparator "response" time, therefore, the upper cut-off frequency of most generators described in amateur radio literature does not exceed 10. ..20 kHz.

(click to enlarge)

In the device brought to the attention of radio amateurs, the K574UD1B OU is used as an integrator (the output voltage rise rate is 50 V / μs, the unity gain frequency is 10 MHz), and the comparator is made on the elements of the K155LA3 microcircuit (delay time - no more than 30 ... 40 ns) . This made it possible to expand the range of generated frequencies up to 1 MHz. The generator generates rectangular, triangular and sinusoidal voltages, as well as rectangular pulses with TTL levels and adjustable duration from 0,5 µs to 1200 ms. The output voltage can be changed within 0...1 V. The harmonic coefficient of the sinusoidal signal is no more than 1,5%. The output impedance of the generator is about 100 ohms.

In addition to the already named integrator (op amp DA1) and comparator (DD1), the generator includes an emitter follower (VT1), a sinusoidal voltage generator (VT2), a scale amplifier (op amp DA2, VT7), a buffer stage (VT4, DD2.1). RS flip-flop (DD2.2, DD2.3). two single vibrators (DD3.1, DD3.2) and three transistor voltage regulators (VT3, VT5, VT6). The device is powered by a bipolar stabilized voltage source ± 12 V. The current consumed from a positive voltage source is no more than 180 mA, negative - 80 mA.

Rectangular pulses from the output of the comparator (pin 6 of the element DD1.2) are fed to the inverting input of the integrator on the op-amp DA1. At the output of the latter, a triangular-shaped voltage is generated, which controls the comparator through the emitter follower on the transistor VT1. With switch SA1, the oscillation frequency is changed roughly, with a variable resistor R1 - smoothly. Trimmer resistor R16 serves to set the amplitude, and R17 - the constant component of the triangular voltage. The required mode of operation of the comparator is ensured by applying to pin 7 (common) of the DD1 microcircuit a voltage of -2 V from the output of the stabilizer on the VT3 transistor, and to pin 14 - a voltage of +3,2 V from the output of the stabilizer on the VT5 transistor.

Triangular-shaped oscillations from the emitter of the transistor VT1 enter the cascade, made on the field-effect transistor VT2, where a sinusoidal voltage is formed from them. From the source of the transistor, a sinusoidal signal is fed to the switch section SA2.2. Here - through resistors R18 and R22 - triangular and rectangular voltages are supplied, taken respectively from the emitter of the transistor VT1 and the output of the comparator element DD1.2. The signal selected by the SA2 switch (its amplitude is regulated by a variable resistor R27) is amplified by a scale amplifier made on the DA2 op amp and the VT7 transistor, and goes to the step attenuator - the voltage divider R24-R26, and from it - through the SA3.2 switch section and the resistor R32 - to the output socket XS1.

Rectangular pulses with a TTL level are fed to the SA3.2 switch section from the output of the buffer stage, assembled on the VT4 transistor and the DD2.1 element. They also run the DD3.1 single vibrator, connected to the output of the device in the second and third (top) positions of the switch. The duration of the pulses is controlled by switching capacitors C9-C12 and changing the resistance of the variable resistor R3 of the timing circuit.

The second single vibrator of the DD3 microcircuit is used in the shaper of single pulses (it is connected to the output of the device in the fourth and fifth positions of the SA3 switch). When the SB1 button is pressed, the RS-trigger on the elements DD2.2, DD2.3 changes its state and, with a positive drop in the output voltage, starts the DD3.2 single vibrator. As in the previous case, the required pulse duration is set by switch SA2.1 and resistor R3.

The device provides the ability to use voltage drops at the outputs of the RS-trigger as an output signal when the SB1 button is pressed (the sixth and seventh positions of the SA3 switch).

Setting up the generator begins with balancing the scale amplifier (DA2, VT7). To do this, the switches SA1-SA3 are set respectively to the positions "0,1 ... 1 kHz", "30 ... 1200 ms" and "1: 1", turn on the power and the trimming resistor R31 achieves zero voltage at the output jack XS1. Then, with a trimmer resistor R19, a voltage of -7 V is set at pin 1 of the DD2 microcircuit. And with a trimmer resistor R33, a voltage of +3,2 V at its pin 14 is set. After that, an oscilloscope is connected to the output of the device, switch SA2 is switched to the upper (according to the diagram) position and the same tuning resistors R19, R33 ensure that the rectangular pulses on the oscilloscope screen become symmetrical (with respect to level 0).

Next, the SA2 switch is set to the second (top) position and, by moving the slider of the resistor R1 to the lower (according to the diagram) position, the triangular signal is balanced with a tuning resistor R6. The symmetry of the latter should not be violated when the slider of the resistor R1 is moved to another extreme position. The absence of a constant component of this signal is achieved by a tuning resistor R17.

Nonlinear distortion of the sinusoidal voltage is minimized by the trimmer resistor R16 by setting the SA2 switch to the third position.

After that, the engine of the variable resistor R27 is transferred to the upper (according to the scheme) position and the resistor R29 is selected until a voltage of 1V is obtained at the output of the device. The same voltages of rectangular and triangular shapes are achieved by selecting resistors R22 and R18.

In conclusion, the capacitor C8 is selected until the upper cut-off frequency of the generated oscillations is equal to 1 MHz.

It should be noted that, if desired, the maximum frequency of the generator can be increased to 2 ... 2,5 MHz. To do this, the capacitor C8 should be excluded, and the resistance of the resistor R16 should be increased to 6,8 ... 10 kOhm. True, in this case, difficulties will arise in obtaining a sinusoidal signal, since with an increase in the resistance of the specified resistor, the amplitude of the triangular voltage will decrease. The way out is the introduction of an amplifier with a linear (in the frequency band of 0 ... 3 MHz) AFC between the integrator and the shaper of the sinusoidal voltage.

A. Ishutinov

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