LF harmonic signal generators. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Measuring technology

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The article describes several simple low-frequency generators made on the basis of an op-amp, tunable by a selective filter based on a Wien bridge or a gyrator and stabilized in signal amplitude. Readers will find useful formulas for calculating a frequency-selective circuit and information about the features of the operation of generators at the boundaries of the operating frequency band.

In low-frequency generators of harmonic oscillations, a Wien bridge is often used and, much less frequently, a low-frequency gyrator. The Wien bridge oscillator is simple, but it requires a dual tunable element - a resistor or capacitor. The gyrator, on the other hand, allows using only one variable resistor for frequency tuning. To obtain a signal with a coefficient of non-linear distortion of about 0.01%, thermistors are used in both versions.

On fig. 1 shows a diagram of a generator with a Wien bridge.

The two frequency-dependent arms of the bridge are connected to the non-inverting input of the differential amplifier. The other two arms are connected to the inverting input. The output voltage is stabilized by a semiconductor thermistor of the PTM-2/0,5 type, commonly used in industrial generators with a Wien bridge. A feature of this element is its low power consumption: in the stabilization mode, a voltage of 2 V drops on it at a current of 0,5 mA. The output voltage of the generator is about 3 V and practically does not change when the frequency is tuned. The generation frequency is determined by the expression

f=1/2(PI*R*C) [1] at R=R2=R4, C=C1=C3.

To tune the generation frequency, double variable resistors or variable capacitors are used.

The stability of the oscillator signal amplitude largely depends on how equally both frequency tuning elements change. However, variable resistors can change their resistance over time. In this regard, it is difficult to achieve the accuracy of setting the frequency on the scale during the entire period of operation. The best results are obtained with a double block of variable capacitors (preferably with an air dielectric), in which the sections are shunted with trimmer capacitors to equalize the initial capacitance and limit the frequency tuning range. Using a conventional block of variable capacitors, you can get a tenfold overlap in frequency in one range. To prevent parasitic self-excitation of the generator at high frequencies, a correction capacitor C5 is introduced. For the same purpose, the output signal is taken through the decoupling resistor R5. Such a device can generate signals with a frequency of up to 500 kHz, however, at frequencies above 100 kHz, its non-linear distortion increases due to a decrease in gain and the appearance of a phase shift in the op-amp. At the lowest frequencies of the audio range, there is an increase in distortion due to insufficient thermal inertia of the thermistor.

In case of difficulty in acquiring a thermistor to stabilize the amplitude, it is possible to use a miniature incandescent bulb (for example, type CMH-10). However, the output current of the differential amplifier to set the light bulb in stabilization mode (when its thread has a dark red color) is not enough, and a more powerful output stage is needed. For this purpose, the generator according to the scheme in Fig. 2 introduced an emitter follower.

A high-quality harmonic signal generator with a gyrator was described earlier in [2]. The advantage of the gyrator is that when using it, there is no need to maintain the equality of the parameters of the elements in frequency-dependent circuits. On fig. 3 shows a simpler generator circuit that does not contain a stabilizing thermistor.

Nevertheless, the amplitude of the output voltage in this generator remains almost constant over a wide frequency range. This is because the gyrator simulates a low-loss inductance at the input of the first amplifier, which, together with the capacitor C1, forms an oscillatory circuit with a very high quality factor. Generation occurs due to the POS circuit introduced into the circuit of the second amplifier (R7, R8, C5). When generation occurs, the voltage on this circuit increases until the shunting of the circuit increases due to an increase in the input current of the first amplifier. As a result, due to a decrease in the quality factor of this circuit and the depth of the POS, a further increase in the oscillation amplitude is impossible. The coefficient of non-linear distortion of the signal also depends on these parameters, therefore, in a particular device, it may be necessary to optimize the parameters of the elements R7, R8, C5. The generation frequency in such a device with a ratio of resistances R3 = R6 can be calculated by the formula

f=1 /2*PI*[(R1+R5)*R2*Cl*C2]^1/2

To smoothly change the frequency of the generator, a variable resistor R1 is used. at the same time, its restructuring by 3-4 times is possible. By choosing the capacitance of capacitors C1 and C2, you can set one or another frequency range of the generator. For simplicity, we should take С1=С2=С. With a capacitance of 0,22 μF, the signal frequency is regulated in the band of 20 ... 70 Hz.

If a double variable resistor is used to tune the generator (the second adjustable element can be R3 or R2), the frequency can be changed by 10 ... amplification. The generator operates up to a frequency of about 20 kHz with an increase in the coefficient of nonlinear distortion at a maximum frequency of up to 8%.

Supply voltage of generators - ±15V.

Literature

1. Horowitz P., Hill W. The Art of Circuitry - M: Mir, 1993.
2. Petin G. The use of a gyrator in circuits of resonant amplifiers and generators - Radio, 1996, No. 11. With. 33, 34.

Author: G. Petin, Rostov-on-Don

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