A simple AF generator. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Measuring technology

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The main technical characteristics are as follows:

Frequency range, kHz ..... 0,01 ... 100
(subranges: 0,01...0,1; 0.1...1; 1...10 and 10...100)
Harmonic coefficient, %, in the subband, kHz:

0,01-0,1;
0,15 - 0,3;
0,1...1 - 0,04...0,05;
1...10 - 0,04...0,1;
10...100 - 0,06...0,4

Frequency response unevenness, dB, not more than ..... ±0,5
Output voltage, V.. .1,2,3,4
Output impedance, Ohm 600

Among the most necessary instruments in the laboratory of a radio amateur, one can rightfully include a generator of sinusoidal oscillations of the AF. Most often in amateur radio literature, generators are described with the so-called Wien bridge in the positive feedback circuit, usually tunable with a double variable resistor. Unfortunately, despite the apparent simplicity of such generators, it is far from easy to repeat them in amateur conditions, especially if we take into account the increased requirements for the nonlinear distortions of the measuring signal. The preservation of the identity of the resistances of the frequency tuning organ necessary to reduce distortion throughout the entire range requires the use of very accurate dual variable resistors, and they are practically inaccessible to most radio amateurs. Attempts to improve the signal quality by introducing various stabilizing circuits (nonlinear dividers, AGC), as a rule, lead to an improvement in some parameters at the expense of others.

The measuring oscillator [1] offered to the attention of readers is tuned by one variable resistor, has fairly good technical characteristics and is easy to set up.

Fig. 1

A simplified circuit diagram of the generator is shown in fig. 1. On the op-amp DA1 and elements R1 - R3, C1, a widely used and described in the literature adjustable phase shifter is assembled, introducing a phase shift of the signal, which is determined by the ratio of the capacitance of the capacitor C1 and the resistance of the resistor R1. From the output of the phase shifter, the signal enters the EL1R4 amplitude stabilization circuit, which compensates for the influence of such destabilizing factors as temperature and non-ideal op-amp parameters.

On the op-amp DA2 and resistors R5 - R7, a conventional inverting amplifier is made. The phase shift introduced by it is constant and equal to 180°. Trimmer resistor R6 is used to set the required output signal level.

Capacitor C2 with the input impedance of the cascade on the op-amp DA1 forms a circuit that additionally shifts the phase of the signal by an angle that, in total with the phase shift introduced by this cascade, is 180 °.

Thus, one of the conditions for the occurrence of generation is fulfilled - the phase balance.

The complete circuit diagram of the generator is shown in Fig. 2

Fig. 2

The adjustable phase shifter is assembled on the op-amp DA1. The signal from its output goes to the emitter follower, made on the transistor VT1. This cascade creates the conditions for the normal operation of the generator for low load resistance and the amplitude stabilization circuit, consisting of incandescent lamps EL1-EL3 and a trimmer resistor R13, with which the voltage of the signal at the output of the generator is regulated. From one subrange to another generator is switched by switch SA1, the required signal frequency is set by a variable resistor R3.

From the engine of the resistor R13, the signal is fed to the inverting amplifier (op amp DA2), the transfer coefficient of which is determined by the ratio of the resistances of the resistors R16 and R14. The R15C10 circuit connected in parallel to the latter compensates for the effect of parasitic phase shifts in the op amp, allowing you to preserve the nature and scale of the frequency change as a function of the resistance of the resistor R3 in the high frequency region of the operating range. (By the way, the introduction of this circuit made it impossible to change the resistance of the resistor in the OOS circuit covering the op-amp DA2, so the output signal voltage regulator had to be included in the amplitude stabilization circuit).

Capacitor C13 compensates for a slight rise in the frequency response in the higher frequencies caused by the introduction of the R15C10 circuit, and reduces the non-linear distortion of the signal at these frequencies.

The output voltage of the generator is set by the switch SA2, connecting the load to one or another part of the divider R7-R11. If necessary, the number of values ​​​​of the output voltage can be chosen by any other by including the appropriate number of resistors in the emitter circuit of the transistor VT1. The total resistance of these resistors should not exceed 150 ohms.

Details and construction

The use of different types of op amps in the phase shifter and inverting amplifier is due to the need to obtain a sufficiently wide operating frequency range with good generator stability. When using two op-amps of the K574UD1 series, the generator turns out to be prone to parasitic self-excitation at higher frequencies, and when using the op-amps of the K140UD8 series in both stages, the upper limiting frequency of the operating range cannot be raised above 20 kHz.

The KT807B transistor can be replaced by any of the KT815, KT817 series. In any case, the emitter transistor must be mounted on a heat sink with a cooling surface area of ​​at least 50 cm².

As a frequency tuning body (R3), it is desirable to use a variable resistor of the brand SP4-2Ma or SP3-23a. To reduce the non-linearity of the scale, this resistor must be of group B. You can also use a group B resistor by turning it on accordingly, however, the frequency in this case will increase when the slider is turned counterclockwise (this applies to the SP4-2Ma resistor). Trimmer resistor R13-SP4-1, SPZ-16a, SP5-16V.

Switches SA1, SA2 - any nut or push-button (for example, P2K with dependent fixation).

It is advisable to take capacitors C1 - C8 of the frequency-setting circuit with the smallest possible (in any case, normalized) TKE and select them in pairs (C1 and C2, C3 and C4, etc.) with an error of no more than +2%. This will ensure the required constancy of the amplitude of the generated oscillations during the transition from one subrange to another.

To power the generator, any stabilized source with output voltages of 4-15 and -15 V at a current of at least 200 mA and a ripple voltage of not more than 25 mV is suitable (for example, the device described in [2] fully meets these requirements).

Setting up a generator start by setting the trimmer resistor R13 output voltage 4V (switch SA1 - in the "I" position, SA2 - in the "4 V" position). Then, setting the slider of the variable resistor R3 to the upper (according to the scheme) position (it corresponds to the lower cutoff frequency of the subrange), by selecting the resistor R1, the generation frequency is equal to 10 Hz, after which the output voltage is measured and, if necessary, set to 4 V times (with the same resistor R13).

Next, the variable resistor R3 is transferred to the lower (according to the scheme) position and by selecting the resistor R2, an oscillation frequency of 100 Hz is achieved. After that, the SA1 switch is set to the "IV" position and the resistor R15 is selected such a resistance at which the output signal frequency is 100 kHz.

Capacitor C13 is selected, striving to ensure that the uneven frequency response of the generator at the highest frequencies of the operating range does not exceed +0,5 dB.

Literature

1. Author's certificate of the USSR, No. 1327263 (Bulletin "Discoveries, Inventions ...", 1987, No. 28).
2. Shityakov A., Morozov M., Kuznetsov Yu. Voltage stabilizer at the OU.-Radio, 1986, No. 9, p. 48.

Author: E. Nevstruev; Publication: cxem.net

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