Nonlinear distortion meter for AF amplifiers. Scheme, description

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Nonlinear distortion meter for AF amplifiers. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Measuring technology

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This device cannot be used on its own; an audio frequency signal generator and an AC millivoltmeter are required for measurements.

The main parameters:

Measurement frequencies, kHz ....... 0,33; one; eight; 1
Frequency tuning limits, %......20
The output voltage of the measured amplifier, Vef ....... 2
The lower limit of measurement of non-linear distortions, %......0,15
Measurement error, %......50
Input impedance, kOhm.......5
Transfer coefficient, times ....... 2

At the input of the measured AF amplifier, a signal is supplied from a sound generator that provides small (less than 1%) non-linear distortion. From the output of the amplifier, the signal, which has undergone distortion in the amplifier path, is fed through the XI connector to the input of the harmonic coefficient meter. The variable resistor R1 sets the required signal level at the bases of transistors VI and V5. The signal is divided into two channels: the upper channel according to the scheme rotates the phase of the signal by 180¦, the lower channel does not change the phase. The phase shifter is assembled on transistors VI-V4; cascades on transistors V1 and V3 create the necessary phase shift, emitter followers on transistors V2 and V4 serve to decouple between the cascades of the device.

The frequencies at which the phase shift is 180¦ determine the capacitance of the capacitors C2-C5, C6-C9 and the resistance of the resistors R7, R11, R12.

The operating mode of all transistors is set by the voltage divider R3, R4 *.

From the output of the phase shifter, the signal through the resistor R13 and the SI capacitor is fed to the input of the lower channel in the circuit - an amplifier (transistor V5) with a gain of about 5. The input of the same amplifier through the resistor R16 receives the signal voltage from the input of the device - resistor R1.

The main and auxiliary signals, given in antiphase, but with equal amplitudes on the basis of the transistor V5, are mutually compensated for the first harmonic. Only harmonics remain, which are amplified by transistor V5. The amplified signal from the load V5 (resistor R20) is fed to an active high-pass filter assembled on the transistor R6. The cutoff frequency of the filter (200 Hz) depends on the capacitance of capacitors C13-C15 and the resistances of resistors R22 - R25. The steepness of the decay of the amplitude-frequency characteristic of the filter is about 15 dB per octave; this means that this filter attenuates 100 Hz interference by 15 dB, and 50 Hz AC hum by 30 dB. This is sufficient for most measurements encountered in practice.

From the output of the filter, the alternating voltage of the harmonics is fed through the X2 connector to the input of the millivoltmeter. The meter can use any high-frequency and low-frequency transistors of the appropriate structure with a static current transfer coefficient h21e=60 (at an emitter current of 1 mA). The circuit uses capacitors MBM, KM (C2-C5, C6-C9, C13-C15) and K50-6, MLT 0,125 resistors, SP-1 variables, buttons S1 - KM1-1, switch S2 - sliding from the Sokol receiver , converted to two-way (four positions).

The adjustment of the device begins with checking the modes of transistors for direct current - they should not differ from those indicated by more than ¦ 20%. Then adjust the filter on the transistor V6 by selecting the resistor R26 *, check the phase and amplitude of the direct and rotated signals by 180¦. After that, you can take measurements. To do this, the millivoltmeter is switched to the 2V limit, the sliders of the variable resistors R16 and R12 are set to the middle position. The SJ button must be in the position shown in the diagram. A signal with an amplitude of 3 ... 5 V and a frequency corresponding to the measurement frequency set by switch S2 is fed to the meter input from a sound generator.

By manipulating the variable resistor R1 and the switch of the millivoltmeter measurement limits, they achieve that the arrow of the device is set in the last third of the scale. Resistor R12 achieves a minimum reading of the device, then, resistor R16 further reduce these readings. After that, the minimum is found again with the resistor R12, and then the minimum is again set with the resistor R16, and so on until the manipulations of the resistors R12 and R16 no longer reduce the readings of the millivoltmeter. After that, they proceed to the calibration, for which the millivoltmeter limit switch is again set to the 2 V position and the S1 button is pressed. With a variable resistor R1, a voltage is set, possibly close to 2 V, then the S1 button is released and the harmonic voltage is read on the millivoltmeter scale.

The harmonic coefficient is calculated by the formula

KG \u1d 2/2 U100 * XNUMX%.

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