Test signal generator for testing UMZCH. Scheme, description

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Test signal generator for testing UMZCH. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Transistor power amplifiers

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When setting up and checking UMZCH, sinusoidal, rectangular and triangular signals are most often used as test signals. Sometimes a mixture of two signals is taken to measure intermodulation distortion, for example. 19 and 20 kHz. The combined signal makes it possible even to evaluate the introduced intermodulation distortions even aurally, since in this case the distortions are isolated in the form of a 1 kHz signal, to which the hearing sensitivity is very high. When testing, it should be taken into account that the signal amplitude doubles on beats.

As the world experience of UMZCH research has shown, the total level of nonlinear distortions of the amplifier (THD - Total Harmonic Distortion) only indirectly speaks of its quality. The thing is. that the low level of distortion is achieved mainly due to the suppression of lower harmonics using deep total negative feedback (CNF) on voltage. In this case, the higher harmonics, as a rule, are not suppressed, but more often, on the contrary, they increase, and their spectrum expands due to the insufficient broadband of the original (without OOS) amplifier. To an even greater extent, this effect is manifested in the UMZCH with a push-pull emitter follower at the output when operating on a complex load (without taking additional measures to reduce the output resistance of the cascade in the event of a “break” in the CNF), which is the “transistor” sound.

In some cases, it is more informative to study the amplifier using a meander-type signal. Using such a signal, you can explore the dynamic properties of the amplifier and its transient response. For high-quality reproduction of the meander, the UMZCH bandwidth must be at least 10 times higher than the frequency of the test signal. Poor dynamic characteristics are manifested by a large surge (more than 3 ... 5%) and "ringing" on the "shelves" of the test signal at its full swing at the amplifier output of about 600 mV.

In principle, the real sound signal is far from any of the test signals in form and is impulsive in nature. To bring the test signal closer to the musical one, I suggest this generator (Fig. 1).

Test signal generator for testing UMZCH

The device consists of:

a master oscillator with a frequency of 4 kHz on the elements DD1.1, DD1.2;
divisor by 5 on elements DD2.2, DD1.3, DD1.4;
divisors by 2 on triggers DD3.1, DD3.2;
adder on DA1.

The power supply of the circuit is bipolar, stabilized. As stabilizers, low-power integrated types IL78L06AC and IL79L06AC are used (not shown in the diagram).

The scheme works as follows. The master oscillator generates a signal with a frequency of 4 kHz (achieved by selecting the resistor R1). Further, this signal is divided by 5, and short positive pulses with a frequency of 11 Hz are formed at pin 1.4 DD800. Then these pulses are fed to a divider by 2, and a meander with a frequency of 3.1 Hz is obtained at the output of DD400. At the same time, the 4 kHz signal is divided by the second divider by 2, as a result, we get a second meander with a frequency of 3.2 kHz at the output of DD2.

Depending on the position of the regulators R4 and R5, the following types of test signals are observed at the generator output:

meander frequency 400 Hz;
meander frequency 2 kHz;
combined signal.

If you bring the slider of the resistor R4 to the top position according to the diagram, and gradually move the slider of the resistor R5 from the bottom up, you can get a switching signal with a frequency of 2 kHz both in the extreme positions of the combined signal and in the region of the zero crossing. Unlike a musical signal, where the picture changes unpredictably, this signal is repeated and well synchronized by the oscilloscope, which expands the possibilities for studying the behavior of the UMZCH in switching modes of operation. For clarity, Fig. 2-4 shows the oscillograms of the combined signal at different ratios of signals with a frequency of 400 Hz and 2 kHz.

(click to enlarge)

In my opinion, the test of UMZCH with a combined signal gives the greatest information content, in which the original signals (400 Hz and 2 kHz) are equal or have a slight difference. It is desirable that at the output of the UMZCH the "troughs" of the signal in Fig. 3 do not reach the zero line of about 0.5 V, and in Fig. 4 they go over the zero scan line by about 0,5 V (but not more than 1 V).

Such signals can be the most "difficult" for UMZCH with a push-pull emitter follower at the output, operating in class "B" or "AB". A combined signal with a small signal component with a frequency of 2 kHz will help to identify the shortcomings of the UMZCH at an output signal level close to limiting. In this case, one of the arms of the output stage is in the cutoff mode or close to it.

Author: A.Petrov, Mogilev

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