Attachments for measuring the coefficient of non-linear distortion - Kg. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Measuring technology

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In [1], an attachment was described for measuring the coefficient of nonlinear distortion on two transistors. The proposed schemes are its development. The prefix is ​​designed to work with standard RF generators and oscilloscopes. It is desirable that the generator output voltage be about 10 V (for example, GZ-35 or GZ-102), which simplifies the necessary mathematical calculations.

The principle of operation of set-top boxes, based on filterless selection of the harmonic component of the signal, was developed by I.T. Akulinichev [2]. The circuit shown in Figure 1 is essentially an inverting summing amplifier. It can be used to measure Kg of a powerful low-frequency amplifier in cases where it inverts the signal and has the following approximate parameters:

• input sensitivity, V - 0,75;
• voltage gain, dB - 26;
• rated output power, W - 50;
• load resistance, Ohm - 4.

It is assumed that the outputs of the generator and UMZCH have zero DC potential. If this condition is not met, the prefix is ​​​​connected to the output of the generator and UMZCH through a separating capacitor with a suitable capacitance (providing the transmission of the required frequency band).

The procedure for working with the device is as follows. The power of the UMZCH and the operational amplifier IS1 of the set-top box is turned on. A signal with a swing of 1 V is supplied to the XP20 input from the AF generator, which corresponds to approximately 7,5 V of the effective value on the generator scale. From the XS1 output of the set-top box, a signal with an effective value of 0,75 V is supplied to the input of the power amplifier. The UMZCH output signal is fed to the XP2 input of the set-top box, and the oscilloscope is connected to the XS2 output.

By adjusting RP2 and RP3, you need to get the minimum signal at the output of IC1. Then the phase of the total signal is adjusted using C1 according to the minimum output signal of IS1. This procedure must be repeated several times, i.e., by adjusting RP3 and C1, to achieve the lowest possible signal. As a result, with fine tuning, the amplitude of the signal at the output of XS2 will be equal to the amplitude of the harmonics.

Of course, it is possible to accurately determine the coefficient of non-linear distortion (harmonics) Kg only by removing the spectrum of harmonics using a low-frequency spectrum analyzer. However, with sufficient accuracy for practice, Kg can be considered equal to the ratio of the signal amplitude on the oscilloscope screen to the amplitude of the output voltage of the amplifier under test. The calculation of Kr is carried out as follows: - the output voltage range (from peak to peak) Uout.p of the amplifier under test is determined by the formula

where Uin is the effective value of the input voltage of the tested amplifier; Ku is the gain of the amplifier.

For ease of calculation, the output voltage of the UMZCH can be set to Uout.r = 40 V by adjusting the input signal using RP1. If the vertical scale division of the oscilloscope is 0,01 V, and a harmonic signal of ±1 division (corresponding to 0,02 V) is received, then the Kr coefficient expressed as a percentage will be equal to 0,05%. In some oscilloscopes, for example, C1-49, C1-73, the main division of the scale is further subdivided into 5 parts. Therefore, it will be possible to determine a minimum Kr of the order of 0,01%. If even higher sensitivity is needed, the 5 kΩ resistor R20 must be replaced with a 200 kΩ resistor, i.e., increase the gain of IS1 by another 10 times, and the minimum measured Kg will be 0,001%. Large values ​​of Kr are measured by switching the oscilloscope to a larger range.

The prefix can be simplified if the goal is not to measure such small values ​​of Kr. IS1 serves solely to increase the sensitivity of the considered device. The measurement technique remains the same, but the oscilloscope input is connected to the connection point of resistors R3 and R4. Then the minimum division of the scale for Kr will become equal to 0,15%, and the smallest measured value of Kr can be 0,03%.

To determine Kg of powerful non-inverting amplifiers, the considered circuit is transformed into the circuit shown in Fig. 2. In it, the parameters of the prefix are similar to those considered. The measurement technique remains the same.

Fig.2. Schematic diagram of the Kg meter of a powerful non-inverting low-frequency amplifier

Both set-top boxes use operational amplifiers of the A741C type. Other ICs are also suitable, for example, 1UO741S, MAA741S, K140UD7 or TL081 (TL071, TL061). It is most convenient to use the combined circuit by installing an additional switch. You can also change the sensitivity of the set-top box by switching R5.

Literature

1. Kostov D., Imanuel I. Prefix for measuring the coefficient of nonlinear distortion. - Radiomir, 2002, N1, S.XNUMX.
2. Akulinichev I. Vector indicator of non-linear distortions. - Radio, 1977, N6, p.42.

Publication: radioradar.net

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