E X T E N S I O N OF T H E S P H E R E OF A P P L I C A T I O N

OF N O N L I N E A R - D I S T O R T I O N M E T E R S

(UDC 621.317.853/1)

B. G. K a d u k

Translated from Izmer i t e l ' naya Tekhnika, No. 10, pp. 95-96, October, 1966

Measurement of the Coefficient of Nonlineax Distortions of Signals with a Voltage Below 0.1 V by Means of Meter $6-1. The widely used meters INI-11 and $6-1 are intended for measuring nonlinear distortions of signals

with an input vol tage leve l exceeding 0.5 and 0.1 V, respect ively, in a range of coefficients from 0.1 to 30% tt is considered that nonl inear-dis tor t ion meters cannot be used for measuring d i rec t ly the nonlinear distortion coefficient of signals with an effect ive vol tage value below 0.1 V.

In such cases the tested signal has first to be ampl i f ied . The requirement for an addi t ional amplif ier which in- creases the measurement errors by the irregulari t ies of its frequency character is t ic and the nonlinear distortions con- tributed by it is a def ic iency of such a measuring method. However, it is possible to measure the nonlinear distor- t ion coeff ic ient of signals with a low vol tage level even without an addi t ional amplif ier .

In measuring the effect ive value of the total signal voltage, the nonlinear distortion meter is cal ibrated at the fixed mark of the 10-V measuring l imi t of the instrument vol tmeter , in order to be able to measure nonlinear dis- tortion coefficients of the order of 0.1%, which corresponds to the higher harmonics effect ive vol tage va lue of 10 mV (the most sensitive fu l l - sca le reading amounts to 10 mV). The nonlinear distortion meter $6-1 can be cal ibrated

for lower measurement ranges of its vol tmeter , for instance, to the same fixed mark but on the 1-V range. A non-

l inear distortion coeff ic ient of the order of 0.1% will then correspond to the higher harmonics effect ive vol tage range of the order of 1 mY. Since the ext reme reading points of the instrument vo l tmeter corresponds to 3 mV, the range of the measured nonlinear distortion coefficients wii1 be l imi ted to 0.3%

The po ten t ia l i ty of the $6-1 nonlinear distortion meter in measuring the nonlinear distortion coefficients of signals with a vol tage level below 0.1 V are shown in the table.

I t should be noted that for input signals below 1 mV it is diff icul t to measure the coeff ic ient of nonlinear dis- tortion owing to the high leve l of the $6-1 instrument 's input noise.

Ut i l iza t ion of Noniinear Distortion Meters for Evaluating the Ampli tude of Harmonics. Certain part icular features of nonlinear distortion meters make them suitable for evaluat ing the harmonic ampli tudes, especial ly in the absence of special instruments used for that purpose, such as analyzers AG-10, S-5-3 , etc. [1].

In nonlinear distortion meter KLM-87 [2], it is possible to de termine the number and phase of the distorting harmonic on the osci l loscope incorporated in the device .

G(:

O2 !

Fig. 1.

50 z5 ~oo %%

Fig. 2,

Voltmeter fsd ranges, in V

10 3 1 0.3 0.1

I Minimum input signal voltage, mV

lO0 :30 10 3 1

Klirr factor meas. range, ~

0 1--30 O. 1--50 O. 3--59

1--50 3--50

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Since, for certain nonlinear distortion meter circuits there exists a functional relationship between the corre la- t ion coeff icient Q and the nonlinear distortion coeff ic ient K f l , i t is possible to use the meter for evaluating the cor- relat ion coeff ic ient [3]:

1

The ava i lab le nonlinear distortion meters are suitable for evaluating the vol tage of the tested signal and the effect ive va lue of the first harmonic vol tage, and under cer tain conditions of higher harmonics.

In fact, the frequency characterist ic of the major i ty of such meters does not correspond to an ideal character- istic of a high-pass fi l ter, but can be represented by an equation of a band-e l imina t ion filter. Band-el iminat ion

fi l ter characterist ics are obtained by including a se lect ive e lement (a Wien bridge or a twin-T bridge for the range of low and infrasonic frequencies, and an LCR bridge for the range of high frequencies) into the feedback circuits of a wide-band amplif ier [4]. If, in addition, a feedback is used for higher harmonics, the frequency character is t ic of the amplif ier acquires the shape shown in Fig. 1.

In such a case it becomes possible to de te rmine the ampl i tude of the first harmonic and that of higher har- monics and subharmonic components without any modif ica t ion to the circuit (for instance, switching of the se lect ive e lement from the positive to the negat ive feedback).

The nonlinear distortion coeff ic ient is evaluated by measuring sequential ly the effect ive vol tage values of the tested signal and the signal with the suppressed first harmonic. Thus, the value of the nonlinear distortion coeff i - c ient is represented by

UI I - - n - 2 U

U I ~ n x

n ~ 2

(1)

The ampl i tude of the first harmonic can be evaluated from (1) by representing the above expression in a more convenient form (Fig. 2) and taking into consideration that K 2 = UII /UI:

(2)

The error in measuring the harmonic ampli tudes in such a case is high (~ 10%), but it is accep tab le for a qual i - ta t ive analysis.

By tuning the nonlinear distortion meter to the frequencies of subharmonic or higher harmonic components, it becomes possible to de termine their ampli tudes by an analyt ica l method, provided there is a heavy feedback which

includes the select ive e lement .

LITERATURE CITED

1. G . I . Gubenko, Avtomat ika i proborostroenie, No. 4 (1960). 2. Elect~onische Rundschau, 13, No. 4 (1959).

3. M . A . Sapozhkov, Akust. zh., No. 3 (1956). 4. B.G. Kaduk, Radio, No. 12 (1962).

All abbreviations of periodicals in the above bibliography are letter-by-letter translitera- tions of the abbreviations as given in the original Russian journal. Some or all o f this peri- odical l i terature may wel l be avai lable in Eng l i sh translation. A complete l ist of the cover-to- cover English translations appears at the back of the first issue of this year,

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