MEASUREMENT

OF ELECTRIC

DIGITAL INSTRUMENT FOR

THE

AND

MAGNETIC

MEASUREMENT

OF PERIODIC INFRALOW-FREQUENCY

QUANTITIES

INTEGRAL CHARACTERISTICS

OF

SIGNALS UDC 621.384.3:006.063

V~ V. Smelyakov, Yu. G. Ol'khovskii, Ao I. Aliseiko, and A. V. Shevchenko

The Soviet industry manufactures various measuring instruments covering ultralow frequencies (ULF). The need in such instruments increases continuously. To test their performance in the 20-10-a-Hz range one has to have sufficiently accurate instruments with a short measuring time capable of simultaneous measurement and recording of the effective value U of the signal per period, the effective value of the first harmonic U~, the dc component Uo, and the nonlinear distortion factor (NDF). The parameters U, UI, and Uo should be measurable with an error less than

6U = 0,15 + 0 , 1 ( - ~ - - 1 ) % , and the NDF within the range 0.i to 100% with an absolute error not exceeding AK=

•

(o.olK+o.1)%.

One of the design versions of such an instrument described in [i] has an input shaper of the measured signal period, a digital frequency multiplier [2, 3] which controls the times the instantaneous signal values u(~) are converted into a digital code u(~i) at n points at the period ~i = 2Ti/n, a sample-and-hold circuit which keeps the instantaneous signal value u(~ i) constant for the conversion time, an analog--digital converter, a digital generator of orthogonal functions cos~ i and sinai, a control circuit which programs the execution of all computing and logical operations, a digital computing unit which computes and stores the measured quantities, and a display device. To measure U, U~, Uo, and NDF all integrals are replaced in the instrument with finite sums and the following estimates are computed:

~, ~ (~);

= ~

(1)

n

(3) i=I

(4) (5)

K = Uhh/Up where Uhh is an estimate of higher harmonics voltage; n

==

-

(6)

u (oci) c o s o~i ,

-

i=1

2

(7)

i=I

In principle,

U~h can be found from another expression P

(8) I=2

Translated from Izmeritel'naya Tekhnika, No. 8, pp. 40-41, August,

658

0543-1972/81/2408-0658507.50

9 1982 Plenum Publishing

1981.

Corporation

where p is the number of harmonics: -2

1

~o

=

5~=

n-B l );

--

ll

~

u (::ti) COS

icti;

i=1

~a= - -2- 2 yt

u ("zi) s i n lot i.

i=l

For signals with a limited spectrum expression (8) gives a more accurate estimate for ~2 Uhh than (4) whereas for signals with an infinite spectrum expression (4) is better. Since in practice signals have, as a rule, an infinite spectrum, the described instrument computes U~h by expression (4), Moreover, implementation of a circuit which comoputes U~h by (4) is simpler than a circuit based on e x p r e s s i o n (8). The main error components of the measurement of effective voltage values and of NDF are: the error of measuring the instantaneous values u(~ i) introduced by the input analog-digital converter, the sample-and-hold circuit, and frequency multiplier; the error a s s o c i a t e d w i t h replacing integrals with corresponding

finite sums;

the error due to rounding off the orthogonal functions cos~ i and sina i . In the course of instrument design the above errors were analyzed and the measuring process was computer simulated for different numbers of bits of the input analog--digital converter, the computing circuits, and the orthogonal functions cos~ i and sina i generator. The simulation was carried out with different input signals with a NDF from 0.i to 100%, with finite and infinite monotonically decreasing and oscillating spectra, and for different numbers of instantaneous signal samples per period. The principal metrological characteristics and design parameters of individual units and of the complete instrument were checked experimentally. The results of a theoretical and experimental study proved the possibility of designing a versatile digital instrument with a frequency range from 0.001 Hz to several kHz with the permissible error values stated before. Such an instrument must include a n l l - o r 12-bit analog-digital converter, a 48- to 50=bit computing unit, a 23- or 24-bit digital generator of orthogonal functions cosai and sinai, and a digital frequency multiplier operating at a reference master generator frequency from 1 to i0 MHz (depending on the input signal frequency). The number of samples n per one input signal period is taken to be 256. The maximum analyzed signal frequency is limited mainly by the digital frequency multiplier error due to uniform signal quantization in time. By increasing the master generator frequency one can extend the upper limit of the analyzed signal frequency. The first experimental model of the instrument built of series K155 integrated circuits passed official tests. These tests indicated that the error of NDF measurements in the range from 2 t o 80% did not exceed 0.1% in the range from 10 -3 to 15 Hz. The instrument has been approved for commercial manufacture. LITERATURE le 2. 3.

CITED

A. I. Aliseiko et al., Inventor's Certificate No. 618695; Byull. Izobret., No. 29 (1978). V. V. Smelyakov et al., Inventor's Certificate No. 663068; Byull. Izobret., No. 18 (1979). V. V Smelyakov , Digital Ultralow-Frequency Measuring Equipment [in Russian], Energiya, Moscow (1975).

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