Calibrated Voltage Source GK-10

Yu. M. Fomichev, Yu. I. Latonov, A. V. Sokolov

(Presented by the scientific seminar of the Department of Radio Engineering)

In connection with the expansion of the production of digital voltmeters, the question of developing reference equipment has become especially acute—equipment that would ensure their calibration and verification at the places of manufacture and operation. Along with high metrological properties, this equipment must also satisfy stringent operational requirements and ensure high work productivity.

In 1971, the Department of Radio Engineering of TPI developed a fully transistorized model of the GK-10 calibrator. As operation of the calibrators under production conditions has shown, this model quite fully satisfies the requirements imposed on reference equipment for alternating current.

The GK-10 calibrator is a source of calibrated sinusoidal voltages of audio and ultrasonic frequency, with a low level of harmonic distortion and background noise and high stability of the output voltage.

The problem of obtaining a stable output voltage calibrated by rms value is solved by developing the GK circuit with combined stabilization and calibration. In this case, a specified level of alternating voltage (in the present case, 10 V) is taken from the GK output with specified error, stability, and nonlinear distortion. This voltage is then divided by means of an inductive divider down to a level of 1 mV. Comparison of the rms value of the alternating voltage of 10 V with a constant voltage is performed by means of a differential comparator using photoelectric converters.

Fig. 1

The functional and schematic diagrams of the generator-calibrator are given in Figs. 1 and 2. As can be seen from Fig. 1, in the GK-10, as in the conducted

Fig. 2. Schematic circuit diagram of the GK-10

in earlier developments, GK-3, GK-4, etc. [1, 2], in order to reduce the resulting instability of the output voltage, a photoelectric comparator was included in the automatic-control circuit.

Description of the GK-10 Schematic Diagram

The master oscillator (ZG) is an RC oscillator with a Wien bridge. The ZG amplifier is assembled on transistors \(T_1—T_4\). The output stage \(T_3—T_4\) is an amplifier with a dynamic load, which makes it possible to obtain a large gain and, at the same time, by selecting the operating mode (carried out by means of \(R_{13}\)), to reduce its nonlinear distortion considerably. The emitter follower on \(T_2\) is intended for matching the output resistance of the first stage and the input resistance of the final one. The ZG amplifier is provided with negative feedback for direct current (\(R_{10}, R_3, R_4, R_6\)) and alternating current (\(R_8, R_9, \text{LN}_1 \parallel R_6\)). To stabilize the output voltage of the ZG, a miniature incandescent lamp of type NSM 12×5 (LN) is used; its time constant in the selected mode is within \(0.3 \div 0.5\) sec, which is quite acceptable at the lower cutoff frequency of 20 Hz.

The power amplifier (UM) is assembled on four transistors: \(T_5—T_8\). The first two stages are followers—a source follower (on a field-effect transistor) and an emitter follower (\(T_6\))—which makes it possible to obtain a high input resistance and drive a powerful final stage. The output stage is an amplifier with a dynamic load. Such a construction of the UM makes it possible to obtain high loop gain when negative feedback is introduced without loss of stability. To reduce nonlinear distortion, the amplifier is provided with deep parallel negative feedback (\(R_{18}, R_{17}, R_{16}, R_{\phi1}\)). This same network is the negative-feedback circuit for direct current. In addition, the output stage operates in a mode close to push-pull, which leads to partial compensation of even harmonics.

To amplify the mismatch signal, a three-stage differential DC amplifier (UPT) is used. As the first stage, an integrated amplifier of type 1UT221 is used, having a small input-referred drift. The subsequent stages (\(T_9—T_{11}\)) are made on matched transistors MP116B. The output stage is an emitter follower, performing the role of a matching link between the output resistance of the UPT and the input resistance of the photoelectric converter (\(CD_1, R_{\phi1}\)). To increase the stability of the automatic-control system and reduce nonlinear distortion in the low-frequency region, a correcting capacitance \(C_{41}\) is introduced. The reference-voltage source is a stabilizer assembled according to a compensation circuit of the series type. The control element consists of transistors \(T_{18}\) and \(T_{19}\), connected in a compound-triode circuit. The UPT included in the feedback circuit is implemented on an integrated differential amplifier of type 1UT221, having a small zero drift (5 μV/1°C), and transistors \(T_{13}—T_{17}\). In addition, the integrated UPT serves as an element for comparing the stabilizer output voltage with a standard constant. As the reference element, a stabilitron of type KS196B is used, having a TKN of \(-0.0005\%/1^\circ\text{C}\) and a guaranteed voltage drift over 2000 hours of operation of not more than 2 mV.

To obtain the specified range of output voltages in the GK-10, a wideband precision inductive divider is used, consisting of four decades and making it possible to obtain output voltages from 0 to 10 V in steps of 1 mV. All decades are autotransfor-

transformers wound by a special method [3] with PELSHO Ø0.23 mm wire on 79NM-type permalloy cores.

For convenience in calibrating digital voltmeters, the GK-10 uses digital indication of the set value of the output voltage ($L_1—L_5$).

The mechanical design of the GK-10 is shown in Fig. 3. In terms of the principal metrological characteristics—the error in setting the output voltage, its time instability, and the harmonic

Fig. 3

content on the main range (10 V) and on the ranges 100 mV–10 V, three generator-calibrators were calibrated against the standard equipment of VNIIM.

The procedure for calibrating calibrators on the main range is described in [4].

Of interest is the method used for testing the GK on ranges below 10 V. Since the inductive divider in the GK has an output resistance of the order of 3–5 ohms and cannot be loaded directly by a thermoelectric converter, an indirect method is used, employing a highly sensitive digital AC voltmeter as a null indicator. The functional calibration circuit is shown in Fig. 4. The measurements were carried out as follows. At the output of the GK-10 a voltage of 10 V was set, which was then applied to the GK inductive divider and to an auxiliary resistive divider. By alternately connecting digital voltmeter 6 to point $a$ or $b$ (Fig. 4), using resistive divider 7, with comparator 3 connected, the voltage $U_7$ is set equal to $U_x$; then the voltage $U_7$ is measured in the usual way with thermoelectric comparator 3. Obviously, in this case the accuracy of calibration will be limited mainly by the sensitivity and stability of the digital voltmeter, as well as by the stability of the voltage $\sim 10$ V.

The harmonic composition of the voltage delivered by the GK-10 was determined by suppressing the fundamental harmonic and measuring the remaining components at all frequency points. The functional measurement circuit is shown in Fig. 5. First, the transfer coefficient of the filter for the harmonics $K_{f2}$, $K_{f3}$, ... was determined (Fig. 5, a); then the circuit of Fig. 5, b was assembled, the calibrator output voltage $U_f$ was set at the frequency under investigation, and the voltages of the second ($U_{f2}$) and third ($U_{f3}$) harmonics after the filter were measured. The harmonic coefficient of the output signal is equal to

\[ K_{\mathrm{r}2}=\frac{U_{f2}}{U_f\cdot K_{f2}}\cdot 100\%;\qquad K_{\mathrm{r}3}=\frac{U_{f3}}{U_f\cdot K_{f3}}\cdot 100\%. \]

The technical characteristics of the GK-10, taking into account the results of tests at VNIIM, are given below.

1. Frequency range: 20 Hz–100 kHz (10 fixed points).

2. Output voltage: 1 mV–10 V in 1-mV steps.

Fig. 4. 1—GK-10 calibrated-voltage source, 2—inductive divider, 3—standard comparator, 4—DC voltage source, 5—potentiometer, 6—digital voltmeter, 7—resistive divider

1. Error in setting the rms value of the alternating voltage on the main range (10 V), \(\delta=\pm 0.03\%\); additional error on the range below 10 V at 1 kHz

Fig. 5. \(G\)—sinusoidal-oscillation generator, \(\Phi\)—filter, \(SV\)—selective voltmeter

\[ \delta=\left(2\cdot 10^{-6}+\frac{5\ \mu\mathrm{V}}{U_{\mathrm{out}}}\right)\cdot 100\%. \]

1. Additional frequency error on ranges below 10 V:

a) in the frequency range 1–100 kHz it does not exceed the value

\[ 10^{-8}\left[(f-1)K+S(f-1)^2\right], \]

where

\[ f\text{—frequency }[\mathrm{kHz}],\qquad K=\frac{U_{\mathrm{in}}}{U_{\mathrm{out}}}\text{—division factor;} \]

b) in the range 20 Hz–1 kHz it does not exceed the value

\[ 6\cdot 10^{-4}(1-f)^2, \]

where

\[ f\text{—frequency }[\mathrm{kHz}]. \]

1. Coefficient of odd harmonics of the calibrator output signal in the audio-frequency region: not more than 0.03%.

2. Instability of the output voltage: less than 0.01% over 10–15 minutes; 0.2% over 8 hours.

REFERENCES

1. M. S. Roitman, Yu. M. Fomichev, E. I. Tsimbalist. Stable source of calibrated alternating voltage. Avtometriya, 1968, No. 3.

2. M. S. Roitman, Yu. M. Fomichev, E. I. Tsimbalist. Standard source of calibrated alternating voltage GK-4. Izvestiya TPI, 1969, vol. 171.

3. A. I. Kramnyuk, M. S. Roitman. Extension of the operating range of inductive dividers. Izvestiya TPI, 1971, vol. 231.

4. R. F. Aknaev, Yu. M. Fomichev, A. A. Kushch. Questions of verification of calibrator generators. Izvestiya TPI, 1971, vol. 231.