GK-12 Generator-Calibrator

M. S. Roitman, Yu. M. Fomichev, Yu. A. Bulatov, Yu. G. Svinolupov

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

The GK-12 generator-calibrator is the first domestic multivalued measure of AC voltages with high long-term stability. It differs from the previously developed GK-8 and GK-10 generators also in its substantially improved metrological properties and operating characteristics. The latter is due to the fact that it has no manual adjustments of any kind. This simplifies maintenance and sharply increases labor productivity. The block diagram of the GK-12 is shown in Fig. 1. The voltage from the output of the master generator ZG is fed to two cascaded voltage-stabilization stages.

The expediency of two independent automatic-regulation loops is explained as follows. Changes in the voltage at the output of the ZG during frequency switching, owing to variation of circuit parameters, can reach 10%, while the instability of the output value must not exceed 0.005%. Consequently, a stabilization coefficient \(G_{\text{st}} \ge 2000\) is required. At the same time, the largest possible stability margin and a minimum coefficient of the third harmonic \(K_{3\text{г}}\) in the low-frequency region are desirable. In a single-loop system this is feasible only with an artificial and very substantial increase in the filter time constant (see expressions (1), (2) in [1]), which sharply raises the requirements on the permissible level of voltage fluctuation at the ZG output. It should also be noted that, when an RMS measuring converter (IP) is used as the sensitive element of the autostabilization system, a substantial increase in the IP time constant is difficult and, in most cases, even impossible. In a two-loop automatic-regulation system, however, the situation is greatly simplified [1], since \(G_{\text{st}}\) is equal to the product of the stabilization coefficients \(G_1\)—of the first stage—and \(G_2\)—of the second—and \(K_{3\text{г}}\) is equal to the sum of the third-harmonic coefficients of the stages. In the first stabilization loop, a full-wave average-value measuring converter (IP) on an operational amplifier is used (Fig. 2). The use of an operational amplifier with low equivalent drift (1UT401A) and high-frequency diodes (GD508B) makes it possible to construct an IP with good frequency response and time stability. As studies have shown, \(\delta_{\text{ip}}(f)\) does not exceed \(\pm 0.2\%\) in the frequency range 20 Hz–200 kHz, and \(\delta_{\text{ip}}(t) \le 0.1\%\).

To ensure a small \(K_{3\text{г}}\) over the entire frequency range, in the IP at frequencies from 20 to 200 Hz a filter with a single-

Fig. 1.

Visible labels in the schematic:

• Output
• DI: \(20\ \text{Hz}–200\ \text{kHz}\)
• DI: \(20\ \text{Hz}–200\ \text{kHz}\)
• U4
• G
• U5
• Cell 2
• U2
• Average-value converter
• \(2\ \text{Hz}–20\ \text{kHz}\)
• \(200\ \text{Hz}–200\ \text{kHz}\)
• U3
• OI
• Output-voltage monitoring
• \(0\)
• \(0.1\%\)
• \(1\%\)
• UM1
• U1
• G2
• G1
• U1
• G3

stage \((R_7 C_1)\) to a two-stage one \((R_7 C_1 R_8 C_2)\). The voltage is adjusted by an optocoupler consisting of an AL102A light-emitting diode and a low-inertia SF3-4B photoresistor.

Thus, the automatic voltage-control system at low frequencies, to a first approximation, is a third-order system.

For the adopted spread of the time constants of the stages, the practical value of the stabilization coefficient exceeds 500, while

Fig. 2

\(G_1 = 100\text{–}150\). The second automatic-control loop with \(G_2 \sim 20\) consists of an rms comparator with automatic drift correction [2]. To increase the stability margin and reduce the duration of transient processes, a non-inertial stage is used for voltage control (a divider using resistors and UD\(_2\) diodes). In the adopted narrow voltage-control range (2%), the nonlinear distortions introduced by the regulating stage are less than 0.005%.

The reference generator (Fig. 3) consists of a voltage follower with a KP303 field-effect transistor at the input, an amplifier with transmission coefficient \(K = 3\), and an inertial nonlinear element (NSM12×5) in the negative-feedback circuit, and an \(RC\) circuit of positive feedback. To reduce nonlinear distortion, the follower in the reference generator is made according to the circuit of a two-stage amplifier \((T_1, T_2)\) with a dynamic load \((T_3)\) and almost 100% series negative feedback. The use of an operational amplifier (1UT401B) in the reference generator makes it possible to apply deep negative feedback to it without loss of effectiveness over the entire frequency range. In addition, by means of resistor \(R_5\) the coefficient of the second harmonic of the follower and of the entire generator can be adjusted and reduced to values on the order of 0.005% at an output voltage of up to 2 V.

The entire frequency range is divided into 4 decades. Decade switching is performed by switching capacitances, while frequency variation within each decade is accomplished by varying paired resistors. Provision is made for remote control of the generated frequency. Scanning within a decade is carried out by two identical photoresistors illuminated by an NSM 6.3×20 incandescent lamp (see Fig. 1). Identity over the entire range is achieved by using

of the photoresistor matrix. Decade frequency changing is performed by automatic switching of the capacitances of the phase-shifting circuit.

To obtain small values of nonlinear distortion and output resistance, additive correction by the instantaneous value of the transfer-coefficient error is used in the power amplifier.

All voltage graduations below the main limit of 10 V are obtained by means of a five-decade inductive divider. The first three decades are made by the stepped winding method [3].

One of the main features of the divider is that, in order to achieve a small frequency error up to 200 kHz, it consists of two

Fig. 3

simultaneously switched decade inductive dividers. The first operates in the range from 20 Hz to 20 kHz, and the second—from 20 to 200 kHz.

The GK-12 also provides for the possibility of deviation of the output voltage within specified limits. The change in the delivered voltage is effected by the first and second autostabilization loops, with a precisely specified deviation of the reference-voltage level.

REFERENCES

1. M. S. Roitman. Multivalued measures of alternating voltages. This collection.

2. M. S. Roitman, Yu. G. Svinolupov, Yu. A. Bulatov. Comparators of alternating voltages with high long-term stability. This collection.

3. A. I. Kramnyuk, M. S. Roitman. Wideband inductive voltage dividers. This collection.