UDC 621.317.7.001.6

GK-12A GENERATOR-CALIBRATOR

Yu. M. Fomichev, Yu. N. Varakuta, Yu. G. Svinolupov

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

A description is given of the modernized source of calibrated voltage GK-12A. A description is provided of the principal units of the calibrator that underwent modernization, and of the reasons that necessitated it.

The fundamental changes affected the setting generator, the controlled divider, and the differential comparator. The influence of transient processes at the comparator output, caused by the inertia of the commutator, on the output voltage is analyzed, and a concrete solution is given that makes it possible to eliminate this drawback. The technical characteristics of the GK-12A generator-calibrator are presented.

Illustrations 4, bibliography 5.

The GK-12A generator-calibrator is a modernized version of the previously developed GK-12 calibrator [1].

The need to modernize the GK-12 was caused by the following reasons:

1. Insufficient inertia of the nonlinear element in the setting generator (an incandescent lamp) led to the appearance of large nonlinear distortions in the low-frequency region.

2. The small transmission coefficient of the feedback circuit of the setting generator with respect to the envelope led to the appearance of considerable nonuniformity in the frequency characteristic of the SG ($\pm 10\%$), which, in fact, necessitated the use of a two-loop stabilization system.

3. When the photoelectric comparator was switched to correction, the second loop of stabilization of the calibrator output level proved to be open, and the signal delivered during the correction time (5–10 sec) was not controlled.

4. At the output of the calibrator there appeared a voltage overshoot due to the transient process arising during switching in the second loop of autostabilization.

Thanks to new solutions for individual links, it proved possible to simplify the structure of the calibrator and at the same time improve its metrological properties.

The setting oscillator (SG), the controlled divider [2], and the differential comparator with automatic drift correction underwent fundamental changes.

In the setting generator (Fig. 1), instead of an incandescent lamp, an amplitude-stabilization system is used with a frequency-independent sharpening link at the input [3]. A reverse-biased diode ($D_1$) is used as such a link. At its output, an error signal exists only at the moment when the signal voltage ($U_{\text{out}}$)

of the reference voltage \((U_{\mathrm{ref}})\), i.e., it consists of clipped peaks of sinusoids of one polarity. The error pulse signal is then amplified by amplifier \((U_2)\), which must be noninverting in order to provide negative feedback with respect to the envelope, and broadband so that the stabilization coefficient remains constant over a wide frequency range. The amplified error signal is rectified

Fig. 1

by an amplitude detector \((D_2)\) and is fed to the gate of the field-effect transistor \((T_1)\). To reduce the nonlinear distortions introduced by the negative-feedback circuit of amplifier \((U_1)\), the drain-gate characteristics of \(T_1\) are linearized by means of feedback from drain to gate with a transmission coefficient equal to 0.5. The use of such an automatic-regulation system makes it possible to obtain a large stabilization coefficient without increasing inertia and nonlinear distortion at low frequencies [3]. The master oscillator in the GK-12A has nonlinear distortion of less than 0.05% and frequency-response nonuniformity of \((0.1—0.2)\%\) over the entire frequency range. This, in turn, made it possible to simplify the functional design of the GK-12A, since, with an allowable calibrator frequency-response nonuniformity of 0.01%, the required stabilization coefficient of the subsequent links, equal to \(10 \div 20\), is easily provided by a single automatic-regulation loop comprising a power amplifier \((UM)\), a comparator with automatic drift correction, and a controlled divider on a field-effect transistor \((UD)\) (Fig. 2).

In the comparator, a method is used for simultaneous correction of two alternately operating measuring channels. The essence of the method is that two measuring transducers are used in the comparator \((R_{\phi1}, LN1, R_{\phi2}, LN2)\), which alternately perform the function of measuring \(U_{\sim}\). While one of them is connected into the measuring path (in Fig. 2, \(R_{\phi1}\), LN1), the second is being corrected \((R_{\phi2}, LN2)\). In the ideal case this makes it possible to reduce the time during which information is absent from the output signal to the transfer time of the switch contacts.

Successful application of such a method is possible, however, only with ideal switch characteristics, i.e., provided that the resistances of the closed switch contacts of both channels are identical (or equal to 0), and that switching is instantaneous. If the compensation error due to failure to satisfy the first condition can be reduced to extremely small values by using sealed magnetically controlled contacts (reed switches) as the switches, then the inertia of the latter leads to the appearance of undesirable transi-

transient processes at the output of the comparator and, correspondingly, at the output of the calibrator when the contacts are switched.*

Indeed, representing the photoelectric comparator as a second-order system for the input power [4] with transfer function

\[ F(p)=\frac{K_{\text{p}}}{(p+a)(p+b)}, \]

where \(a=\dfrac{1}{T_{\text{l}}}\), \(T_{\text{l}}\) is the time constant of the incandescent lamp; \(b=\dfrac{1}{T_{\text{f}}}\), \(T_{\text{f}}\) is the time constant of the photodetector; \(K_{\text{p}}\) is the transfer coefficient of the comparator, and representing the action on this system at the moment of switching the contact from an alternating voltage to a constant voltage equal to it in power as

\[ F_{2}(p)=\frac{1}{p}\left(e^{-p t_{0}}-1\right), \]

Fig. 2

where \(t_{0}\) is the contact switching time, we obtain the transform of the output signal

\[ F(p)=\frac{(e^{-p t_{0}}-1)K_{\text{p}}}{p(p+a)(p+b)}, \]

which corresponds to a change in the output signal

\[ f(t)=\left[\left(\frac{1}{ab}-\frac{b e^{-a(t-t_{0})}-a e^{-b(t-t_{0})}}{ab(b-a)}\right) -\left(\frac{1}{ab}-\frac{b e^{-at}-a e^{-bt}}{ab(b-a)}\right)\right]. \]

* The use of hybrid-type switches (reed relay—transistor) for the purpose of eliminating contact inertia [5] is inexpedient in this case, primarily because of the complexity of such an implementation.

Fig. 3 shows a graphical representation of \(f(t)\), constructed for the averaged value of the time constants of a photoelectric converter based on the SF2-5 and NSM \(8\times 60\). As can be seen from the graph, when

Fig. 3

the condition \(t_0 \leq (0.1 \div 0.05)T_\ell\) is fulfilled, the transient process is practically completed after \(t \simeq T_\ell\).

In this connection, in the GK-12A, strobing of the output signal of the comparator with storage of the output voltage (transistor \(T_3\), capacitor \(C_1\)) is used for the duration of the transient processes. The timing diagrams for switching the commutators (relays \(P_1, P_2, P_3\)) are given in Fig. 2.

Technical Characteristics of the GK-12A

The frequency range \(20\ \text{Hz} \div 200\ \text{kHz}\), with smooth tuning, is divided into 4 subranges:

\[ \begin{aligned} 20\ \text{Hz} &\div 200\ \text{Hz},\\ 200\ \text{Hz} &\div 2\ \text{kHz},\\ 2\ \text{kHz} &\div 20\ \text{kHz},\\ 20\ \text{kHz} &\div 200\ \text{kHz}. \end{aligned} \]

Output voltages are \(100\ \mu\text{V} \div 10\ \text{V}\) in steps of \(100\ \mu\text{V}\).

Deviation of the output voltage is provided within \(\pm 0.5\%\) and \(\pm 0.05\%\) of the set value.

Instability of the output voltage is \(\pm 0.01\%\).

Nonlinear distortion is $\leq 0.05\%$ at frequencies from 20 Hz to 100 kHz, and no more than 0.15% up to 200 kHz.

The mechanical design of the GK-12A is shown in Fig. 4.

Fig. 4

REFERENCES

1. M. S. Roitman, Yu. M. Fomichev et al. Generator-calibrator GK-12. Izvestiya TPI, vol. 270, Tomsk, Tomsk State University Publishing House, 1973.

2. Yu. M. Fomichev, Yu. N. Varakuta. Control devices with low nonlinear distortion (present collection).

3. M. S. Roitman. Theory of amplitude-stable self-oscillators with low nonlinear distortion (present collection).

4. M. S. Roitman, Yu. G. Svinolupov et al. RMS comparators (present collection).

5. M. S. Roitman, Yu. G. Svinolupov, Yu. A. Bulatov. RMS comparators with high long-term stability. Izvestiya TPI, vol. 270, Tomsk, Tomsk State University Publishing House, 1973.