AC VOLTAGE COMPARATORS WITH HIGH LONG-TERM STABILITY

M. S. ROITMAN, Yu. G. SVINOLUPOV, Yu. A. BULATOV

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

One of the important tasks of measurement technology is the creation of precision comparators of the rms value of voltage, capable of functioning effectively in automatic-control systems and possessing high long-term stability. The urgency of this task is due to the fact that the most accurate method for measuring an alternating voltage is the method of comparing it, by rms value, with a known direct voltage, while all broad-band frequency comparators have comparatively low time stability.

One approach to solving the problem under consideration is the use of the method of periodic conversion [1]. In this case the measured voltage \(U_x\) and the reference voltage \(U_0\) are fed alternately, through the switch \(P\) at a frequency \(f_k\), to the measuring converter \(IP\) (see Fig. 1). If the units \(P\) and \(IP\) are ideal, then, for

Fig. 1

\(U_x \ne U_0\), a signal with frequency \(f_k\) will appear at the output of the measuring converter; after amplification by the amplifier, this signal is rectified by the synchronous detector \(SD\). The low-frequency filter \(\Phi\) suppresses the alternating component of the rectified signal, and at the output we obtain a direct voltage \(U_{=}\), whose value is proportional to the deviation of \(U_x\) from \(U_0\), while its polarity depends on the sign of this deviation. Such a construction of the comparator imposes very stringent requirements on the switch, since the errors introduced by it enter fully into the resulting comparison error. The required dynamic characteristics of the switch are determined by the lower limiting frequency \(f_n\) of the measured voltage \(U_x\) and by the permissible error of the measuring converter \(IP\) at this frequency, \(\delta_f\). For example, if \(f_n = 20\) Hz and \(\delta_{fn} = 0.01\%\), then in the case of photoelectric

for a thermal transducer the time constant of the filament lamp \(\tau_l\) must be no less than 200 msec [2], and the maximum switching frequency

\[ f_{k\max} \ll \frac{1}{5\tau_l}=1\ \text{Hz}. \]

Lowering the switching frequency, however, makes it difficult to amplify the signal and especially to filter the ac component at the output of the synchronous detector (to avoid instability in the closed-loop voltage-control system, the filter \(F\) can only be single-link), and ultimately reduces the speed of response of the comparator. Therefore the frequency \(f_k\) has to be chosen within the range \(0.5 \div 1\) Hz. Hence we obtain an allowable switching duration of less than 1000 μsec. Studies carried out on magnetically controlled contacts (reed switches) of types KEM-1, KEM-2, and KEM-2B showed that the transfer time of the contacts and the bounce exceed the indicated value. The use of transistors in inverse connection in the commutator does not give the desired results, primarily because of the considerable resistance of the closed switch \((r_k > 3\ \Omega)\) and because it is not identical for direct and alternating currents. One more difficulty arises, associated with the need to have a source of the switching voltage \(G\) with small capacitance relative to “ground.” Such a need is absent if the transistor commutator is made according to the circuit shown in Fig. 2. This comparator is simple and effective when a comparison error of the order of 0.05% or more is permissible.

Fig. 2

The requirement for low resistance of the open switch is sharply relaxed if a matching amplifier (MA) or a voltage follower is inserted between the measuring transducer TP and the commutator P (see Fig. 3). It is obvious that the transfer coefficient of the MA for the voltages \(U_x\) and \(U_0\) must be the same.

Fig. 3

Unfortunately, at present there are still no implementations of amplifiers with an invariant transfer coefficient for dc and ac. This makes it necessary to take an ac voltage as the reference voltage \(U_0\). This is precisely the approach used in the Hewlett—Packard 745A calibrator [3]. As \(U_0\), a square-wave voltage with a fixed frequency of 400 Hz is used. It is clear that, all other conditions being equal, the potential metrological properties of such a comparator are limited by the metrological characteristics of the reference ac-voltage generator \(U_0\) and by the stability of the frequency-phase characteristic of the MA. The latter is the principal factor limiting the frequency range (110 kHz in the 745A calibrator) and causing the error at 100 kHz to increase to 0.05%. To reduce the frequency error, we developed a matching

...ing follower using additive correction. The schematic diagram of the follower is given in Fig. 4.

In the frequency range 20 Hz–100 kHz, the transfer coefficient of the follower \(K_{\text{f}} \geq 0.99995\), and its instability is less than 0.005%. The frequency error at 200 kHz is 0.02%. The output resistance \(R_{\text{out}} \leq 0.05\ \Omega\). Owing to the high accuracy of the transfer coefficient and its stability, as well as the very low output resistance, the follower described can find application in many precision devices. Further extension of the frequency range encounters greater difficulties.

Fig. 4

The comparator shown in Fig. 3 also has a fundamentally important shortcoming—the impossibility of direct “self-checking.” All this makes it necessary to seek other approaches to the solution. In [4] a commutator was proposed in which parallel-connected magnetically controlled contacts and transistors connected inversely are used as switches (see Fig. 5).

Fig. 5

A special transistor-control circuit makes it possible to sharply reduce switching errors due to the inertia of the magnetically controlled...

...of the relay contacts and their chatter. But even with an ideal commutator, at low frequencies of \(U_x\) surges arise at the output of the MC because of the arbitrary nature of the switching instant \(t_{n}\).

Figure 6 shows plots of the power of the alternating voltage (1) and the direct voltage (2). It is clear that even with instantaneous switching of the MC, surges will occur, depending on the switching instant and the value of \(\tau_\ell\). In order to reduce these surges, it is necessary to synchronize the operation of the commutator with the alternating voltage \(U_x\) applied to the comparator.

Fig. 6

Taking into account the complexity of the technical implementation of a precision modulation-type comparator operating at low frequencies, and its inertia, we propose using comparators with automatic adjustment (see Fig. 7). The voltage \(U_x\), through contact \(a\) of relay \(p\), is applied to the input of an MC consisting of a low-power incandescent lamp and a photoresistor. From the direct-current source IO, the reference voltage \(U_0\) is applied to the input of the second measuring converter.

Fig. 7

Both MCs, together with resistors \(R_1, R_2, R_3\), form a differential comparator of the voltages \(U_x\) and \(U_0\). The unbalance amplifier \(K\) is connected to points \(c, d\). When the relay contacts are in positions \(a\) and \(a'\), the voltages \(U_x\) and \(U_0\) are compared. If the parameters of the comparator elements are identical and \(U_x = U_0\), then there will be no signal between points \(cd\). If now \(U_x\) changes, a mismatch signal will appear and it will be amplified by amplifier \(K\). But because of the instability of the measuring converters, even when \(U_x = U_0\) a signal \(U_{cd}\) will appear. In order to eliminate the time instability of the differential comparator, in the position of relay contacts \(b\) and \(g\) the direct voltage \(U_0\) is applied to both inputs of the comparator, and if the converters are identical, then \(U_{cd}=0\). If the converters are not identical, then \(U_{cd}\ne 0\), and this unbalance voltage, after amplification by amplifier \(K\), is applied through contact \(g\) to the storage capacitor \(C\) and the gate of the field-effect transistor \(T\). Depending on the unbalance signal, the current of transistor \(T\) changes and, consequently, the degree of shunting of resistor \(R_2\) changes. In this way the circuit is automatically balanced to the value

the static error in the system, which is equal to \(\dfrac{\Delta H}{H}\cdot\dfrac{1}{K_{\mathrm{st}}}\), where \(\dfrac{\Delta H}{H}\) is the relative instability of the converter, and \(K_{\mathrm{st}}\) is the relative stabilization factor in the loop. This process of balancing the differential comparator lasts two to three seconds. Then relay contact \(P\) is automatically switched to position \(a\), and the differential comparator measures \(U_x\). This stage may last 2–5 minutes. During this time the comparator parameters change only insignificantly. Then the comparator is again corrected for 2–3 seconds.

In the implemented prototype, the 1UT401B microcircuit was used (as amplifier \(K\)), along with a K77-1 storage capacitor and a TN-12 insulated-gate field-effect transistor. If the comparator operates in a self-monitoring system, the correction intervals may be programmed, or correction may be combined with other switchings in the system. Such a construction of the comparator does not lead to an increase in its inertia; it is simple and makes it possible, to the fullest extent, to put into practice the metrological properties of the measuring converter, including its frequency range.

REFERENCES

1. Yu. A. Skripnik. Methods of converting and extracting measurement information from harmonic signals. Kiev, “Naukova dumka,” 1971.

2. M. S. Roitman. Multivalued measures of alternating voltages. Izvestiya TPI, vol. 270, Tomsk, 1972.

3. “Calibrator 745A.” Hewlett—Packard company prospectus, 1968.

4. M. S. Roitman, Yu. A. Bulatov, Yu. G. Svinolupov. Generator-calibrator with high long-term stability. Second Republican Scientific and Technical Conference on Electronic Measuring Instruments and Systems with Commutation-Modulation Converters. Abstracts of reports. Lviv, 1971.