A SIMPLE SYNCHRONOUS DETECTOR WITH A STABLE ZERO

M. S. ROITMAN

(Presented by the scientific seminar of the Faculty of Automation and Computer Engineering)

A synchronous detector (SD) using four semiconductor triodes [1], owing to its high metrological properties [2], is becoming increasingly widespread. However, the need to use four triodes and, most importantly, two galvanically isolated windings in the commutation circuit is a substantial drawback of the circuit. This drawback is felt especially acutely in multichannel systems, for example in multichannel strain-gauge equipment. In cases where the SD must operate not in the vector mode, but in the amplitude-demodulator mode (i.e., the angle \(\varphi\) between the measured \(U_c\) and the switching \(U_k\) voltages is close to \(n\pi\) [3]), the SD circuit using two triodes is often employed (Fig. 1). For \(\varphi = n\pi\) and \(U_k > U_{c\max}\), the zero drift of this circuit is determined to a significant extent by the residual voltage \(U_p\) and the currents \(I_p\) of the triodes.

From consideration of the simplified equivalent circuit of the SD (Fig. 2, a, b), for both half-periods of the switching voltage it is seen that the zero drift is equal to

\[ U_0 \simeq \frac{U_{p1}+U_{p2}}{2}\cdot \frac{R_{\mathrm{H}}}{R_{\mathrm{H}}+r_i} +\frac{I_{p1}+I_{p2}}{2}\cdot \frac{R_{\mathrm{H}}\cdot r_i}{R_{\mathrm{H}}+r_i}, \]

The SD circuit can be simplified still further, and its metrological characteristics improved, if different-type triodes are used (Fig. 3) [4].

A voltage of positive polarity applied to the bases opens the \(p\text{-}p\text{-}p\) triode \((T_2)\), while a negative polarity opens the \(p\text{-}n\text{-}p\) triode \((T_1)\). The presence of resistances \(R\) in the circuit is absolutely necessary. The SD can function normally-

...operate only under the condition that the blocking voltage at the base of the closed switch is greater than \(U_{c.\max}\). If the resistances are absent, then the closed junction is shunted by the open one and \(U'_{k.\max}=U_{\text{base--collector}}\leqslant 0.15\text{ V}\). In the presence of resistances, \(U_{k.\max}=I_b\cdot R+U_{\text{base--collector}}\). The zero drift of the SD, as is not difficult to see from the equivalent circuits of Fig. 4, is equal to

\[ U_0 \approx \frac{U_{p1}-U_{p2}}{2}\cdot\frac{R_n}{R_n+r_i} +\frac{I_{p1}-I_{p2}}{2}\cdot\frac{R_n\cdot r_i}{R_n+r_i}. \]

\(U_0\), when alloy germanium triodes are used in inverse connection and \(R_n \leqslant 500\,\Omega\), practically does not exceed \(0.5\text{ mV}\), even without selection of triodes. It should be noted that the described SD can also operate in a vector mode, but with switching by a rectangular voltage or a sinusoidal one and \(U_k \gg U_{c.\max}\).

Fig. 3.

Fig. 4.

A particularly great advantage of the circuit is that in an \(n\)-channel system only \(n+1\) wires are required for switching the triodes, whereas for an SD using 4 triodes a minimum of \(3n+1\) wires would be required.

On the basis of the circuit of Fig. 3 we developed and experimentally investigated two modules (Figs. 5, 6). Both modules are a combination of an SD with an inverter amplifier. The first module is intended for operation with electric measuring instruments or other devices with input resistance \(R \geqslant 500\,\Omega\). The second is for operation with loads below \(500\,\Omega\), and also in devices requiring galvanic isolation of circuits.

Fig. 5.

Fig. 6.

The output voltage of the SD depends linearly on the input voltage up to values \(U_{c.\max}\leqslant 0.7\,U_k\) for a sinusoidal switching voltage and up to \(U_{c.\max}\leqslant U_k\) for a rectangular one. The voltage \(U_{ac}\), determined by the asymmetry of the controlled elements in the circuit of the measured

voltage [5], is a function of both the magnitude \(U_k\) and \(U_{\text{noise}}\). The maximum value of \(U_{ac}\) at \(U_k = 3\ \text{V}\) and \(U_{\text{noise}} = 2\ \text{V}\) does not exceed 10 mV. The voltage transfer coefficient of the synchronous detector is 0.8. Consequently, the noise immunity [4] at \(U_k = 3\ \text{V}\) and \(U_{\text{noise}} \le 2\) is greater than 100. In the second module, the resistors \(R\) are soldered in only after the load values and the permissible switching power are known. The required value of \(R\) is calculated by the method set forth in [3]. The secondary winding of the transformer has a number of taps, which makes it possible to match the synchronous detector to a load whose reduced value should be about 2 kilohms. The transformation ratio for the sections is respectively equal to 0.5; 0.25; 0.125; 0.07.

The author is very grateful to student Petrov for assistance in carrying out the experimental work.

REFERENCES

1. R. L. Bright. AIEE Transactions, 74 pt. 1, p. 111—121, 1955.

2. M. S. Roitman, S. A. Gofman, A. P. Olomutskii, A. N. Karmadonov. On the zero stability of synchronous detectors using semiconductor diodes and triodes. Izmeritel’naya tekhnika, No. 2, 1961.

3. M. S. Roitman. On the calculation of a synchronous detector using semiconductor triodes. Proceedings of the Tomsk Institute of Radio Electronics, Vol. 1, 1963.

4. M. J. Weight. A transistor phase sensitive demodulator of high performance. Electronic Eng., No. 10, 1962.

5. M. S. Roitman. On the theory of the synchronous detector. Proceedings of the Tomsk Institute of Radio Electronics, Vol. 1, 1963.