PULSE FREQUENCY DIVIDER WITH A VARIABLE DIVISION RATIO

A. P. Zaitsev, A. I. Zaitsev

The proposed new method of dividing pulse frequency makes it possible, in a comparatively simple way, to obtain voltage or current pulses with an adjustable repetition frequency from an alternating sinusoidal voltage of unregulated frequency. Most of the known circuits of pulse-frequency dividers are complex, do not make it possible to obtain a large output power in the pulse without the use of additional amplifying devices, and have a limited range of variation of the frequency-division ratio \(k\). The circuit described below is free of these shortcomings. It permits frequency division in the ratio \(\frac{f}{n}\), where \(f\) is the initial frequency of the supply voltage,

\(n\) is any integer.

Smooth regulation of the division ratio is accomplished by changing the constant setting voltage in the grid circuit of the thyratron.

The frequency divider is a combination of a storage capacitance with a thyratron switching circuit and is built on two thyratrons (Fig. 1). An alternating voltage of unregulated frequency is applied from the supply source to the anode of the thyratron \(L_2\), operating in switching mode, and voltage or current pulses with adjustable frequency are taken from the cathode. In the initial state the thyratron \(L_2\) is closed by the negative setting voltage \(U_z\), applied to the resistance \(R_2\) in the grid circuit of \(L_2\). The discharge thyratron \(L_1\), connected in parallel with the storage capacitor \(C_n\), is also closed by the negative voltage \(U_{\text{zap}}\), applied to the resistance \(R_1\) in the grid circuit of \(L_1\). From the moment the circuit is switched on (the sinusoidal voltage \(\sim U\) is applied), voltage pulses from the resistance \(R_3\) pass to the storage capacitor \(C_n\) through the rectifier \(B\) and the charging resistance \(R_{\text{zar}}\). The character of the charge and discharge of the capacitor \(C_n\) is determined by the charging time constant \(T_z = R_{\text{zar}} C_n\) and the discharge time constant \(T_r = R_r C_n\), and is shown in Fig. 2. The voltage \(U_C\) on the storage capacitor rises stepwise and is subtracted from the setting voltage \(U_z\). When the resultant grid voltage \(U_d = U_z - U_C\) becomes equal to the ignition voltage, the thyratron

\(L_1\) fires (Fig. 3). During one half-period, a current flows through the resistances \(R_{k1}\) and \(R_{k2}\), under the action of which an output voltage pulse \(U_{\text{out}}\) is produced at the cathode of \(L_1\). In Fig. 3, the half-periods during which the thyratron \(L_2\) conducts current are shaded. The load may be connected in place of the resistance \(R_{k1}\). The resistance \(R_{k2}\) is selected in such a way that the voltage drop

Fig. 1.

Fig. 2.

Fig. 3.

\(U_k\) across it is equal to, or somewhat greater than, the ignition voltage \(U_{\text{ign}}\). When the thyratron \(L_2\) conducts, the voltage difference \(U_{\text{ign}} - U_k \approx 0\) acts in the grid circuit of the discharge thyratron \(L_1\). The discharge thyratron opens, and the storage capacitor is rapidly discharged through \(L_1\) and the small resistance \(R_{k3}\), which limits the current through the thyratron.

within permissible limits. After the end of the conducting half-period, \(U_c \approx 0\), \(U_k = 0\), the thyratrons are locked. The storage capacitor again “counts off” the specified number of pulses, and the process described is repeated. By changing the magnitude of the setting voltage \(U_z\), it is possible to vary over wide limits the number of pulses being “counted off,” and consequently also their repetition frequency.

In order to ensure ignition of thyratron \(L_2\) at the beginning of the conducting half-period, the charging pulses applied to the storage capacitor must be shifted toward advance with respect to the conducting half-periods of the anode voltage of thyratron \(L_2\). This is achieved by forming the charging pulses by half-wave rectification of the alternating voltage \(U_{R_3}\), phase-shifted with respect to the supply voltage by means of the phase-shifting circuit \(C_2 R_3\).

The range of variation of the frequency division ratio depends substantially on the amplitude of the charging pulses and on the ratio of the time constants \(T_z\) and \(T_p\). The greater the amplitude of the charging pulses and the smaller the voltage increment on the storage capacitor during the time interval between two adjacent charging pulses, the larger the range of variation of the frequency division ratio.

To ensure a linear dependence of the frequency division ratio on the setting voltage, it is necessary that the time constants \(T_z\) and \(T_p\) be as large as possible, but differ only slightly from one another.

The advantages of the circuit described include:

1. Achieving a wide range of pulse-frequency regulation by simple means.

2. Obtaining pulses with an adjustable repetition frequency directly from an alternating-current source of unregulated frequency.

3. The possibility of obtaining electrical pulses of high power (on the order of tens of kilowatts and more) through the use of controlled valves.

The latter circumstance makes it possible to use this circuit in various tracking systems with stepper motors, in systems for pulse control of the speed of direct-current motors, and also in many other areas where regulation of the pulse repetition frequency is required.

The operation of the frequency divider was tested experimentally in the laboratory of the Department of Electric Drives and Industrial Installations of the Tomsk Polytechnic Institute. As the supply alternating voltage, the voltage of an industrial-frequency network was used. The pulse repetition frequency was smoothly regulated within the range \(0 \div 50\) cps.

Received by the editors
in May 1962.