MEASUREMENT OF PHASE DIFFERENCE IN PULSED MODE BY THE RING SWEEP METHOD

I. D. Zolotarev, V. P. Denisov

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

In a number of cases it becomes necessary to measure the phase shift between the high-frequency fillings of short radar pulses received by spaced antennas [3].

Questions of such measurements—especially in the case of a signal comparable with noise—have been little developed and are of considerable interest [1].

The method considered here for measuring phase shift was developed for radio observations of meteor reflections. The short lifetime of these reflections and their low power impose a number of specific requirements on the apparatus [3].

The stated problem was solved by using a special circuit of a phasemeter with ring sweep. A simplified block diagram of the phasemeter is shown in Fig. 1.

Fig. 1.

The voltages whose phase shift is to be measured are fed to the input of the two-channel receiver 1.

From the output of the amplifier of the first channel, the reference voltage is fed to the phase-shifting circuit 2. From the phase shifter, two sinusoidal voltages shifted by 90° are applied to the terminal amplifiers of the ring sweep 3, 4, from which they are fed to mutually perpendicular deflection plates of the tube.

As a result of the action of both voltages on the electron beam, a circular sweep is obtained on the tube screen, the period of which is equal to the period of the high-frequency filling.

Fig. 2.

A distinctive feature of the developed pulsed ring-sweep circuit is:

1) the use in the ring-sweep circuit of resonant systems with a wide passband ($\Delta F$ comparable with $f_0$, where the passband is $\Delta F = 500$ kc, the signal frequency $f_0 = 2$ Mc);

2) tuning to resonance of the circuits included in the phasemeter circuit¹.

The voltage from the second channel of the receiver is fed to the circuit for forming marker pulses 5. The shaped peaked pulses are applied to the modulating electrode of the cathode-ray tube to produce a phase marker.

The shaping circuit developed as a result of experimental investigations generates pulses of duration on the order of $10^{-8}$ sec with a repetition frequency of $2 \cdot 10^6$ pulses/sec. With such a duration of the marker pulses, the probable measurement error (in the absence of noise) does not exceed $2^\circ$.

The schematic diagram of the indicator of the phasemeter with ring sweep is shown in Fig. 2.

At the top is shown the ring-sweep circuit; below, the circuit for forming marker pulses.

Fig. 3.

From the output of the reference channel of the receiver, the signal voltage is applied to a preliminary amplifier assembled on tube $L_1$ (6T9), whence, through a push-pull amplifier $L_2$ (GU-29), it is fed to a phase rotator assembled according to a bridge circuit. Phase trimming is carried out by capacitors $C_{20}$ and $C_{21}$. Overall adjustment of the amplitude of the ring sweep is performed by potentiometer $R_{11}$. Adjustment of the deflection amplitude along the axes is carried out by potentiometers $R_{19}$ and $R_{24}$. From the output of the second channel of the receiving device, the voltage is applied to the input of the shaping circuit. The first tube of the shaping circuit, $L_6$ (6T9), operates in limiting mode. The shaping stage is assembled on tube $L_7$ (G-807). In this stage a shock-excitation circuit of low quality factor is used, connected in the anode circuit of the tube. The circuit parameters are chosen so that, over a time equal to half the period of the signal, the voltage on it rises to a value comparable with the voltage of the source

supply. When the circuit is excited by rectangular current pulses of signal frequency, a voltage of triangular form will arise on it (Fig. 3). From these voltage pulses, as a result of their subsequent “sharpening” in stages assembled with tubes \(Л_8\), \(Л_9\), and \(Л_{10}\), we obtain sharp-pointed marker pulses (Fig. 4).

Fig. 4.

Fig. 5.

From the output of tube \(Л_{10}\), the negative marker pulses are applied to the cathode of the cathode-ray tube.

Comparison shows that, under the stated conditions, this shaping circuit gives a gain in pulse amplitude, in comparison with circuits using differentiating networks, by a factor of 3–4 [2].

Adjustment of the marker amplitude is performed by potentiometer \(R_{60}\). The remaining elements of the phasemeter circuit require no special explanation.

The purpose of the experiment was to study the operation of the indicator part of the phasemeter circuit in pulsed mode in the presence of noise, and also to estimate the accuracy of the phasemeter.

Fig. 6.

To check the linearity of the pulsed ring sweep, electrical calibration was used. For this purpose, a calibrating sinusoidal voltage of frequency 48 megahertz was applied to the modulating electrode of the tube. The appearance of the phasemeter indicator screen in the calibration mode is shown in Fig. 5.

It was shown experimentally that the indicator part of the phasemeter with ring sweep makes it possible to measure phase shift in pulsed mode in the presence of noise. The phasemeter with ring sweep makes it possible to determine the position of the phase marker with a probable error of:

a) in pulsed mode without noise, with pulse duration
\(\tau_i = 5\ \mu\text{sec}\) — \(\Delta\varphi_{\text{ver}} \simeq 2^\circ\) (Fig. 6);

b) in pulsed mode with noise, with pulse duration
\(\tau_i = 5\ \mu\text{sec}\)

and

\[ \frac{U_c}{U_{\text{sh}}} \simeq 5 \quad — \quad \Delta\varphi_{\text{ver}} \simeq 3^\circ \]

(Fig. 7)

Fig. 7.

LITERATURE

1. Bonch-Bruevich A. M., Shirokov V. I., “Some problems of phase measurements,” Journal of Technical Physics, vol. XXV, issue 10, 1955.

2. Meerovich L. A., Zelichenko L. G., Pulse Technique, Sovetskoe Radio Publishing House, 1954.

3. Zolotarev I. D., “Phasometric apparatus for meteor investigations” (published in the present collection).

4. Essman D., Zeitschrift für angewandte Mathematik und Physik, Bd. 6, H. 3, S. 115–120, 1954.

1. In pulsed phasemeters the circuits must be tuned to the filling frequency of the signal radio pulses. Otherwise, the voltage phase shift of the signal arising during transients will lead to distortion of the phase marker on the screen of the phasemeter indicator and, consequently, the accuracy of measurement of the phase difference will be impaired [4]. ↩