Automatic strobes and recurrence-frequency selectors

Abstract

Automatic strobes and recurrence-frequency selectors are both systems devised for radar purposes in which a locally-generated repetitive pulse, called a “strobe,” is held in coincidence with an incoming repetitive pulse. In the automatic strobe, the incoming pulse is an echo of a pulse transmitted by the radar set. The function of the apparatus is that of holding the time interval elapsing between the occurrence of the transmitted pulse and the strobe pulse equal to that elapsing between the transmitted pulse and the echo pulse. This result is achieved by controlling the delay time between the transmitted pulse and the strobe pulse. The system may be used to indicate the range of a given echo continuously, since this delay time is a measure of the range of the object producing the echo. The circuit arrangements used allow the wanted echo to be selected from among others present at different ranges. Any modulation present in the selected echo may, therefore, be detected without interference from the others, which are rejected.In the recurrence-frequency selector, incoming pulses are not, in general, echoes, but are pulses received from a remote transmitter with a basically fixed repetition frequency which may, however, have small amounts of frequency- or phase-modulation superposed. In these circuits there is no local transmitter pulse, and the strobe pulses are derived from a local self-oscillator. The requirement is still to hold strobe pulses and incoming pulses in coincidence, but now the result is achieved by controlling the frequency of the local oscillator.In both cases a device is needed to detect the error in the timing of local pulses relative to the incoming pulses; this device is called a time discriminator, and practical examples are given. The resulting error signal is used to control either the delay time between transmitter and strobe pulses, or the frequency of the local oscillator.Both systems are analogous to automatic control systems met with in other branches of engineering, and after a brief introduction the paper goes on to summarize that part of the mathematics of control systems that is relevant to these particular applications. It is shown that some “processing” of the error signal is necessary before it can be used to give stable control with desirable characteristics. This processing is performed in a “function unit” and the various mathematical forms it can take are outlined. The realization of these forms is then discussed; feedback amplifiers are used for the purpose and suitable types are described in some detail.In both parts of the paper the closing Sections contain descriptions of practical circuits with some design detail. The predicted performance is verified experimentally.