Frequency control using binary rate multipliers for automatic testing on c.w. radar systems

Abstract

R.F. signals with certain defined characteristics have to be generated for ground testing on f.m. c.w. radar airborne-guidance systems. Normally in flight, two r.f. signals would be received; one direct from the ground transmitter and the other originating at the same transmitter, but reflected from a target. A lowfrequency wide-deviation frequency modulator is applied to the carrier at the ground transmitter. Owing to a difference in the path lengths of the two signals, a difference in phase angle of the modulating signal results, and, owing to the Doppler effect, the carrier frequencies are slightly separated. For testing therefore two r.f. signals with a precise and variable difference frequency have to be generated. Also both signals must be frequency modulated at the same frequency to the same deviation, but with a variable phase angle between the two modulating signals. For this purpose, a study has been made of the behaviour of binary rate multipliers used partly to shift the frequency, and partly to frequency modulate the r.f. carriers. A common crystal oscillator would feed into two b.r.m.s each shifting the carrier by a different amount, and each frequency modulating the shifted carriers with the same deviation. The b.r.m. operates by removing pulses from an uninterrupted pulse train according to the digital address. Thus the mean output frequency is lower than the input, but is contaminated with phase jitter. Jitter magnitude varies considerably with the address pattern, and it increases with the number of address lines. A graph of maximum jitter is given for increasing numbers of address lines up to 13. It isshown how phase jitter may be reduced to an acceptable limit, and how a system to meet the requirements may be designed.