Performance of a Receiver during the Recovery Period from Strong Bursts of Broadband Interference
P. de Bruyne, Wolfgang Bachmann · 1977
The design of automatic gain control (AGO) and noise limiter circuits of radio receivers is concerned with the conflicting requirements of rapid recovery, low distortion and low intermodulation within the range of modulation frequencies. The design is often such that the performance characteristics during and after short periods of strong broadband quasi-impulsive interference are affected and the result of such interference requires special methods of measurement [l]. A method depending on intermodulation measurements of modulated noise and an IP signal is suggested which permits the evaluation of the receiver output under a variety of input conditions. Intermodulation between a demodulated signal and noise has, as far as we know, not been measured in the past because the amplitudes of the sum and difference frequencies can not be determined. We are developing a method using noise, which permits evaluation of the performance during the recovery period of the AGO and noise limiter circuits. It may also be used to record the effects induced by simulated rapid fading or flutter. The instrument which has been developed uses a microprocessor to average repeated cycles of coherent signals. It displays the average waveform of such cycles and thus records the average changes which take place during the recovery period from short bursts of interference. The signal variance or noise power curve is displayed together with the signal curve so that a possible correlation between changes in the noise level and the momentary value of the signal amplitude may be observed. The cycle length may be chosen in the range of 100 ms to 10 s to investigate the recovery effects associated with most AGC and limiter circuits. It is thus possible to evaluate the effect of time constants and levels of attack for different circuit configurations. The simulated interference may be changed in type, duration and amplitude to fully investigate the performance under different conditions. With the averaging technique noisy low level output signals may be displayed free of noise whilst this noise is separately displayed for evaluation of the dynamic characteristics of the circuitry. The effects from flutter or rapid fading may be investigated by modulating the carrier with a superposition of an LP flutter waveform and a small pilot signal consisting of LF white noise. Intermodulation between these two components results in a modulation of the variance curve. This may for instance occur when the AGC lags behind the fluctuations in carrier strength which were induced by the fading.