Viii. Communication Research A. Multipath Transmission

As mentioned in the October 1950 report, three-path reception is troubled by impulsive noise. Each noise impulse is accompanied by an ultrasonic burst. This suggests that the ultrasonic signal be rectified and used to operate gate circuits for repairing the audio output of the receiver. A block diagram of the receiving system emphasizing the repair circuits is shown in Fig. VIII-1. The receiver output is passed through a high-pass filter to separate the ultrasonic bursts. These are rectified and shaped into rectangular pulses which are used to operate a gate circuit in the "first-order repair circuit". The gate circuit follows work done by Robert Wilson in 1948 while an M. I. T. cooperative student at Philco Corporation. The general plan is indicated in Fig. VIII-2a. The input signal is connected to a condenser through two series tubes that act as rheostats. When the tubes are biased by the gating pulse the resistors become infinite and the condenser voltage is held constant for the duration of the trouble. The circuit is indicated in Fig. VIII-2b. Wilson experienced difficulty in isolating the gating pulse circuit from the series triodes. The trouble has been avoided by using the gating pulse to modulate a radio-frequency oscillator as shown in Fig. VIII-3. The modulated radio signals are rectified and fed to the grids to supply two thoroughly isolated gating voltages. In effect the oscillator-rectifier combination serves as a d-c transformer, and has many advantages over conventional pulse transformers. It operates on direct voltages and can be made to have a very short rise time. Figure VIII-4 shows some of the waveforms involved. It will be noticed that the first-order repair circuit just discussed in effect cuts off the noise impulses, avoiding the discontinuity in the signal. An attempt has been made to go further than this by removing the discontinuity in the derivation. The scheme is shown in Fig. VIII-5. The original wave (a) is differentiated (b) and then the derivative is repaired (c). At times of trouble the derivative is gated (d).