The FM Demodulator with Negative Feedback

The FM demodulator with negative feedback (FMFB)* is an outgrowth of an FM receiver circuit first described in 1937.1 A demodulator of this type was used during the communication tests with the Echo I satellite, which started in August, I960.2 Compared with a conventional or standard FM demodulator, the principal advantage to be derived from this circuit is its ability to improve the threshold at which "breaking," resulting from excessive noise, will occur. The receivers mentioned above were capable of demodulating a single 4-kc voice channel. Since it was required that the Telstar system be capable of handling television and other wideband signals, the design of feedback receivers became a problem of much greater complexity as a consequence of the relatively wide baseband. The F M F B receiver to be described in this paper is used in the Andover ground station for the reception of wideband signals from the Telstar satellite. 3 A different F M F B receiver, described elsewhere,4 was used in the Holmdel station during Telstar experiments. The basic elements of the feedback receiver are shown in the block diagram of Fig. 1. The incoming RF signal is combined in a mixer with the output of a * Sometimes called " F M feedback r e c e i v e r " or " f r e q u e n c y compression demodulator." 1109