FM terminal transmitter and receiver for the TH radio system

The FM terminals form an important subsystem of TH radio, as the link between the 0.10 mc baseband signal and the 74.1-MC FM signal. Severe requirements arise from the design objective of 16 terminal pairs in tandem in 4000 miles. The FM transmitter design uses a 6-kmc reflex klystron as a frequency modulator, the output of which is heterodyned down to 74 mc by another 6-kmc source. Automatic frequency control, with a 74.13-MC crystal oscillator as reference, provides the required frequency stability. In the FM receiver, an IF amplifier-limiter is followed by a balanced FM discriminator which uses parallel resonant discriminator networks. Improved linearity is obtained by special design of a common interstage network. The video amplifiers are balanced and use high-performance electron tubes. The over-all gain of a terminal pair is 8 db, between 124-ohm balanced video circuits.