TD-3 System: 3A FM Terminal Transmitter and Receiver

FM terminals form an important subsystem of long-haul microwave radio systems as the link between the baseband signal and the 70 MHz FM signal. Designed primarily for use on the TD-3 system, 3A FM terminals are also compatible with the TD-2 system on which they are finding wide application. Development objectives included the use of solid-state circuitry throughout, emphasis on reliability, and performance consistent with TD-3 objectives with up to 16 terminal pairs in tandem in 4000 miles. The FM transmitter uses two voltage controlled oscillators at 186 and 256 MHz which are frequency modulated by the baseband signal and heterodyned down to the 70 MHz IF. Transmitter frequency stability is ensured by controlling the oscillator's environmental temperature and stringent pre-aging of critical circuit elements. The FM receiver uses a balanced, parallel resonant type discriminator preceded by two limiters which ensure good AM suppression and wide dynamic range. The gain of a terminal pair is 16 dB with 124 ohm balanced baseband input and output impedances.