Active IF Units for the Tranmitter and Receiver

The TD-3 microwave radio system uses heterodyne repeaters in which the IF frequency band is centered at 70 MHz. System equalization, and part of the selectivity is provided at 70 MHz along with over half of the maximum available gain. The IF gain is controlled by an automatic gain control (AGC) circuit to compensate for variations in received signal power at the input of the microwave receiver. The repeater also includes an IF limiter and an IF carrier resupply unit. The limiter suppresses incidental amplitude modulation which may be added to the frequency modulation signal by transmission deviations. The resupply circuit automatically inserts a substitute carrier in the event that the transmitted carrier fails or suffers a very deep fade. Loss of the transmitted carrier would cause IF amplifiers in succeeding repeaters to go to maximum gain, causing a noise build-up in the failed channel that would spread into the adjacent radio channels. The design and performance characteristics of the IF preamplifier, the IF main amplifier and AGC circuit, the limiter, and the carrier resupply unit are described in this paper. The design of the IF driver amplifier for the transmitter modulator is covered in a companion paper. 1 Equipment described in this article is manufactured by the Western Electric Co. for Bell System use only. 1227