Transmitter Modulator and Receiver Shift Modulator

In the T D - 3 radio repeater, two upconverters are used: the transmitter modulator and the receiver shift modulator. 1 Although the electrical requirements for these modulators are quite different, the microwave circuit is similar for both. In a T D - 3 microwave transmitter, an FM modulated 70 MHz IF signal is converted by the transmitter modulator to a microwave signal before amplification and transmission. Because the transmitter modulator is a true upconverter, the output contains two sidebands either of which may be used depending on the channel frequency of the particular repeater. The T D - 3 frequency plan requires a 40 MHz difference between the received and transmitted signals in a repeater bay. 2 Consequently, separate local oscillator signals are required for the receiver and the transmitter. Because of system frequency stability and economy, however, it is very desirable to use the same microwave generator for both applications.* Therefore, the second local oscillator signal is provided by shifting the common generator frequency by 40 MHz * For reasons explained in Ref. 1, separate microwave generators are used for the receiver and the transmitter of a main station bay. 1289