TH-3 Microwave Radio System: Modulators

The T H - 3 radio repeater makes use of three modulators: a receiver modulator for frequency downconversion, a shift modulator for frequency translation, and a transmitter modulator for frequency upconversion. These are unbalanced solid state modulators having different IF circuits and electrical requirements but similar microwave structures. Each of these modulators consists of two die-cast aluminum parts, one for the microwave structure and the other for the IF circuits. The receiver modulator is used to downconvert the received microwave signal and to preamplify the 70-MHz IF signal before it is fed to the IF main amplifier for further amplification. Since the receiver modulator is the first active device to act upon the received microwave signal, it was designed primarily to obtain the highest possible carrierto-noise ratio consistent with good transmission performance and reliability. T h e transmitter modulator converts the 70-MHz frequency-modulated IF signal to a microwave signal having an upper and a lower sideband. The upper sideband is selected by an external microwave filter and fed to the T W T amplifier for further amplification prior to 2155