TD-3 system: Transmitter modulator and receiver shift modulator

The frequency upconverter provides one of the basic building blocks of heterodyne radio systems. In the TD-3 repeater, two waveguide modulators based on a balanced tee hybrid structure provide the required upconverter functions. The transmitter modulator uses high efficiency varactor diodes to convert the IF signal into an RF output. The second modulator, called the shift modulator, provides a 40 MHz shift in frequency between the transmitter local oscillator signal and the receiver local oscillator signal. This circuit uses varistor diodes to obtain simple broadband performance.