While not limited thereto, the present invention is particularly adapted for use as a dual mode transponder for a position location radar such as that used to guide an aircraft to an airborne tanker. In such a radar system, a transponder is provided on one of the aircraft which receives a signal modulated with range tones at the radar frequency from the other aircraft and transmits back a signal that is similarly modulated, but displaced in frequency with respect to the radar frequency. In the passive mode, the transponder behaves as a lossy frequency translator wherein a microwave mixer pumped by an oscillator at frequency f.sub.o generates two new signals on each side of that which is received by the frequency f.sub.o. One of these is transmitted back to the other aircraft and becomes the radar received signal. In the active mode, the transponder mixer is used to convert the received radar signal to an intermediate frequency where its modulation is detected, and processed for retransmission back to the radar with a net gain through the transponder. The dual mode apparatus described requires the radar range tone modulation to be either FM or phase modulation. Hereinafter, if neither is specifically mentioned it will be assumed that the modulation is phase modulation.
A transponder of the type described above preferably utilizes a four-port dual mode mixer which will perform the passive as well as the active mixer function. In the past, four-port dual mode mixers have been provided which will perform the passive as well as the active mixer function; however they require the use of RF and IF switches which increases size and costs and degrade the mixer-IF amplifier integration for noise performance, bandwidth and conversion loss.