I don't know - taking a stab in the dark
are your 2 frequencies low level signals presented to the transmitter,
or are they a transmitted frequency and a reflected frequency, as betwixt describes, which look like a Doppler radar
the return frequency goes up if the object is approaching and goes down if the object is receding
if they are low level signals presented to the transmitter, it may be a minimum shift keying modulations scheme
i worked at a place that used MSK to transmit binary (not a radar system)
MSK is like a frequency shift system, but the phase is maintained when you change frequency
it may help if you provide the circuit diagram of the fets and diode, the duplexer and the mixer