Dopplerizing a signal of varying frequency

A transponder for shifting all frequancy components of signal frequency, f.sub.i, by a factor K, to simulate a true doppler frequency shift in a complex signal, said transponder comprising;

• means for receiving said signal, f.sub.i,
• a closed loop comprising an amplifier with a feedback circuit, the gain of said amplifier being sufficient to generate self-sustained oscillations in the loop,
• said loop including a first balanced modulator, a second balanced modulator, a band pass filter, and two side band selector means connected, respectively, in the outputs of said modulators for selecting one side band at the output of each of said modulators,
• said band pass filter having a center resonant frequency, f.sub.f, and having an extended substantially linear phase-frequency characteristic extending above and below the zero phase at said resonant frequency,
• a linear phase shifter,
• means for applying said signal, respectively, to said first balanced modulator, and to said second balanced modulator, the signal circuit to said second modulator including said linear phase shifter so that the phase of the selected side band at the output of said second modulator shifts as the frequency f.sub.i shifts, so that the reactive network of said band pass filter will shift the frequency of oscillation, f.sub.f, of said loop as the frequency of said input signal f.sub.i is shifted to kf.sub.i, and
• means for extracting the frequency f.sub.i + kf.sub.i in said loop comprising a fixed frequency oscillator of frequency f.sub.o equal to said center frequency, f.sub.f, of the band pass filter and means for adding said f.sub.o to f.sub.f + f.sub.i in said loop to yield true doppler frequency shift, f.sub.i + kf.sub.i.