We have implemented a FMCW radar, using ADF4159 and HMC951B and other ADI parts.
In our software setting, we tried both continue sawtooth and continue triangle ramping, frequency from 5.5 GHz to 7.5 GHz and time per ramp is 10 ms or 25 ms, respectively.
First, in order to simplify the test case, we used cable to create time delay from Tx to Rx(to simulate a reflection situation ), ramp mode is "continue sawtooth" and time per ramp is 25 ms. we have found harmonic in full spectrum:
Then We opened Tx and Rx (without transmit and receive) and measured IQ from mixer. A periodic harmonic was found which the period is same as time per ramp. Any idea what causes the harmonic ?
Time per ramp : 10 ms
Time per ramps : 25 ms
We also used spectrogram to measure Tx signal, there are some interference appeared instantaneously at different frequency when ramp frequency from 7.5 GHz back to 5.5 GHz.