I am presently testing with a PCB I designed on which the ADI ADF4159 PLL / ADI HMC738 are used to generate a chirped FMCW signal at 21.75-22.75 GHz. This is the 2nd version of this PCB and this section of the board was working ok on version 1.
I am initially testing with a fixed frequency signal however it is intermittently switching to a different frequency from that which it is programmed at, i.e. from 21.75 GHz to 22.35 GHz or from 22.75 GHz to 23.35 GHz. This happens every 20 to 100 seconds and lasts for approximately 1-2 seconds before switching back to the correct frequency. Why might this be happening?
There is some noise from a switch-mode voltage regulator leaking onto the voltage rail for the op-amp in the loop filter, which is causing harmonics to appear at multiples of the switching frequency away from the carrier (this wasn’t the case in V01 of the PCB), I’m not sure if this could be related to the frequency switching issue?