I have two evaluation boards of HMC764 that one is derived using its 50MHz internal clock and the other PLL is using this 50MHz clock as an external clock. When I set PLLs for the frequency around 7.5GHz with 1MHz separation in frequency and just looking at one of the PLL's output using spectrum analyzer, I can see some spurious signals and its harmonics very close to fundamental frequency, 45KHz away from it and 35 dB below of it. Also, by changing the 1MHz separation between two PLLs the position of the spurious changes as well. it seems that some kind of modulation is happening between them.
What do you think is the reason of the phenomenon? are there any way to suppress it?