I'm working with HMC703 to design an optical PLL. However I find that the CP only sources current out. I use an 100 MHz ref and RF in(frequency around 100 MHz) to test the CP output. When the RF frequency is lower (about 99.9 MHz), the phase is like the figure below which is below zero meanning the phase is 0-2pi. When increasing the RF frequency to 100.1 MHz, the phase is like figure 2 below. It seems like that some signal is cut for some reasons. And still no signal above zero is observed. I've tried setting the PFDUpEn, PFDDnEN still cannot solve this problem. Is there any possible settings should be taken?
The CP output is like this :