When I use the HMC984, I input a 100MHz sine wave from OCXO into it’s VCOs pins. And the output PLL+VCXO input into it’s REFs pins. Just like below.
The circuits of the input of VCOs pins just like below. The amplitude of the VCOs input of HMC984 is between 625mVpp to 825mVpp(Single-Ended).
Q: I find that the input amplitude of VCOs affects the phase noise of the PLL out. When the input amplitude reach the 1.2Vpp(Single-Ended). The output of PLL will reach the best phase noise . But it is exceed the requirement of datasheet.
So I guess that may be the skew rate of the input VCOs is not enough(sine wave). So if I want to get the best performance of the HMC984,I should increase the input amplitude, and the EDS diodes in VCOs pins of HMC984 will cut the peak of the input signal ,so the skew rate will increase.
Do I correct? And how can I design the circuit better?
look forward for your reply.