PFD Phase Noise Question

[Engineer4ever]

Hi,

I ran a PNoise and Pss analysis on Cadence to get the phase noise for my PFD. There is a peak at the reference frequency. Could anyone explain to me the meaning of that peak?

Thanks in advance,

 

[kspalla]

Hi,
read below link.
https://www.analog.com/library/analogDialogue/archives/33-05/phase_locked/PLLs_2.pdf
After loop locking, PFD gives pulses of reference signals that must appear at the output.

Regards,

 

[Engineer4ever]

Thanks for the link, but I still have a question: you said that this spur takes place after phase locking and the same is written in the file, but this phase noise curve is for PFD only. The PFD wasn't in the loop. I put square waves as inputs that represent the reference and feedback frequencies.

 

[erikl]

... There is a peak at the reference frequency.

Parasitic coupling from the reference frequency input to the output (directly or via the power supply). You could verify this by analyzing the rejection ratio between this input and output.

And you should find further peaks at the odd harmonics.