pll frequency response

hi,
I am working on PLL frequency synthesizer.
Now I need to get the frequensy response of the whole system to find the phase margin and stabiltiy..
I can perform ac or stb analysis.

But I am not getting the correct output.

My concern is that is this due to the digital blocks (PFD and divider circuit),
or something else.
Can somebody suggest a better method.

My block is consisting of PFD, Charge Pump, 2nd order passive Loop Filter, VCOand a divide by 192 circuit.

Regards
Shaikh Sarfraz

hi sarfaraz
for phase noise PM and stabiltiy
why dont u directly use the easy PLL option on www.national.com
go to their wirelss section enter details of the PLL u are using and the VCO and specify the PM, PN etc
and it automaticaly gives you lopp filter components

HTH
-ashutosh

hi,
I am working on PLL frequency synthesizer.
Now I need to get the frequensy response of the whole system to find the phase margin and stabiltiy..
I can perform ac or stb analysis.

But I am not getting the correct output.

My concern is that is this due to the digital blocks (PFD and divider circuit),
or something else.
Can somebody suggest a better method.

My block is consisting of PFD, Charge Pump, 2nd order passive Loop Filter, VCOand a divide by 192 circuit.

Regards
Shaikh Sarfraz

Hi Aashutosh,
Thanks for the reply.
Actually my concern is something else. I have already designed PLL frequency synthesizer.
Now I am simulating it for all the process corners. With each process corner the phase margin will change, for that I need to perform ac analysis for the whole block.
But the ac analysis is not producing the desired output.

What I need to know is that do we perform ac analysis with all the blocks(analog + digital ) or only the analog blocks.

Thanks & Regards
Shaikh Sarfraz

better utilise MATLAB for the simulation
you can write a small program for PLL as a feedback system.

and can manupulate the parameter changes.