PSS analysis: using variable for beat frequency

When simulating an oscillator in PSS, you need to check the box labelled "oscillator" and then tell it where the output of the oscillator is (this should be the nodes where the fundamental can be found - so if you have an oscillator followed by a divider, it should be the output of the divider).

You have to give an estimate of the oscillation frequency to give the simulator a clue, but it doesn't have to be very accurate. PSS will solve for the actual oscillation frequency and so there is no need to be terribly accurate here - the chances are that you wouldn't have been able to enter it accurately enough anyway.

So there is no need to try to calculate it yourself, unless you're trying to come up with a better estimate - but calculating it from a transient simulation is pretty pointless. It would only be if you have a very wide range of frequencies and can estimate it from one of the variables (e.g. a control word) for the design that it would be worthwhile.

Note: Do not use the "oscillator" mode unless it is a free running oscillator. I've seen people use this when they have a driven circuit and they can't be bothered to work out the beat frequency. Well, this will mean that the simulator is trying to solve an unknown which is not unknown, and will lead to an ill-conditioned matrix and probably convergence difficulties...

Regards,

Andrew.

Hey Andrew, Thanks for the detailed answer :). Yes, the oscillator is free running but I do have an extra circuitry through which I can input different combinations to trim my oscillator output and that is why I want to calculate the the oscillator frequency first and then I want to use it in pss to calculate the phase noise and jitter. There can be 8 different combination for the extra circuitry and then 8 different trimmed frequencies of the oscillator so I guess the I should use calculated frequency from the oscillator output rather the estimated value. What would be your suggestion for this scenario ?

EDIT: I also found a nice explanation regarding the beat frequency in another thread
http://community.cadence.com/cadence_technology_forums/f/38/t/2661

PSS should find the oscillator frequency anyway - it's only really to help the algorithm find the frequency more efficiently. So if you have control variables used to set which mode it's in, you can adjust your estimate. It shouldn't be absolutely required though.

The phase noise and jitter should then be measured relative to whatever frequency it finds.

So I'm not sure you have to do anything...

Regards,

Andrew.