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Simulators >> RF Simulators >> TRAN+STB or PSS+PSTB ?
https://designers-guide.org/forum/YaBB.pl?num=1289836565

Message started by the_wavefunction on Nov 15^th, 2010, 7:56am

Title: TRAN+STB or PSS+PSTB ?
Post by the_wavefunction on Nov 15^th, 2010, 7:56am

Circuit:
A VCO + simple 2-stage opamp that feeds the VCO comprise my entire circuit. Also, a feedback path is taken from the VCO back to the opamp.
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Question:
Now, which method is the appropriate one to find out the stability of the opamp loop using iprobe component - tran+stb or pss+pstb ?

Is it tran+stb because its an opamp loop? or is it pss+pstb because there is an oscillator?
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Since I'm unsure, I curiously tried simulating with both methods.
The loop gain plots seem totally different and so do the corresponding gain and phase margins! For e.g: the loop gain magnitude for pss+pstb almost always starts from close to 0 dB and rises midway around, irrespective of the opamp open loop gain.

Could anyone kindly suggest the right analysis that suits this task?

Title: Re: TRAN+STB or PSS+PSTB ?
Post by the_wavefunction on Nov 15^th, 2010, 9:27am

Thank you pancho_hideboo.

Let me take a humble bow for your enlightening and thought-provoking suggestions!

The circuit simulator is Spectre 7.1.1 - 32 bit, and the tech lib is 0.18um TSMC.

And I sincerely hope, the names 'VCO' and 'opamp' stay the same immaterial of Cadence or Synopsys or ADS! Kindly correct me if I'm wrong.

Thanks once again :)

Title: Re: TRAN+STB or PSS+PSTB ?
Post by Ken Kundert on Nov 15^th, 2010, 9:29am

You would use pss+pstb because the effective operating point is periodically varying. With tran+stb you effectively freeze the oscillator at a certain point in its limit cycle, which would give the wrong result.

-Ken

Title: Re: TRAN+STB or PSS+PSTB ?
Post by the_wavefunction on Nov 15^th, 2010, 10:31am

Thanks a lot for your inputs, Ken!

I'll proceed that way.