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PSS not converging for a psuedo differential ring oscillator (Read 1742 times)
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PSS not converging for a psuedo differential ring oscillator
Dec 20th, 2016, 2:47pm
 
Hi

I am not sure if anyone of you has faced this issue. I have a three stage inverter based single ended ring oscillator(SRO) which converges if pss is done. Once I connect it to an exact replica to create a psuedo differential ring oscillator(PDRO) using cross coupled resistors, it fails to converge. I'm unsure why this is happening. I found the frequency of oscillation for the oscillator from transients and gave it in the pss. There is no issue with transients for the PDRO. Is there any strange attractor kind of behaviour expected in a PDRO which doesn't exist in SRO helping it achieve limit cycle? Please help. Thank you.
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Andrew Beckett
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Re: PSS not converging for a psuedo differential ring oscillator
Reply #1 - Dec 29th, 2016, 10:22am
 
Without seeing the circuit and exactly how it is failing to converge, it's hard to tell. I wonder if you've effectively ended up with two oscillators which are not as coupled in frequency as you think - and maybe they are then operating at (very) slightly different frequencies? If that was the case, PSS wouldn't converge.

That's a wild guess though in the absence of any more information...

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Re: PSS not converging for a psuedo differential ring oscillator
Reply #2 - Dec 29th, 2016, 10:45pm
 
Hi Andrew,

Your prediction was perfectly correct. I had been sweeping my inverter sizes and load cap in order to see the frequency variations. While I checked for the nominal case that the differential outputs were perfectly out of phase, on a closer inspection I found that at higher frequency the resistance was too large to create good coupling that the oscillators were slightly off. This resulted in lower frequencies to pass PSS while higher frequencies to fail. This was only evident after passing them through a set of buffers and looking at the output. Having reduced the resistance, I have been able to do the pss analysis. Thank you very much. Smiley

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