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Stability analysis of a fully differential ring oscillator

MoranFish

Hi, 

I'm trying to verify Barchausen criteria for a 4 stage differential ring oscillator and I'm trying to understand the different results I'm getting from stb to pss+pstb using shooting-newton/HB (With diffstbprobe from the analogLib). 

Running stb analysis:

As I saw in your workshop (/tools/spectre/examples/SpectreRF_workshop/PSTB.pdf) I should get gain>1 when phase=0.

Is it correct also for differential ring oscillator analysis?

In my analysis the phase doesn't get to 0 at the relevant frequency range.

Running pss+pstb using Shooting-Newton:

Barchausen criteria is met - gain=1 when phase=0.

DC phase is 180 as expected.

Running pss+pstb using HB I'm getting a totally different results:

DC phase is 0. Phase=~60deg when gain=1

 

What can be the reason for the different results?

Thanks,

Moran

  • To be honest, I have never really understood why pstb should be useful for examining a free-running oscillator and why Cadence chose this example. I came up with the idea for pstb in the context of a control loop for a DC/DC converter which is driven by a periodic source (see https://designers-guide.org/forum/YaBB.pl?num=1107716368). I am still waiting for an example from Cadence that shows this application.

  • in reply to Frank Wiedmann

    Frank - you mean like this: Analysis of Stability in DC to DC Converters? This is an updated version of a presentation I gave (with examples) at a National Microelectronics Institute (now called Techworks) event in the UK in 2008. In fact the request for a periodic stability analysis came a couple of years or so before you first requested it. I agree, an oscillator is a poor choice because the only way to find out whether it's stable is if it's actually oscillating and PSS/HB can find a stable oscillation, so it's a bit "chicken and egg".

    Moran In this specific case, stb is never going to work as there's no DC operating point where the signal will transfer around the loop. I have my doubts about its usefulness with a ring oscillator because the feedback loop is highly nonlinear. Most likely with HB you don't have enough harmonics, but I'd never simulate a ring oscillator with harmonic balance anyway...

    Regards,

    Andrew.

  • in reply to Andrew Beckett

    Yes, Andrew, I was thinking of something like your presentation, but in the form of a Rapid Adoption Kit or as an example in the Spectre installation with an attached database, so that one has a working setup and can get some experience with running this kind of simulation.

    Regarding the periodic stability analysis: I did not just log a request, but I proposed a solution that the questioner then successfully used to analyze his circuit.

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