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Simulators >> RF Simulators >> Phase Noise of Oscillator after capacitor
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Message started by martin88 on Dec 15th, 2016, 5:27am

Title: Phase Noise of Oscillator after capacitor
Post by martin88 on Dec 15th, 2016, 5:27am

Hi,
I build a oscillator with a buffer stage after it. To cancel out the common voltage I also added a capacitor before the load. My problem now is why the phase noise is almost constant over the offset voltage after this capacitor and doesn't show the decreasing behaviour. See the picture attached.
Hopefully someone knows why the phase noise is looking like that.
Thanks
Regards,
Martin

Title: Re: Phase Noise of Oscillator after capacitor
Post by vroy_92 on Dec 15th, 2016, 5:54am

Can you please share a snapshot of the circuit?

Title: Re: Phase Noise of Oscillator after capacitor
Post by martin88 on Dec 15th, 2016, 6:12am

The setup looks like this:

Title: Re: Phase Noise of Oscillator after capacitor
Post by vroy_92 on Dec 17th, 2016, 10:02pm

This capacitor should not change the phase noise significantly. The phase noise is mostly introduced by the device noise of the ring devices.

Title: Re: Phase Noise of Oscillator after capacitor
Post by martin88 on Dec 19th, 2016, 2:36am

But before the capacitor the phase noise looks like this:

Title: Re: Phase Noise of Oscillator after capacitor
Post by Ken Kundert on Dec 20th, 2016, 12:50pm

The first result you gave is clearly wrong. This appears to be a simulator issue. How big is the capacitor you are adding?

-Ken

Title: Re: Phase Noise of Oscillator after capacitor
Post by martin88 on Dec 22nd, 2016, 2:33am

The capacitor is 8pF.
Thanks for the answers, I will check the testbench again.

Title: Re: Phase Noise of Oscillator after capacitor
Post by Andrew Beckett on Dec 29th, 2016, 10:20am

Which MMSIM version are you using (the spectre version will be shown near the top of the spectre log file)? There were some bugs in shooting pnoise (in oscillator mode) prior to MMSIM15.1 which could lead to bizarre results in certain situations (the results were generally obviously wrong when it occurred - it didn't happen all the time), so I'd certainly recommend switching to something more recent.

Andrew.

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