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tdnoise overestimation ?

So I have a basic inverter circuit and the input signal is 1 GHz clock.

I ran PSS + PNoise analysis (shooting engine) for time domain noise.Being a clock signal I should have used full spectrum but we use a rather old version of Spectre (7.0.1.179.isr16).

So I plot my integrated noise in frequency range from 50 MHz to 7GHz.But I noticed that integrating over this bandwidth the noise voltage was in order of Volts !

And so I plotted noise spectrum (Pnoise>tdnoise) and found that I have a huge spike (spur) at 5 GHz.For reference :

Integrating noise from 50MHz to 1050MHz yielded noise of ~5.1 mV

Integrating noise from 50MHz to 7GHz yielded noise of ~170 V

What might be the problem ?

Andrew Beckett over 9 years ago

With tdnoise (and PMjitter which uses tdnoise underneath) there is an ideal sampler at the output of the circuit which takes care of strobing the noise at the PSSfund frequency - this is strobing the noise at the particular instant during the period. Because of this ideal sampler, all noise gets aliased into the bandwidth from DC up to PSSfund/2 - so you should never sweep the frequency beyond half the PSS fundamental (so not beyond 500MHz in your case). Otherwise you end up double counting the noise. Also, if you happen to go very close to an exact multiple of the fundamental, you'll end up with an "infinite" (it's not really infinite, but very large) noise contribution because of the up conversion of the 1/f (flicker) noise.

So change your sweep to be 50MHz->500MHz and you will capture all the noise - the sampler is doing that.

Regards,

Andrew.
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analogy over 9 years ago

Okay. This sounds difficult to grasp - if signal through system is square wave (hence odd harmonics), as a quick notion , integrated noise bandwidth would be frequency of a particular harmonic slightly above noise floor. But if I integrate noise until PSS/2 , how does that correspind to "right" bandwidth for total integrated noise ?
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Andrew Beckett over 9 years ago

I don't understand your question, sorry. The primary purpose of tdnoise is for when the subsequent stage will be sampling the output of your block - and essentially you're then looking at the effect of that sampling (which will alias the noise). Not sure my sentence here helps - but then again, I don't really understand what you're asking me...
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analogy over 9 years ago

Oh. So isn't tdnoise suitable for time varying systems like inverters to determine integrated noise during transition period ?
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Andrew Beckett over 9 years ago

Yes, it is suitable for that. That's exactly what you might use it for!
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Frank Wiedmann over 9 years ago

You might want to take a look at http://www.designers-guide.org/Analysis/sc-filters.pdf, figure 3 shows the aliasing.
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analogy over 9 years ago

The classic Kundert paper !
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