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Measurements >> Phase Noise and Jitter Measurements >> flicker noise folds in a strobed noise sim?
https://designers-guide.org/forum/YaBB.pl?num=1213355760

Message started by chenyan on Jun 13^th, 2008, 4:16am

Title: flicker noise folds in a strobed noise sim?
Post by chenyan on Jun 13^th, 2008, 4:16am

I did a simple test as follows:

(1) 4GHz squarewave drives a hard switching buffer.
(2) pnoise simulation with enough sidebands, compare simulation with (timedomain at threshold) and (source).

The flick noise contribution is increased by almost the same factor as white noise. Flicker noise corner is at 100K

What I don't understand is :

flicker noise corner is much lower than f0/2=2GHz, so theoritcally there would be no folding for flicker noise, then why would the flicker noise contribution also increases dramatically?

Any reply is highly appreciated.

Title: Re: flicker noise folds in a strobed noise sim?
Post by chenyan on Jun 13^th, 2008, 5:47am

Maybe the question I should ask first is:

what the 'timedomain' pnoise simulation result really means?

Is it just the average noise spectrum (simulated using 'source') sampled by f_0, which results aliasing and folding? I think it is more than that, since different time points give different result, but what really this strobed pnoise does? What is the relation between noise simulated using 'source' and 'timedomain'

Title: Re: flicker noise folds in a strobed noise sim?
Post by Ryan Cheung on Jun 13^th, 2008, 8:44pm

Hi,

The strobed noise and the time avged noise tell different stories. The former is used to describe discrete noise while the other is describe the continuous time noise. Strobe noise only gives the noise at the threshold voltage and ignores the noise at the other time of the period, but the time avged noise means the time avged noise of the total noise in the whole period. That's why you got a larger noise (both thermal & flicker) from the time avged pnoise simulation.

-Ryan

Title: Re: flicker noise folds in a strobed noise sim?
Post by pancho_hideboo on Jun 13^th, 2008, 9:52pm

rfmems wrote on Jun 13^th, 2008, 5:47am:

what the 'timedomain' pnoise simulation result really means?
Is it just the average noise spectrum (simulated using 'source') sampled by f_0, which results aliasing and folding? I think it is more than that, since different time points give different result, but what really this strobed pnoise does? What is the relation between noise simulated using 'source' and 'timedomain'

You posted resemble questions.
It seems you couldn't understand the following.

http://www.designers-guide.org/Forum/YaBB.pl?num=1207830622/13#13

Title: Re: flicker noise folds in a strobed noise sim?
Post by chenyan on Jun 14^th, 2008, 2:10am

Thanks guys, I think now I undertand better.

But in my simulation, I always get higher strobed noise (after converted as shown in Kundert's jitter+noise) than average noise, not like Ryan said. So which noise should be higher exactly?
I think that is related to the situation that, now not only the noise upconverted to the fundamental tone affect the zero-crossing, but also those around harmonics. While with average noise, we count only the noise around the fundamtenal tone. Am I right?

Also with ripple counter, I tried with several different settings

(1) simulate it as whole with strobed feature.
(2) Simulate it stage by stage, and sum it up using the equaiton given in Kundert's jitter+noise

These two simulations give similar results, however they are all much higher compare to simulate it as whole with 'source'. So is it necessary to simulate the ripple counter stage by stage as Kundert suggested?

Title: Re: flicker noise folds in a strobed noise sim?
Post by Ryan Cheung on Jun 14^th, 2008, 5:35am

Hi,

I do not think you are right when you said avged noise count only the noise around the fundamental tone. Avged noise count all of the harmonic when you defined it in the 'number of harmonic' entry. On the other hand, the strobe noise did count noise around harmonics, but this will not increase noise at flicker noise dominated part. It only increase noise floor through aliasing.

-Ryan

Title: Re: flicker noise folds in a strobed noise sim?
Post by chenyan on Jun 14^th, 2008, 7:20am

Hi Ryan,

What I mean by "avged noise count only the noise around the fundamental tone", is the noise after conversion. And 'number of harmonic' counters only for the mixing (up conversion) effect. Bbut when you plot your phase noise around the foundamental, it is actually only the noise around the fundamental tone after conversion. Whereas the timedomain noise simulation really counts all the noises around the harmonics which affect thr zero crossing.

I have done a number simulation with quite basic circuits like inverters and so on, always get a higher noise with strobed simulation. But give it a second thinking, I think wether the stobed noise should be higher or lower really depends on the slew rate, it you have a infinite slew rate, then strobed noise will be actually 0 after converted to phase noise. As you said, avergae noise and strobed noise are differnent stories, so we should not compare them anyway. One is physical, one is not.