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Design >> Analog Design >> Flicker Noise Noise Folding Question
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Message started by teem on Jun 5^th, 2015, 6:03am

Title: Flicker Noise Noise Folding Question
Post by teem on Jun 5^th, 2015, 6:03am

Dear Friends:

Flicker noise is infinite at DC. If I pass flicker noise through an on/off switch with switching frequency w, what the output spectrum would be? Would the spectrum be infinite at w, 3*w, 5*w…etc?

Thanks.

Title: Re: Flicker Noise Noise Folding Question
Post by loose-electron on Jun 5^th, 2015, 11:08am

If you want to observe for long periods of time there is some truth to the infinite at DC concept.

Give this a read:

http://www.edn.com/electronics-blogs/the-signal/4408242/1-f-Noise-the-flickering-candle-

The spectral content at (and near) DC does get up converted when you multiply, but the infinite amplitude you mention, in a practical realization situation, is not there.

Generally, when you up convert flicker noise, you will get sort of equilateral triangle shaped noise profiles centered at the mixer (multiplication signal) frequency.

Title: Re: Flicker Noise Noise Folding Question
Post by RobG on Jun 9^th, 2015, 12:07pm

loose-electron wrote on Jun 5^th, 2015, 11:08am:

From the excellent article...
Quote:

From the spectral plot, you might infer that 1/f noise grows boundlessly as you measure for increasingly long periods. It does, but very slowly. Noise from 0.1 to 10Hz doubles (approximately) with a lower bandwidth extended to 3.17e-8 Hz (a one-year period). Add another 6% for ten years.

I believe it takes 10 billion years for it to go up 50%. I don't get too worried about the lower bound ;).

Title: Re: Flicker Noise Noise Folding Question
Post by teem on Jun 10^th, 2015, 7:48am

Dear loose-electron:

Thanks a lot for the good article in the link. It provides much insight worth to read.

But after reading the article and your comment, I am still confused. Do you mean flicker noise spectral density does go infinity ideally at the mixer(multiplication signal) frequency, but in practical situation it will become sort of equilateral triangle shaped noise profiles? How and why?

Why does it become triangle shaped noise files?

How to calculate the triangle shaped noise profiles?

It seems the article does not mention this part. Could you elaborate or do you have any articles discussing the behavior you explained?

Thanks again.

loose-electron wrote on Jun 5^th, 2015, 11:08am:

Give this a read:

http://www.edn.com/electronics-blogs/the-signal/4408242/1-f-Noise-the-flickering-candle-

The spectral content at (and near) DC does get up converted when you multiply, but the infinite amplitude you mention, in a practical realization situation, is not there.

Generally, when you up convert flicker noise, you will get sort of equilateral triangle shaped noise profiles centered at the mixer (multiplication signal) frequency.

Title: Re: Flicker Noise Noise Folding Question
Post by carlgrace on Jun 10^th, 2015, 9:20am

If you model the 1/f spectrum as starting at "infinity" at DC and then a straight line down to the corner frequency (the frequency at which thermal noise begins to dominate you get the standard frequency domain plot of noise density.

Then, imagine reflecting the plot about the y axis. Then you will have a triangle in the middle. The peak is a DC and it hits the thermal noise level at the fc and -fc. Mixing shifts the spectrum up to the mixing frequency. That is how you get a triangle at the mixing frequency and is also the main source of oscillator phase noise.

Title: Re: Flicker Noise Noise Folding Question
Post by teem on Jun 10^th, 2015, 11:32pm

Dear loose-electron, RobG and carlgrace:

Thanks a lot for all your explanation. Sorry that I am asking the same question again and again because my brain cannot work with that. Please excuse my poor knowledge in some fundamental concept.

Allow me rephrase my question, I also draw the figures to assist:

1. Like fig.1 shown, original flicker noise spectrum is infinite at DC. After upconversion, noise spectrum is also infinite at mixer frequencies. This seems to be the ideal case as what you explained. Am I correct?

2. I read replies from all you sirs, suggesting that practical flicker noise maybe more like fig.2 and fig.3 present (fig.2 and fig.3 are the same thing, just fig. 2 freq is in linear scale, fig. 3 is in log scale). Am I correct?

3. If fig.2 and fig. 3 are the actual case, how to determine values of each peak in the spectrum?

Thanks a lot for all your time.

Title: Re: Flicker Noise Noise Folding Question
Post by loose-electron on Jun 11^th, 2015, 8:08am

Figure 3 seems to be the closest to what I have seen in the lab.