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Design >> Analog Design >> Noise folding in mixer, but no penalty in chopper
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Message started by Jacki on Jun 17^th, 2019, 11:15pm

Title: Noise folding in mixer, but no penalty in chopper
Post by Jacki on Jun 17^th, 2019, 11:15pm

Hello,

When we do noise analysis for mixer, we can see the noise folding, and the noise figure is higher. But for chopper, we don't have any noise penalty. I wonder why one has the noise folding but another one doesn't have, and their principles are very similar. Thank you for any discussion.

Title: Re: Noise folding in mixer, but no penalty in chopper
Post by Jacki on Jun 18^th, 2019, 2:42am

My thought is mixer focuses on down-conversion only when we talk about the noise analysis. But chopper has both, it has aliasing too, just be suppressed.

Title: Re: Noise folding in mixer, but no penalty in chopper
Post by R.kumar on Jul 30^th, 2019, 4:35am

I think for a chopper the input is baseband with the chopping frequency much larger than the 1/f corner frequency. If the chopping frequency is set lower than the 1/f corner frequency, then the noise folded components start to appear. Intuitively, if the slowly varying noise is sampled and subtracted at a rate faster than the rate at which the noise itself varies, then the flicker noise is eliminated.

Title: Re: Noise folding in mixer, but no penalty in chopper
Post by Jacki on Jul 30^th, 2019, 5:29am

Hello Kumar,

Thank you for your reply. Yes, for the flicker noise, due to the bandwidth is too narrow, after chopping (fch >> fc) no noise spectrum overlapped. How about the thermal noise? Because in mixer we normally consider the thermal noise folding, not flicker noise. Also I think due to the frequency converting and the scaled amplitude of the harmonics and the limited bandwidth of the LPF, the contribution of noise folding is not big, so we can skip it.

Title: Re: Noise folding in mixer, but no penalty in chopper
Post by Horror Vacui on Aug 12^th, 2019, 6:41am

Chopping is mixing. The noise at multiple f_chop will be translated to DC output. In precision amps the offset and the 1/f noise poses a serious threat to the small signal to be sensed, but we have no such thing for an AC coupled RF mixer. Furthermore in chopped amps the bandwidth is limited therefore the high frequency noise is also filtered, and this filtered noise is folded back.