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Design >> RF Design >> Effect of Flicker noise in SSB and DSB NF
https://designers-guide.org/forum/YaBB.pl?num=1161194311

Message started by Prabhu on Oct 18^th, 2006, 10:58am

Title: Effect of Flicker noise in SSB and DSB NF
Post by Prabhu on Oct 18^th, 2006, 10:58am

Hi,

I have a basic doubt regarding mixer ( Gilbert cell ) noise figure.
People say,
Typically NF ( SSB ) = 2*NF ( DSB )

I want to know if the above holds only for thermal noise or for flicker noise also.
Since the flicker noise at the mixer output is due to mechanisms entirely different from the thermal noise folding back from the signal band and the image band, I would think that the flicker noise contribution should be same in both NF ( SSB ) and NF ( DSB ) computation.

But when I plot the spot NF ( DSB ) and NF ( SSB ) of a mixer in cadence , I find a 3 dB difference in the flicker noise region ( low freq. ) also.

Another question :

In considering the noise contribution from mixer transconductor and load in computing NF ( DSB ) and NF ( SSB ),
I would think that the load should contribute equally in both the cases and
the transconductor should contribute twice in the SSB case.

Am I right ?

Prabhu

Title: Re: Effect of Flicker noise in SSB and DSB NF
Post by aaron_do on Oct 19^th, 2006, 1:07am

It seems your logic is right about the flicker noise not being doubled since is not in the same band as the RF signal. However, the fact is that for DSB, you have double the signal level of SSB, therefore you will definately have a 3 dB better NF (SNRout/SNRin) than SSB. You have the same output noise but double the signal level ;)

Same applies to the transconductor point...