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Design >> RF Design >> questions about noise figure for mixers
https://designers-guide.org/forum/YaBB.pl?num=1258675545

Message started by eecs4ever on Nov 19^th, 2009, 4:05pm

Title: questions about noise figure for mixers
Post by eecs4ever on Nov 19^th, 2009, 4:05pm

Hi everyone,

I have a few questions about the noise figure for a mixer. How is the noise figure for a mixer defined? If the mixer has a given conversion gain and noise figure, how much noise is added at the output? Does the noise figure take noise folding into account? Since the mixer has 2 inputs, does it make certain assumptions about these inputs?

Definition of Noise Figure:
http://en.wikipedia.org/wiki/Noise_figure

Thank you.

Title: Re: questions about noise figure for mixers
Post by pancho_hideboo on Nov 20^th, 2009, 4:13am

eecs4ever wrote on Nov 19^th, 2009, 4:05pm:

How is the noise figure for a mixer defined?
If the mixer has a given conversion gain and noise figure, how much noise is added at the output?

Your questions are too vague, so I can't understand meaning of your questions.

eecs4ever wrote on Nov 19^th, 2009, 4:05pm:

Does the noise figure take noise folding into account?

Yes.

eecs4ever wrote on Nov 19^th, 2009, 4:05pm:

Since the mixer has 2 inputs, does it make certain assumptions about these inputs?

What do you mean by 2 inputs ?
You mean USB and LSB for RF ?
Or simply you mean RF and LO inputs ?

http://edocs.soco.agilent.com/display/ads2009/Harmonic+Balance+for+Mixers

http://www.designers-guide.org/Forum/YaBB.pl?num=1239925339
http://www.designers-guide.org/Forum/YaBB.pl?num=1234783833

Title: Re: questions about noise figure for mixers
Post by eecs4ever on Nov 20^th, 2009, 10:28am

Hi Pancho_hideboo,

Thanks for responding. I'll try to clarify my questions. The definition for noise figure is SNRin/SNRout assuming an input noise floor of kTB (typically -174dBc/Hz). For an amplifier, the output noise floor is raised by the amplifier gain plus the noise figure in dB. Does this hold for a mixer as well? I was wondering if the mixing operation or noise folding makes it more complicated for a mixer than for an amplifier, or if I can assume that the noise floor at the mixer output is raised in the same way as for an amplifier.

For the two inputs, I meant the RF and LO. Is the noise folding different if the RF and LO are the same frequency and we are mixing down to baseband as oppose to an IF? If these are different, is there a separate noise figure for each case? Also are we assuming that the input noise floor is only applied at the RF signal and the LO signal is pure when taking the noise figure of the mixer?

Thanks again.

Title: Re: questions about noise figure for mixers
Post by pancho_hideboo on Nov 20^th, 2009, 11:07pm

eecs4ever wrote on Nov 20^th, 2009, 10:28am:

The definition for noise figure is SNRin/SNRout assuming an input noise floor of kTB (typically -174dBc/Hz).
For an amplifier, the output noise floor is raised by the amplifier gain plus the noise figure in dB.
Does this hold for a mixer as well?

Yes, if B=1Hz although you have typo.

But you have to consider "Spurious Response Frequencies" for RF inputs.
For example, Half-IF, SSB(USB, LSB) and DSB.
Again see http://www.designers-guide.org/Forum/YaBB.pl?num=1239925339

eecs4ever wrote on Nov 20^th, 2009, 10:28am:

For the two inputs, I meant the RF and LO.
Is the noise folding different
if the RF and LO are the same frequency and we are mixing down to baseband as oppose to an IF?

Both for zero-IF and non-zero-IF, they are different.

eecs4ever wrote on Nov 20^th, 2009, 10:28am:

If these are different, is there a separate noise figure for each case?
Also are we assuming that the input noise floor is only applied at the RF signal
and the LO signal is pure when taking the noise figure of the mixer?

Generally we don't define NF for LO input.
Noise effect from LO is also included in NF for RF Port. Here noise folding from LO is also included.
Consider Numerator and Denominator of NF definition.

Most important things of NF for Mixer is that there are many RF frequencies which yield same IF frequency.
These frequencies are called as "Image Frequencies" or "Spurious Response Frequencies".

Again see http://edocs.soco.agilent.com/display/ads2009/Harmonic+Balance+for+Mixers