The Designer's Guide Community Forum

The Designer's Guide Community Forum
https://designers-guide.org/forum/YaBB.pl
Modeling >> Behavioral Models >> noiseless mixer has 3db NF?
https://designers-guide.org/forum/YaBB.pl?num=1169135976

Message started by Jochen on Jan 18^th, 2007, 7:59am

Title: noiseless mixer has 3db NF?
Post by Jochen on Jan 18^th, 2007, 7:59am

Dear all,

I have simulated the Demodulator (Mixer) from Jesse Chen's Paper "Modeling RF Systems" (available on this webpage) in order to examine the problems with my mixer-model (http://www.designers-guide.org/Forum/YaBB.pl?num=1166542399thread).

With all simulation setups (qpnoise and pnoise) I achieved an noise figure deviation of +3.01db (at 27°C). Even if the nf-parameter of the model is set to 0, I get 3.01db NF.
And the Noise Contribution from the print menu displays the correct noise ratios according to the parameter settings.

Then I read the chapter "SSB and DSB Noise Figures" in B. Razavi's book "RF Microelectronics" (p. 183):
There is written that "the noise figure of a noiseless mixer is equal to 3db" due to the noise floor at the image band which is folded to the IF-frequency.
The Cadence documentation for qpnoise-analysis says that it calculates the SSB-Noise Figure.

Is this the reason for the deviation? Therefore 3db NF is correct for model-parameter nf=0?
So the software does not automatically subtract these 3db?

Thanks for your time,
Jochen

Title: Re: noiseless mixer has 3db NF?
Post by pancho_hideboo on Jan 19^th, 2007, 6:27am

SpectreRF gives you both SSB_NF and DSB_NF.

SSB_NF and DSB_NF are following.

vn^2/Rl + kTo(G1+G2+...+Gn)
Fssb = -------------------------------------
kToG1

vn^2/Rl + kTo(G1+G2+...+Gn)
Fdsb = -------------------------------------
kTo(G1+G2)

kTo is the noise power at the input termination.
G1 is the primary frequency conversion from RF input frequency to IF output frequency.
G2 is the other frequency conversion from RF input image to the same IF output frequency.
G3 and higher are the higher order, low efficiency conversions
that occur as other input frequencies mix with harmonics of the LO to
appear at the IF output frequency.

If mixer is noiseless, vn=0.
G3 and higher are all zero in mixer behavioral model.
So NFdsb=0dB and NFssb=3dB.

Also see followings.

http://www.designers-guide.org/Forum/YaBB.pl?num=1162399761

Title: Re: noiseless mixer has 3db NF?
Post by Jochen on Feb 6^th, 2007, 1:18am

Thanks for your quick answer pancho_hideboo!

Now I believe that my mixer delivers 3db NF_SSB without internal noise!

I have found another (old) thread where this is also discussed:
http://www.designers-guide.org/Forum/YaBB.pl?num=1042537924

Jochen