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Design >> RF Design >> Receiver Noise Figure
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Message started by aaron_do on Sep 4^th, 2006, 1:26am

Title: Receiver Noise Figure
Post by aaron_do on Sep 4^th, 2006, 1:26am

Hi everybody,

i have a question receiver noise figure. Basically I have a system which looks like this:

--------------LNA------|--------------|-------I+ }NF
| | Double Bal |_____I- }
Port-----Ideal Balun | Quad |
| | Mixer |-------Q+
--------------LNA------|--------------|_____Q-

hope that primitive drawing helps. Simple anyway. Anyway I want to calculate the required Input Referred Noise (IRN) of the IF blocks. My LNA/Mixer combination has a gain of 17.4 dB in simulation and a noise figure of 9 dB when simulated across I+ and I- (differential output). The 9 dB is a SSB NF result. Here's my calculation

NFsystem = 19 dB
Input Noise (50 ohm) = 4kTR = 8.004 e -19 V2/Hz
LNAMixer Output Noise due to source = Input Noise * Gain(LNAMixer) = 4.44 e-17 V2/Hz (A)
LNAMixer Output Noise = A * NFLNA = 3.53 e-16 V2/Hz (B)
Required IRN at LNA output = NFsystem * A = 3.53 e-15V2/Hz (C)
IRN of IF Block = √(C – B) = 56 nV/√Hz

So what i've done is calculated the noise at the output port of the LNA/Mixer with the LNA NF and then the same noise with the system NF and taken the difference. The result is my required IRN of the IF blocks. I decided to double check this result by adding ideal resistors to the output ports (I+, I-, Q+, Q-) with the other end left as an open circuit. I then simulated the system NF again and swept R until the NF was 19 dB. I found this R to be 25 kohm. Since I have 2 Rs for a differential output, the total noise is sqrt(2*4ktR)=sqrt(2*4*1.28e-23*290*25000)=28 nV/√Hz which is obviously not what i calculated. Can anybody offer any insight as to why my calculation is so off? Here's a pic of how i resimulated the NF.

I used the PSS and PNOISE analysis in spectre to simulate the nosie figure.

IF+--------R-------open circuit } NF
IF----------R-------open circuit }

thanks,
Aaron

Title: Re: Receiver Noise Figure
Post by ACWWong on Sep 4^th, 2006, 2:31am

Your problem is you have the wrong amount of input noise.... the available noise power from a conjugately matched input (ie your LNA input) is kT, ie 4e-21 watts @ 290k.
So th input noise should be 2e-19 V2/Hz on your first line....

please see p18 of the following reference for detailed explanation.

cheers

alan

Title: Re: Receiver Noise Figure
Post by ACWWong on Sep 4^th, 2006, 2:32am

opps, classic mistake i forgot to add the link... p18 of

http://kabuki.eecs.berkeley.edu/~jrudell/papers/specs/gsms6.pdf

Title: Re: Receiver Noise Figure
Post by aaron_do on Sep 4^th, 2006, 8:58am

thanks,

yeah i see... learn something new every day :)

thanks for the paper too

Aaron

Title: Re: Receiver Noise Figure
Post by ACWWong on Sep 4^th, 2006, 9:50am

Its my pleasure...

with regards to the paper, it covers the system design and noise, gain linearity lineup of RFIC transceivers very well.
I recommend it to anyone who whats to do receiver system design, it provides the bridge between RF guys who use power based Friis Equation to practical IC design where one uses voltage gain and volatge noise. The subject coverage is better than i've seen in any book.... in fact i've based many successful designs upon it !