In ZIF recievers I have worked on (for GSM/EDGE) we always measured noise figure on I or Q (the results being the same). This is what we quoted on datasheets/what the baseband DSP guys wanted.
DBRF1 wrote on Nov 21st, 2006, 11:41am:Also wrt the DSB/SSB NF comments. My understanding was that for a ZIF rx there is NO image band so in fact for this type of receiver the DSB and SSB NF are the same as ther is in fact only one Signal band . { Image = Wanted +/- 2*IF , IF = 0 thus image = wanted } ??
I agree with Jess's comments, i have measured the NF of a ZIF transciever in two ways:
1) C/N method: Using a CW carrier one side of the LO, so had to adjust the NF by -3dB to account for the fact the modulated input signal would occupy both sides (the noise was both side of the LO... its a bit diffcult for it not to be).
2) Measuemnt with noise diode (Y-factor method) gives the same result... without the need for the 3dB adjustment.
The treatment of this in Razavi's "RF Microelectronics" p183 is quite good, and aglient do a good app note on y-factor noise measurement.
DBRF1 wrote on Nov 21st, 2006, 11:41am:Jess thanks for the reply
A few questions to double the signal you assume we shift the I {or Q by } by 90 degs then we can add 2 signals in phase hence get 6dB increase in voltage.
I used to think of it as adding signal I and Q in vector form, so you add as squares (I^2+Q^2).... noise also adds as squares... not sure if my loose thinking is strictly correct but i managed to convince myself quite easily ;)
Best way to convince yourself is if you have a dual channel VSA (Vector signal analyser) like HP89441 or R&S FSIQ... then you can measure NF on I or Q using C/N method with -3dB correction factor, and then put the VSA in I+jQ demodulation mode to see it give you the same/similar NF or C/N.
cheers
aw