The Designer's Guide Community Forum

The Designer's Guide Community Forum
https://designers-guide.org/forum/YaBB.pl
Measurements >> RF Measurements >> Noise Figure Measurement Question
https://designers-guide.org/forum/YaBB.pl?num=1188661855

Message started by YYou on Sep 1^st, 2007, 8:50am

Title: Noise Figure Measurement Question
Post by YYou on Sep 1^st, 2007, 8:50am

Hello,

I am now testing the front-end circuit (LNA+Mixer+Synthesizer+Filter) with differential outputs at low frequency.
To measure the noise figure, I used noise source, spectrum analyzer(with NF function), and one active differential probe(Tektronix P6046+amplifier for differential to single ended conversion).

Case 1: unit gain in differential probe, calibrate noise source, connect all together, check the results of NF and gain from Spectrum Analyzer
---- gain: expected value
---- NF: much higher than expected

Case 2: set the attenuation of the diff. probe to get gain of -20dB
---- gain: ~20dB less
---- NF: ~3dB less than that in case 1.

The only thing I can conclude is that the diff probe is very noisy. Ideally, since I was adding the probe at the end, there should be no difference between the two cases if the noise from the diff probe was low. Since this is not the case, I keep the noise from diff probe low by keeping the gain low.

Even in that case, if the noise were to scale with gain linearly we shouldn't see the results I was seeing. I don't know how the noise from the diff probe changes with different gain settings.

Could anybody give some insights on above situation ?

Thanks,

YYou

Title: Re: Noise Figure Measurement Question
Post by didac on Sep 3^rd, 2007, 12:32pm

Hi YYou,
Have you included the probe into the calibration sequence?For what I understand on the steps for measurement it looks like that you didn't include it, in theory all noise not generated by the DUT should be removed via calibration. Also have you checked if the IF falls inside the ENR table?,if not the measurements will not be reliable.
Depending on the instrument manufacturer they usually also have applications notes for measurements that can be useful,agilent for example has a huge database of applications notes for measurements.
Hope it helps,