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