Why the NF is negative dB?

[tony_lth]

Hi, Gurus,
I designed a LNA 1st stage with CE3520.
Simulation is done, and test results showed 8dB gain with VNA test.
But when I tested NF with 346C noise meter in the spectrum analysis, which has NF function, I got -1 or -2dB NF with 8dB gain.
I checked that no self-osc at the output, which used FBP180 waveguide to coaxial converter.
So what caused this situation?
Best,
Tony Liu

 

[vfone]

If there is a large mismatch loss between DUT and VNA, noise analysis will lead to a negative NF.
An LNA (<0.5dB NF, >10dB gain) in front of the VNA will help somehow solving this issue, and also reducing the noise floor of the measurement.
After the LNA is put in place, the system needs to be calibrated before doing the NF measurement.