I am trying to measure Noise figure with Spectrum Analyzer 9 to be specific MXA N9020A) using Gain method.
I used the following formula for Noise Figure calculation:
NF= N1 - (G1+G2) - ktb + 1.7dB
N1 - measured Noise power with DUT and driver amp (-119.44dB/Hz)
G1 - DUT Gain (21.82dB)
G2 - Driver amp gain (29.4dB with noise figure around 0.7dB)
KTB - for room temperature -174dBm/Hz
1.7dB - sum of correction factor for bandwidth, log amp and detector correction noise power.
I use Noise Marker to get 1Hz power bandwidth. At 1GHz Span 4 MHz, 4 MHz RBW, 1Hz VBW.
So my calculation is as follow:
NF= -119.44 dBm/Hz - (21.82dB + 29.4dB) - 174 dBm/Hz +1.7 dB = 5.04dB
The the DUT measured with Agilent Noise Figure Analyzer N8975A show Noise Figure of 2.6dB.
So why I am getting such big error?
From What I deduct is the the KTB is not the correct value because the Spectrum Analyzer have some offset power in the measurement that is unknown.
I find some solution.
It look like I do not need KTB. I just need to know what the spectrum analyzer read when you apply kTb power, like thermal noise of termination.
1. I measure thermal power of the termination on spectrum analyzer when it is boosted by driver amp. (N2 = -142.16 dBm/Hz)
3. I connected my DUT with terminated input and measure output power thru boost amp on spectrum analyzer (N1 same as above = -119.44 dBm/Hz)
4. I calculate Noise Figure as follow:
NF= N1 - N2 - G1+1.7 dB =
-119.44 dBm/Hz -(-142.16 dBm/Hz) -21.82dB + 1.7 dB = 2.6 dB
Look like my second approach give me same reading as I got from NFA.
So what exactly I am missing in the first formula? Does my second formula is correct?
The only indication that I find is the the Spectrum Analyzer receiver mixer adding own noise power (in old HP application note 150-9 it around 6dB) , that probably can be anywhere from 5 to 15 dB base on mixer conversionloss.
I see that in Y-factor measurement you measure the Spectrum Analyzer noise power of setup with Noise Source off state ( same as attaching termination) during calibration.
When you see the description of Gain method nothing is mention about that.
Any comment welcome.