# Is there error in Noise Figure Gain method formula?

Question asked by mirek on Jan 31, 2018
Latest reply on Mar 23, 2018 by mirek

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.

mirek