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

Update:

The second formula not good too. When I expand the frequency from 500 MHz to 1400 MHz my Noise Figure values start falling off, that have no sense. My be it is because my G2 gain dropping from 29 dB at 1 GHz to 24dB at 1.4 GHz.

I wonder what the gain of driver amp (G2) should be to get proper data.

In both formulas I should not add 1.7dB as I think that the MXA N9020A Spectrum Analyzer Correcting that when enable Noise Marker.

So the Noise Figure from first formula should be:

NF= N1 - (G1+G2) - ktb = -119.44 dBm/Hz - (21.82dB + 29.4dB) - 174 dBm/Hz = 3.34 dB

That mean that I am still 0.74dB off from what I measure on NFA (2.6dB).

Not sure what I am missing. The setup on the driver amp side (G2), have 3 ft cable before and after the driver amp. That mean 0.4dB loss from each cable at 1 GHz, however the gain of G2 include that losses.

Regards,

mirek