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MXA N9020A - absolute power measurement: stability problem above 3.6GHz

Question asked by mirek on Nov 19, 2013
Latest reply on Nov 22, 2013 by tabbott
I have a problem with measurement absolute power amplitude on MXA N9020A Spectrum Analyzer above 3.6GHz.
The Spectrum Analyzer appears to read different value for the same frequency and power. You set the source generator at fix frequency and power and Spectrum Analyzer to the same frequency and read the power.
If you change the frequency on the Spectrum Analyzer and then return the previous one, you will see that the amplitude start varied from the previous value (you have to repeat few time to see that). It only shows at few particular frequencies above 3.6GHz. My first assumption was that that particular Spectrum Analyzer is damage but I check several others MXA and I see similar behavior. The amplitude level varies randomly and in some cases I is up to 0.3dB or higher from previous values. That appears too high for many measurements.
My original task was to use Spectrum Analyzer as power meter to level several RF sources, so I used PSG E8257D reference power generator.
1. I measure the absolute power (set arbitrary 0dBm on PSG) using power meter (U2000) directly on output of the PSG. The USB power sensor show values below +/- 0.05dB from set power - very good).
2. I connect together output of RF generator to input of Spectrum Analyzer  with 3ft RF cable and read the values across the frequency.  The cable shows some slop up to 1.5dB.  The obtain values were logged for future measurement correction.  
3. I set on PSG the arbitrary power (0dBm) again, and measure the power over frequency using the same MXA hover now I subtract logged correction values.
The assumption was that I should get very close reading from MXA (after correction)  to power meter, however I getting several  power spikes at specific frequencies. If you repeat the test several time you get different results from that particular frequencies.  See the block diagram of the measurement setup and the readings between 8MHz to 6400Mhz.
I am running the latest firmware on MXA (A13.12), old firmware show (A10.52) showing similar behavior.  Each MXA showing the spikes at slightly different frequencies.
Setting on Spectrum Analyzer:
Span: 20Khz 
RBW: Auto
VBW: Auto
All setting the same for referencing and measurement
Does anybody know any solution? Is there any bug in the firmware?
Error of more than +/- 0.1dB for the same power and frequency look too high.