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MXA N9020A - Unexpected spurse 2561.25 MHz below feed frequency

Question asked by mirek on Oct 17, 2014
I am measuring non-harmonics spurs over very wide band frequencies on RF generator using MXA N9020A Spectrum Analyzer and I found some add spurs generated by the MXA. 
I am sweeping the RF generator between 2575 MHz to 2800MHz at 0dBm and I am seeing on MXA approximately -60dBc spurs at frequencies that are  2561.25MHz below the RF generator frequencies.  For example if I set on RF generator 2600MHz I will see the spur on 38.75MHz. Adjusting the Span do not effect the spur amplitude or frequency. I also try that with several RF generator (Agilent PSG and ESG series and some other manufactures) and different MXAs - 13.5GHz  and 26.5GHz version showing that.  
The -60dBc is when the attenuator is set to 0dB, at attenuation of 10dB  it is -70dBc and so on. I is clearly generate by MXA.

Any able body from Keysight can explain what the reason for that.
Any fix - software/hardware?