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ESA option 1DR and 856xE series zero span amplitude problems.

Question asked by dhamilton on Apr 2, 2014
I have two E4407Bs and 8565E which exhibit a unusual failure in measurements below 50 dB down from full scale.  For example I am providing a 50 MHz 0 dBm, TB phase locked signal to the input. When the analyzer is in the default 10dB log mode,1 KHz span or lower, RBW/VBW auto and at 0 dBm reference I get the correct amplitude from FS to -96 dBc.  However, when I put the unit into zero span at about -50 dBc I get a very unstable display which get worse as I try to decrease the signal.  @ 300 Hz RBW, I have a 20 dB swing and @ 10 Hz RBW, I have a 3 dB swing.  When I put the instruments to a 1 KHz RBW the instruments reads correctly and stable.  The system F/W is A.14.00 and A.14.06. The 856xE series has 1999 F/W. All three do not have FADC option.  I tried this test on a PSA and it works as one would expect all the way down to -98 dBc in all bandwidths.  Is this an inherent H/W design problem for the earlier series.  I have end user that is insisting the instrument is broken. 

Thanks, Dave