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E4440A odd behavior

Question asked by MADENGR on Jul 21, 2009
Latest reply on Jul 23, 2009 by KeithLeitch
I'm inputting a very clean tone (from a PSG) of 3700 MHz at 0 dBm into my E44404A.  If I set the span wide, then I see two distinct bins with a null in the center.  If I zoom in on the data the null is clearly visible.  I would not expect this behavior.  With the RBW at 1 MHz I would expect the power to be contained in one or two adjacent bins, but not two bins 3.3 MHz apart with a deep null between.

Both instruments are locked to an external reference. I tried an ESG as the source but get the same result.  Using internal reference makes no difference.

Of course if I vary the span or frequency it changes somewhat.  Also changes if I change the # of points in the sweep.  Once I narrow the span it goes away.  Is this typical behavior, or is something wrong with the sampling or DSP?  Agilent tech support has confirmed this is normal operation, but has not given an explanation what is going on.  I assumed it was windowing effects, but they say the FFT is not used at this wide of span, but if that's the case then this even more disconcerting.  The reason this bugs me is it causes confusion if I'm looking for spurs over a wide band.