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N5182B gating signal

Question asked by johnw on Apr 6, 2013
Latest reply on Apr 6, 2013 by johnw
I'm using a N5182B vector signal generator, with its RF output connected directly to a spectrum analyzer.  I'm using the Agilent N7622B Signal Studio Toolkit version to write Matlab waveforms to the VSG.

I used the MATLAB sample code that comes with Toolkit to generate a 1000 sample tone - relevant code summarized below:
phase = (2*pi/100)*\[0:999\];
Idata = cos(phase);
Qdata = sin(phase);

This works perfectly; a tone appears where expected.  The VSG indicates that the segment occupies 1000sa in BBG memory, as expected.  However, when I make the tone 500,000 samples long (well within the device's 32Msa limit), the VSG indicates the segment occupies 488ksa, rather than the expected 500ksa.  The noise floor becomes elevated, a copy of the tone appears (mirrored about the center frequency), and the tone gates on and off many times per second.  The only change to the MATLAB code was using "phase = (2*pi/100)*\[0:499999\];" instead of "phase = (2*pi/100)*\[0:999\];"

Can anyone tell me what might be causing this?

I've attached screenshot showing the spectrum analyzer set to zero span at the expected frequency, while the 500ksa tone is being played back at a 50MHz clock rate.  The period of this pattern is exactly 10ms.  The higher-amplitude step at the start of the pattern is 1.3ms long.  The total on time of the pattern is 7.76ms.