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Something strange with my DSOX2012

Question asked by Coris on Jun 8, 2013
Latest reply on Jun 10, 2013 by Coris
I should FFT measure an unmodulated (sine) 100Mhz frequency from an oscillator.  I`ve got down in time base to see some picks in FFT mode. I were very surprised to see a clear LF modulation  (216Hz) of my HF signal. It shouldn't be there. I was thinking it were something wrong with my oscillator, which it may be perturbed in one way. Had to use enough time to find out what it may be wrong with my signal device. So, suddenly I came on the idea to switch off Math - FFT. My signal it were very clear again without any LF modulation. 
I did an experiment with WaveGen on 20Mhz. The same result: nothing wrong with 20Mhz displayed on 20ms time base, but with Math - FFT on it came up a very fine sine wave of 208Hz very well displayed on 20 - 50ms time base (see the picture here).
I can not remember I had this problem before in the last time. I could go up and down on whole time base scale in FFT mode without any modulation of the measured signal. It is enough clear that this low frequency it come over the measured signal when the Math function is on. I did an User calibration, and a Hardware test, which both it passed alright, but this issue still be present.
Now I really can not be sure what I see in FFT mode, if the usual signal is perturbed in some way from  inside the scope...
Any idea about what it could be wrong?  

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