Ghost frequencies in spectrum mode

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Ghost frequencies in spectrum mode

Post by wk_emc » Mon Sep 10, 2012 3:49 pm


The spectrum of a square signal showing ghost frequencies in comparison to the expected result which could be proved by using a spectrum analyzer.

This also happens in demo mode.

Demo mode :
square (de: Quadrat) 2MHz, 1V, no offset
Expected frequencies with Duty Cycle 50%: 2MHz and odd harmonics

Result: Much more frequencies than expected, partially with high levels.

Using Blackman Harris, BINS 262144, total frequency 9.766MHz, modus: (de:) Größe

This effect caused me some time, looking for the sources of these ghost-emissions in real installations.
This seems to be bug in the FFT-software.


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Re: Ghost frequencies in spectrum mode

Post by Martyn » Tue Sep 18, 2012 12:49 pm

Is that not the aliasing effect of the harmonics in the logarithmic scale
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