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Spectrum Analyser Settings

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I am using the spectrum analyser in SIMULINK, to view the spectrum of one cycle of a square wave (50% duty cycle and an amplitude of 1 V).

In the first case I take 40 samples over the single period and I get what I expect a nice smooth sinc x function (albeit plotted in dBm).

However when I increase the sampling by a factor of 50 i.e. 2000 samples over one period, I get straight lines and pointy peaks. The values in terms of peaks and troughs in dBm's and Hz are as before, but the actual graph does not look like what it should be ?


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more samples will yield narrower peaks. if you are describing what i think you mean, which is a wide bell curve with the peak at the harmonic, then when you increase the sample rate, you get a narrow peaked curve. it's because with fewer samples, there is some ambiguity as to where that peak should be on the X axis, so you get a bunch of spread-out points making a bell curve. when you have more samples to work with, the ambiguity goes away.
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