Since the harmonic is always a multiple of the fundamental frequency
I guess you used ideal digital values, thus there is no "distortion" in the digital value. No overtones.
Does your switching DAC include some "nonlinearities" that cause overtones?
I asked before "do you simulate frequency dependent distortion"?
I mean: A simulation does only simulate what you tell it to do.
With AD converters every frequency above nyquist becomes an alias frequency, but the amplitude does not get reduced.Nor does the combined RMS change.
With a "switched" DAConverter... (without an analog post filter) it is similar. It surely causes sone "switching noise". But as long as you don't use an analog post filter - you just shift the frequencies but not amplitueds and nor the RMS.
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Example:
Let's assume you don't use ideal sine values, but square wave instead. (I know this is not what you want to do)
Sample rate always is 10MSPS.
Now let's say you use a 1kHz square wave. So 1kHz fundamental, + overtones: 3 kHz, 5kHz, 7kHz...
Nothing is to suppress the overtones, so you get a defined signal_to_overtones ratio.(if I'm not mistaken 100% fundamental + 44% overtones)
No change from 1kHz to 1MHz. So 1MHz fundamental, + overtones: 3 MHz, 5MHz, 7MHz...
--> you get the same signal_to_overtones ratio as with 1kHz
But as soon as you use an analog post filter everything changes:
Let's say you use a 5MHz post filter.
With a 1kHz signal you get the fundamental and all overtones up to the 500th about not suppressed.
The signal_to_overtone ratio does nit change much with respect to the non filtered signal
But using a 1MHz signal, now you get the 3rd overtone about not suppressed, the 5th is suppressed to 70% and every higher overtone is even more suppressed.
Now the signal_to_overtone changes a lot with respect to the unfiltered value. And with respect to the 1kHz signal.
The S/N R now is frequency dependent ... because you use an analog post filter.
The question is "what" exactly do you simulate?
Klaus