The duration of your measurement is 2^18 /200MHz = 1.31072ms
One cycle of your 20MHz signal is 50ns.
--> there is no integer fit of your signal within your measurement window
--> you need to perform a windowing function (hanning, hamming...) before you run the FFT to get plausible results.
The duration of your measurement is 2^18 /200MHz = 1.31072ms
One cycle of your 20MHz signal is 50ns.
--> there is no integer fit of your signal within your measurement window
--> you need to perform a windowing function (hanning, hamming...) before you run the FFT to get plausible results.
In the figure above, those tones look like the odd order harmonics however the locations are varying with the input frequency. (maybe I should chose some other example)
In some cases (some other input frequencies), tones appear on non odd order harmonic locations and even at dc. So I was guessing signal folding or some other issues.
The sine wave is generated by sine function (MATLAB) and vsin(cadence) and ofcourse there were no preamps.