Scope display bug? distortion of sine wave at high frequency

Hi all,

I found sine waves will be terribly distorted when they are generated at
relatively high frequency(above 5khz) from signal source in GRC. I set
the
sampl_rate of scope sink to 2Ghz(high enough, I think), and there are
always
spikes on generated sine wave. This problem is especially distinct when
generated sine wave reaches 20Khz and the waveform displayed on scope
sink
is almost “saw wave”. There will be nothing to dispaly if I keep on
increasing frequency to 1Mhz. Is there anybody who met this problem
before?
or it might only be the display problem in GRC? I guess it might be the
problem of scope sink which could not achieve that high sweep (sampling)
rate even I set it correctly.

Attached are snapshot of generated sine waves at different frequencies:

Your comments will be very much appreciated.

Thanks,

Milo

what is the sample rate of your signal source? If its samp_rate, which
defaults to 32KHz, then the plots attached make perfect sense.

-Josh

Milo-

Attached are snapshot of generated sine waves at different frequencies:

Your plots are probably fine, depending on what is your sampling rate
(Fs). Think of it this way: if you display a
sine wave near Fs/2 then you only have 2 samples for each sine wave
period (1 per peak) and connecting those dots with
straight lines on a scope graph will look like “spikes”, to use your
terminology.

In the case of your 20 kHz plot, it looks to me like you might have 3 or
4 or so samples per sine wave period, so my
guess is your Fs is somewhere between 60 and 80 kHz.

-Jeff

Thank you all! I’ve fixed this problem.

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