Re: [Discuss-gnuradio] scope display bug? distortion of sine wave athigh frequency

[Jeff Brower]

Milo-

> 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 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