How to get a good sine waveform for frequency bigger than 20HZ(qwt Oscillocope)

[Herr_schneider]

Hello Everyone ,
this is my first post on this forum. I'm a qwt beginner. I would like to plot some curve in real time , I've seen today the example of oscilocope . Its 60% what I want. I've ran this example and I find out that when the frequency bigger than 20Hz , its difficult to get a "good sine waveform" I've tried to change sample interval and time but waveform is still not good when the frequency bigger than 20Hz .
my question : what schould I do to get a "good waveform " the frequency are for example 1000Hz , 3000Hz?
Any help woulb be appreciated

[Uwe]

The example is intended to show how to retrieve values from some device in periodic intervals and to plot them with low CPU usage. It has not much to do with creating a special waveform, beside that it uses the sinus function as placeholder for the real device.

Uwe