How to view a frequency jittered switching waveform on scope?

[grizedale]

Supposing you are viewing a frequency jittered switching waveform of the drain of a flyback.

The switching edge is seen to wander back and forward on the scope screen as it is jittered.

Is it possible to set the scope up so that the switching edge is made stationary on the scope screen (without freezing the waveform)?

 

[emontllo]

Just set the trigger at rising or falling edge (depending on which switching edge you want to analyze) and it will work. There is no way you can see the rising and falling edges fixed with a jittered waveform.
There are also (expensive) oscilloscopes specially designed to work with jittering singals such as this one LeCroy WaveExpert Sampling Oscilloscopes ...

 

[grizedale]

OK thanks.....when you say "it will work", i take it you mean the switching edge can be seen but not stationary?

 

[emontllo]

If the trigger is properly adjusted and you focus only in one commutation cycle, then the edge you want to analyze should be stationary on the screen. The trigger should be in the same commutation cycle, maybe you are using the zoom option and triggering in an other cycle...
If you want to see more than one commutation cycle in the same screen of course they will be swinging.

 

[grizedale]

I am only looking at one edge of the drain-source voltage.......but it goes back and forwards with the jittering......how to stop this? (without freezing the waveform)

 

[mtwieg]

With frequency dithering the resulting PWM waveform will practically never be periodic, you'll always see jitter on the scope. The only thing you can fix is the edge that you are actually triggering on. That will always be at t=0.

 

[emontllo]

The answer is also in the datasheet of an IC that uses frequency jittering:

http://www.powerint.com/sites/default/files/product-docs/lnk302_304-306.pdf
(Page 3):
The frequency jitter should be measured with the oscilloscope triggered at the falling edge of the DRAIN waveform.