HF oscillation on the scope?

[Ethan]

What does HF oscillation look like on a scope?

I always thought it looks like little lumps along the sine wave. Is that correct?
Could anyone tell me what is the best way to see if an amplifier is oscillating at HF?

Thanks

 

[NewYorkDave]

There's no good short answer to that question, but here's a short answer that isn't terrible:

With the sweep set to, say, 2mS/Div: if you get a fuzzy trace with no input signal to the amp, and you speed up the sweep--adjusting the vertical gain as necessary--and see a periodic rather than a random (noise) signal, your amplifier is oscillating at HF. If you have to slow down the sweep, you have LF oscillation (or hum).

Sometimes an amplifier will only oscillate when a stimulus (input signal) is applied, and this can show up as fuzziness, bumpiness or "squeeging" superimposed on the output signal.

Sometimes bad grounding in your test setup can produce the above symptoms (and others) as seen on the scope, even when the amplifier is actually working as it should.

This is not easy to describe verbally. But with some experience, you'll know it right away when you see it.

 

[Ethan]

Thanks Dave!

 

[NewYorkDave]

Forgot to mention: the amp could also be picking up RF. But you learn what the different types of spurious signals look like...

 

[sodderboy]

[quote author="NewYorkDave"]
Sometimes bad grounding in your test setup can produce the above symptoms (and others) as seen on the scope, even when the amplifier is actually working as it should.
[/quote]

That is SO true. I have had more incorrect ground reference signals than actual malfunction signals. When I am jumping around in something with digital and analog grounds, or multiple voltages with multiple 0vl's, I use a simple 600:600 transformer just to isolate grounds and make sure of what I am looking at. I can rig my scope into a tech patchbay and insert xfmr, A/B switcher, or whatever.
Mike

 

[Gus]

A related thing to check

Check your probes using the built in calibration if your scope has it before use.

The newer cables seem to bend and deform more than the older fatter cables sometimes changing the response of the probe cable.

 

[Guest]

[quote author="Ethan"]What does HF oscillation look like on a scope?
[/quote]
You can see feedback-related oscillations,
but in real HF oscillation (like input tube grid - cathode oscillation)
you will not see nothing (while circuit really oscillates - changes anode
current while puts finger on the grid)
Some equipment - Broadband sampling voltmeter - can help.
Very old equipment by Hewlett -Packard. I have one
on the table and does good service.

xvlk

 

[bobkatz]

[quote author="sodderboy"][quote author="NewYorkDave"]
Sometimes bad grounding in your test setup can produce the above symptoms (and others) as seen on the scope, even when the amplifier is actually working as it should.
[/quote]

That is SO true. I have had more incorrect ground reference signals than actual malfunction signals. When I am jumping around in something with digital and analog grounds, or multiple voltages with multiple 0vl's, I use a simple 600:600 transformer just to isolate grounds and make sure of what I am looking at. I can rig my scope into a tech patchbay and insert xfmr, A/B switcher, or whatever.
Mike[/quote]

Just a devil's advocate response: What if the 600:600 transformer does not have enough bandwidth and masks the HF oscillation! Oh, I wish someone else could do my troubleshooting for me