Pictures of oscillation for a newbie

[markiemrboo]

Hi all,

I just got my first 'scope, free from an uncle. It's a really old one, a "Scopex 4D 10B", quick search seems to suggest it's 10MHz. I'm not sure how good it is and all that, but I can't afford to buy one really so I will have to make do! It seems to all work ok, though I think the probe connectors might need cleaning or something. They need a bit of a wiggling about to get a stable image

I was just wondering if anyone would be so kind as to point me in the direction of 'scope pictures of any sorts of oscillation, so I know what I am supposed to be looking for.

I've only been brave enough to play a 30Hz tone, with a spare set of speakers connected (probably not wise, but it worked...), and connected the ground of the probe to the speaker terminal -ve, then probed the speaker terminal +ve. All I can say is that I seemed to get a very clean sine wave show up.

I presume that if it were oscillating it would stand out and wouldn't be a clean wave? I presume that I should be testing a whole load of frequencies though, and possibly not just sine waves (I fear I may be quite limited here, as I am using a computers sound card for the tone generation, I measured a 1KHz square wave straight from the sound card output and it had some 'ringing' and 'overshoot').

Any good links for a newbie would be much appreciated. I tried searching for ages for any pictures of an actual scope showing some form of oscillation, but couldn't really find anything!

[janneman]

If you get your signal from the soundcard you will see no oscillations. Speakers cannot oscillate. Oscillations need some active circuitry like amplifiers.

Oscillations occur if the output from an amp gets back to its input in such a phase that it reinforces the input signal; the output gets higher and higher until it hits the supply limit, then it starts to fall until it hits the lower supply limit etc. You always need to feed in energy to sustain an oscillation.

Does that help?

Jan Didden

[markiemrboo]

Originally posted by janneman
If you get your signal from the soundcard you will see no oscillations. Speakers cannot oscillate. Oscillations need some active circuitry like amplifiers.

Oscillations occur if the output from an amp gets back to its input in such a phase that it reinforces the input signal; the output gets higher and higher until it hits the supply limit, then it starts to fall until it hits the lower supply limit etc. You always need to feed in energy to sustain an oscillation.

Does that help?

Jan Didden

Oh, sorry. I probably badly phrased something (or don't understand what you're telling me yet!).

The 30Hz tone I measured from my newly built GainClone amplifier outputs, basically (at the speaker terminals, with spare speakers connected). I was using a sound card here as a tone generator to feed the amplifier.

The square wave I measured from the soundcard output just experimenting

But basically, from what you're saying, it sounds like oscillation should be really quite obvious....?

[I_Forgot]

A sine or square wave IS oscillation. It sounds like you are interested in seeing whether your amp is oscillating due to instability.

If your amp is oscillating at a very high frequency, 10s - 100s of MHz, you probably won't see it on a 10 MHz BW scope. If it oscillates at lower frequencies, you should be able to see it clearly, even without injecting a signal from your sound card. Try shorting the input of the amplifier and look at the output with the scope. If the amp is working properly and your grounding is done well, you should see just a tiny bit of noise on the amplifier output. If the amp is oscillating, you will probably see some large amplitude waveform like a sine or square or something more complex. You may see some evidence of poor grounding in the form of a 60 and / or 120 Hz wave of probably complex shape. That is what makes a buzz or hum in the speaker.

I_F

[markiemrboo]

Originally posted by I_Forgot
A sine or square wave IS oscillation. It sounds like you are interested in seeing whether your amp is oscillating due to instability.

Ah yes, sorry. This is what I meant!

If your amp is oscillating at a very high frequency, 10s - 100s of MHz, you probably won't see it on a 10 MHz BW scope.

I saw someone state that a 100MHz scope was good for up to about 10MHz, so is what I have probably roughly good for about 100KHz?

I've also read that such high frequency oscillation can destroy tweeters apparently. After how long might you expect this to happen if I were to play at a fairly loud volume constantly?

If it oscillates at lower frequencies, you should be able to see it clearly, even without injecting a signal from your sound card. Try shorting the input of the amplifier and look at the output with the scope. If the amp is working properly and your grounding is done well, you should see just a tiny bit of noise on the amplifier output. If the amp is oscillating, you will probably see some large amplitude waveform like a sine or square or something more complex. You may see some evidence of poor grounding in the form of a 60 and / or 120 Hz wave of probably complex shape. That is what makes a buzz or hum in the speaker.

I_F

Oh I see! Well I believe my grounding is ok. AndrewT was very helpful there and I can't hear anything at all coming from the speakers with the inputs shorted

I'll give that a go and see if there's anything to see there anyhow, thanks very much!

[lndm]

Originally posted by markiemrboo
I saw someone state that a 100MHz scope was good for up to about 10MHz, so is what I have probably roughly good for about 100KHz?

By this reasoning, your scope would be good up to 1MHz. However, you should be able to see 100MHz on your scope, it just might appear a little shorter on the screen than it actually is.

If your amp is prone to oscillation, it may need a push to get it started. An oscillation is the repeated amplification of something and if there was no something to begin with, an unstable amp may sit there like a stone. Often the noise that all amps have is enough to trip an oscillation, sometimes a turn on thump and sometimes it just takes a while to warm up. Sometimes touching the inputs will do it and leaving them open may help in this respect.

An oscillation usually happens at one specific frequency. It is usually higher than 20kHz. It usually causes full scale deflection so that you can either see it cover the oscilloscope screen from top to bottom, or you can see the amp's upper and lower limits on the screen usually in the form of a slightly brighter region at the top and at the bottom.

[I_Forgot]

An oscillating amp will destroy a tweeter in a matter of seconds or even less. Crossover networks to tweeters have high-pass response (that's why it works as a crossover) so it will easily pass the high frequency oscillation, including one that may be well above audible range. Tweeters are made with thin wire coils to keep the mass down so they can respond to high frequencies. Music doesn't normally require a lot of power in the highest registers, so a tweeter in a speaker system rated for 300 Wrms may only be rated for 1 or 2 watts. An oscillating amp can easily deliver quite a bit more than the 1 or 2 watts required to cook the tweeter.

I_F

[markiemrboo]

Originally posted by I_Forgot
An oscillating amp will destroy a tweeter in a matter of seconds or even less. Crossover networks to tweeters have high-pass response (that's why it works as a crossover) so it will easily pass the high frequency oscillation, including one that may be well above audible range. Tweeters are made with thin wire coils to keep the mass down so they can respond to high frequencies. Music doesn't normally require a lot of power in the highest registers, so a tweeter in a speaker system rated for 300 Wrms may only be rated for 1 or 2 watts. An oscillating amp can easily deliver quite a bit more than the 1 or 2 watts required to cook the tweeter.

I_F

Wow! I didn't realise it was that bad. Looks like I am most likely OK for any serious oscillation. I've been playing it at around half volume (~20W?) for about about 3 or 4 days straight now, and the tweeter isn't in ashes yet.

Cheers again I_F.

[lndm]

Originally posted by markiemrboo
half volume (~20W?)

Just FYI (if I didn't misunderstand you), a 40W amp with the volume control turned half way should need much less than 20W on average. This is because of the logarithmic nature of both the control and our ears.

[Netlist]

Apart from the useful info here, have a look at this scope tutorial.
https://www.cs.tcd.ie/courses/baict/bac/jf/labs/scope/

/Hugo