scope procedure for volts/div dials question

[deci belle]

I've been fiddling around and wondering what standard procedure is for monitoring the two sine waves (one from the sigGen and one from the amp.

For example, I have ch1 from the amp set to 1 volt/div and ch2 from the sigGen set to .1 volt/div. Now they overlap very closely when I dial in the amplitude from the sigGen to fill out the scope's screen.

That was a good exercise— but I don't know how relevant it was …I wonder what considerations I am not aware of that would determine how I should be using the two volts/div dials when comparing traces.

[Tom Phillips]

Hi again,
Good procedure opening a new thread. I was wondering what you were doing about a scope.
There is a lot of literature available. A book comes to mind titled "Audio Measurements" by Norman Crowhurst. The original was published in 1958 but there is a reprint put out by Audio Amateur Press beginning in 1998. I think it has lots of good info on using your test equipment. Someone might even have a scan published.
Cheers,
Tom

[nashvillebill]

There's a couple of Crowhurst books available for free downloading here Technical books online though I didn't specifically see the "Audio Measurements" one. There's some good o-scope books, though they are a tad dated.

I don't pay lot of attention to the exact voltage readings I see when I look at a scope, it's the shape of the wave I'm generally interested in. (As long as I don't have the volts/div set so sensitive that I'm picking up extraneous noise that might be confused for a signal)

Note also: I keep the scope probes on 10x. Very important on tube amps!

[deci belle]

Oh boy! Thanks for the titles!! I have three books so far:

"Audio Equipment Tests", by Gordon King 1979
"Troubleshooting with the Oscilloscope", by Robert Middleton 1962 & 1988
"How to Use Oscilloscopes and Other Test Equipment", by R. A. Penfold 1989 & 2010
—but they haven't clicked for me much yet. I'll look at these too!

Ya, Bill— I'm using a 10x and now I feel comfortable around my 470v plates, and thanks for the link!!

I don't know what I'm looking at yet, to be honest…

[Tom Phillips]

There's a couple of Crowhurst books available for free downloading here Technical books online ...

I recommend any books you can find by Norman Crowhurst. He published in the 50's & 60's mostly and he had a very good way of explaining how tube circuits functioned.

[deci belle]

Yup! I downloaded the 3 volumes!

[Gtr_tech]

You can cap couple the probe if in doubt. I'll do this via the clip end probe tip attachments with a .047 cap. Cheap insurance. Granted DC won't get thru so you can't use the scope to gage DC voltage, but I never do that anyway....

[deci belle]

Oh great, Gtr-tech, cuz I was wondering about that— also, the sigGen manual said to do that when using square wave. Thanks!!

[Enzo]

On my scope, ch2 has a regular probe hanging from it. Ch1 I have a cable terminated in dual banana connected to it. That is compatible with the posts on my load panel. So at any time I can flip over to Channel 1 and watch what is on my loads or bench speaker. And I also have a couple posts connected to the output of a small stereo receiver, so i can drive speakers with a couple watts of audio. Most of the time, I leave ch1 plugged into that stereo receiver output, just for eye candy. Customers come into the shop and there is some waveform dancing on the screen. OOOOh, very scientific looking.

Once in a while I might compare input and output signals, but mainly I look at outputs just to see what is happening. Clipping or not? COmplete waveform or not? Crossover distortion? Other distortion? Noise or ripple?

And with the flip of a switch I am looking at the probe in ch2. That is what I use to look around in the circuit.

If I WANT to compare two waveforms, I can put the scope on dual and set the two displays to the same size and location on screen. If they were perfectly identical, they'd overlap and look like one. ANy distortion, and the one would be peeking out from behind the other. On the other hand ther could be times when I'd rather they were separate, so I'd make both smaller and watch one in hte top half of the screen and the other in the bottom half.

I usually run everything with AC coupling. If ther is no DC offset in the circuit where I am looking, then AC and DC coup;ling will look the same. If there is DC offset, then AC coupling can ignore it and just watch the waveform. A good example of that is ripple on a DC supply.

I mainly use DC coupling in low voltage circuits, in particular logic or control circuits, which are all about DC. For example the Fender footswitch decoding circuits. Set my scope up for 5v/div (Yes, remembering I am using a X10 probe) and centered. NOw +15v is three divisions up from center, and -15v is three divisions down on the screen. Now as I click the switches on the amp, I can watch the circuit nodes switching from +15 to ground or +15 to -15 or whatever. You probably won;t be doing much logic work, but little TTL gates going high and low is made for scope display.

You set the vertical gain on your scope to suit what you are looking at or for. Of the display is going way off the screen top and bottom, turn it down. SImple as that. And if you last looked at 30v on a speaker, you will have to turn up the gain to see the half a volt input signal. On the other hand, don't turn it up so high you start picking up radio stations.

As long as you can get the scope trace to sweep continuously, let's not worry about sync just yet.

And horizontal rate - sweep rate. You can slow it down to something stupid like 1 second per division. A bright spot will slowly cross the screen. Each clockwise click of the sweep rate control makes it move faster. Your sweep rate knob probably says TIME/DIV for time per division on the screen. The faster you set the sweep, the faster the spsot will move, and then the spot will start to look like a line. That line will flicker, until you get up to a certain speed that makes it look steady. Somewhere around 2ms/DIV is where that flickering stops and it looks steady to me. Until I need to resolve some other image, I usually watch audio on the slowest sweep rate that doesn't flicker to my eyes. Someone else may feel differently.

You can turn it up faster, but doesn;t have to go too fast before you can no longer see an entire waveform cycle on the screen. Ther may be situations where this is useful, but in general I like to see at least one complete cycle on the screen, and usally several. Try it, set your sweep to 1ms/DIV and your audio to 1kHz - (1000Hz.) Should wind up with about 1 complete cycle per screen division. Now turn up the sweep speed and watch those sine waves get wider, but fewer of them visible. Keep going. Soon you no longer see sine waves, you see a line slowly curving upwards or downwards. And at that point soon you will find you have no idea what you are looking at. High sweep speeds like that are useful for viewing RF oscillation or something and resolving it into an actual waveform, and even calculating its freq. But mostly it is pointless up there for audio. Try it with music too. Music is a nonrepeating waveform, so the result will be a bunch of wiggling lines stretched way out.

One of your channels probably has an invert button. Like a phase reverser on a mixer channel. Turns the waveform upside down. You won;t need to do this much, but it demonstrates some principals. Earlier I mentioned setting two waveforms up with the exact same size and all so they could overlap. Well most dual channel scopes have a mode switch which lets you select ch1 or ch2, or dual display. But there is also usually a fourth step called ADD. WHat that does is add two channels together into one trace. Input two identical sine waves in phase and add them adn you get a sine wave twice the size of either one. Now press the invert button. that channel is now reversed in polarity, so when ch1 goes positive, ch2 then is going negative instead. ADD them together and they should exactly cancel. COnnect ch1 to the amp output, and ch2 to the amp input, then ADD them, but inverting the one. Now if the amp perfectly reproduced the input at the output, then other than amplitude, they should be the same, so setting the amplitudes on the screen equal and adding them with one inverted means the one should exactly cancel the other. There would be a flat line. Not useful? Any difference betwen the inout and output would result in some non-zero point on the screen. SO what you now would se would be the distortion ONLY.

That is probably way past what you will be doing for a while, but it is cool.

Oh, and X-Y. That connects ch1 to vertical and ch2 to horizontal. No sweep in this mode. If both get exactly the same signal, and the gains are the same, you get a 45 degree line on the screen, with amplitude aiming at the corners. It gets interesting when the two signals are not the same. Like left and right of a stereo signal. It will get all thredimensional on your ass. Just don;t leave the bright spot sitting still in the center of the screen for any length of time

[Gtr_tech]

The X-Y mode is useful for tape machine head alignment and wow/flutter checks. Useful for testing the video outputs on Asteroids boards too

[deci belle]

Customers come into the shop and there is some waveform dancing on the screen. OOOOh, very scientific looking.

hahaha!!

Sheer scope-n00b nirvana!! Thanks, Enzo and Gtr-tech!!

[Enzo]

AH yes, is there a game tech from our era that didn;t run Asteroids on their scope at some point?

I once put the scope in XY and connected it to the sweep circuits of a video monitor, intending to get a crude raster display on the scope. Almost worked. Got a crude and skewed image of some sort. Not good enough to want to try it since.

I agree, tape head alignment is probably the one thing I might use it for. To the extent tape decks will remain in use.

[deci belle]

I did try all those things you said. The ADD feature was cool. I'd used the invert button yesterday, cuz the 5f6a doesn't get inverted and I wanted to match the waves. My scope doesn't have the XY feature that I can tell …is it really fun?!! haha!

[Enzo]

Deci, using a scope is like playing a guitar. We can point out the basics, just like I can show you how to tune a guitar and where some basic chords are. Then practice and experience will teach you the ever widening vista of scope use. Right now the trigger functions are just a way for you to get the thing showing a picture, but eventually they will become more useful for syncing up display to something.

You may have a "Z-axis" input on the rear. It actually has some uses, though I can;t recall needing it ever, other than just to play with it. That input modulates the brightness of the trace spot. SO apply a sqaure wave there and the trace blinks on and off at that rate. If you know the sweep rate, then the number of blinks per sweep will tell you the frequency. Or allow you to match two exactly. I never have needed it, but it is something more to have fun with.

[g1]

The x-y setting is likely the one marked CH.B at the far end of the time/div. knob. As mentioned by others, aside from setting azimuth on tape decks, it will not get much use. The top row of the figures here: Oscilloscope Phase Shift Measurements - Oscilloscope Measurements Techniques - Electronic Circuits & Tutorials is what you would see when performing the azimuth adjustment.
For fun, try scoping the 2 outputs of a stereo sound source playing stereo program material, like a cd player. Scope the left ch. with one probe and the right ch. with the other. Switch to x-y mode. You should get a "fireball" effect, not sure if that's the right name for it.
Something to dazzle the customers, like Enzo mentioned. Don't know why they never use that in TV/ movies, anytime I've ever seen a scope in use on TV it's showing a sine wave.