Oscilloscope mini-tutorial.

[DrNomis_44]

Hey everyone,

Just thought that I would do a new mini-tutorial thread for the DIY Pedals section, so here we go.....


Most of the time, when we are building our DIY pedals, and we end-up having to do some trouble-shooting to get our pedals working properly, we can usually get by with a multimeter, and our deductive reasoning, to sort things out.

But, occasionally we might come across a fault in our pedal builds where we need some way of seeing how our pedals are behaving electrically so we can narrow-down where the fault is most likely to be, this is where a piece of test equipment called an Oscilloscope comes in handy, because it gives us a way to see how an electronic circuit is actually behaving, by drawing a graph on a cathode-ray screen, or an LCD screen if you're lucky enough to own a more modern Oscilloscope.

To start with, here's a pic I took of the front-panel of my old DSE Q-1804 20Mhz Dual-Trace Oscilloscope, I have had this Oscilloscope since about 1998, or so:



Here's a close-up pic of the Oscilloscope's controls:




It's actually not as intimidating as it looks....


Starting at the top row, and going from left to right....

Intensity.....This controls how bright the display is.

Focus...This is used to focus the trace to a thin line across the cathode-ray screen.

Trigger Level....This control is used to adjust the triggering-level so that you can get a stable display on the screen.

Coupling....This controls the trigger coupling, most of the time I usually have it set to either Normal, or Auto.

Source....This controls where the Oscilloscope gets it's trigger signal from...Ch-A, Ch-B, Line, or External.

Holdoff....This control is used to help stabilize the display.

Mode....This control is used to put the Oscilloscope into different modes....Main, Mix, Delay, and X/Y.

Position.....This control is used to position the display horizontally left or right, if you pull the knob, that puts it into X10 magnification mode.

Next row of controls.....

Power On and Illumination...This is pretty self-explanatory, turns the Scope on or off, there's also an illumination function that illuminates the grid markings (called graticules) on the screen, very handy in low lighting.

Ch-A Position....This control shifts the Ch-A Trace vertically up and down, it also engages an alternate trigger function when the knob is pulled.

Ch-B Position.....Same as for Ch-A except that it controls the positioning of the Ch-B trace, does not have the alternate trigger function.

Delay Time Position....Controls the position of the delayed Trace.

Delay Time/Div.....Controls the delay time per division sensitivity.

Next row of controls...

Ch-A Coupling....This controls the Ch-A coupling....AC, Gnd (Ground), DC.

Ch-A Volts/Div......This controls the sensitivity of Ch-A.

Vertical Mode....This controls the vertical mode of the Scope.....Ch-A, Ch-B/X-Y, Dual, and Add.

Ch-B Volts/Div....Same as for Ch-A, but for Ch-B.

Ch-B coupling....Same as for Ch-A.

Ext. Trigger.....This is an external input for a trigger signal, useful for when doing frequency response tests of a circuit.

Bottom row.....

Ch-A Input....Self-explanatory.

Component Test....This can be used to test some electronic components, like resistors, caps, diodes, and transistors.

Gnd.....This is a simple grounding-point.

Cal/Beam Find.....The Cal terminal puts-out a 2Vpp Square Wave signal that is used to calibrate Oscilloscope probes that are switched to X10......Beam Find is used to find where the trace is if it is off-screen.

Ch-B Input....Self-explanatory.

Main Time/Div.....This sets the main Timebase for the onscreen display, each horizontal cm graticule corresponds to the Main Time/Div. setting.


More to come, so stay tuned.....

[JohnH]

This is great Doc, looking forward to more.

[jugglindan]

This is great, thanks. I have been eyeing some USB scopes as a reasonably cost-effective option. Second-hand would be OK too, but they don't come up on Gumtree that often.

[DrNomis_44]

This is great, thanks. I have been eyeing some USB scopes as a reasonably cost-effective option. Second-hand would be OK too, but they don't come up on Gumtree that often.

Yep, you can get some pretty decent usb scopes online, I think Jaycar Electronics stock them as well.

[Marcel]

Awesome Doc.
And now for the good bit... how each control works....

[DrNomis_44]

Awesome Doc.
And now for the good bit... how each control works....

Yep...lol.....I'll deal with it tomorrow......

[DrNomis_44]

Ok, we had a look at all the controls on my DSE Q-1804 20Mhz Dual-Trace Oscilloscope, and I explained what each of them do, lets look at how you go about setting-up the Oscilloscope so you can actually use it to do some serious fault-finding in an electronic circuit, for a signal-source, I'm going to see if I can use my Acer laptop and my Scarlett 2i2 2nd Gen usb audio interface, and I'm going to see if I can find a free Function Generator app on the internet that I can download and run on the laptop, I'll also post some screenshots of what I see displayed on my Oscilloscope's screen.


Found a good app that should do the job, it's called SigJenny, and best of all, it's free to download.


OK, the first thing you want to do when you get yourself an Oscilloscope in good working condition, is to make sure you have some good probes to go with it, if your Oscilloscope happens to be a single-trace type that has a 5Mhz, or 10Mhz bandwidth, it'll be perfectly usable for audio work, and you'll only need one good probe, you can buy 100Mhz X10 probes from Jaycar Electronics for around $30, you might be able to buy some cheaper ones on eBay, since I needed two new probes for my Oscilloscope, I bought them from Jaycar.

Assuming that your oscilloscope is in good working condition, plug it into a convenient wall power socket, making sure that the Oscilloscope is turned-off first, next, connect your probe/s to the Oscilloscope channel inputs, again if your Scope is a single-trace you'll only have to connect one probe up to it, and then switch the Oscilloscope on.

After letting the Scope warm-up for maybe 5 minutes, adjust the Intensity, Focus, Trace Horizontal Position, and Trace Vertical Position controls till you see a thin horizontal line going across the Scope's screen roughly about midway, as shown in this screenshot:





You might need to set the main Time/Div. control on your Scope to something like 2ms or 1ms/Div to get the thin line if you see a small bright dot moving from left to right.


Note: Make sure that the input coupling switch is set to Gnd while you're adjusting the trace so you see a thin line.

[jugglindan]

I had always assumed I would need a bench top signal generator. That's a great idea to use the computer. Audacity can generate sin waves too I think.

Sent from my moto g(7) using Tapatalk

[DrNomis_44]

I had always assumed I would need a bench top signal generator. That's a great idea to use the computer. Audacity can generate sin waves too I think.

Sent from my moto g(7) using Tapatalk

Yep, you can pretty much use anything that can generate a steady signal, even a mobile phone that's running a signal-generator app will do, as long as the signal is steady.

[DrNomis_44]

OK, once we have a thin line displayed on the Oscilloscope's screen, the next thing we need to do before we start using our Scope is to adjust the Scope-probe's compensation when it is switched to X10 mode, the reason for doing that is to prevent any measuring errors when we are doing signal-amplitude measurements while the probe is switched to X10 mode.

To adjust the X10 mode compensation for the probe, connect the tip of the probe to the Cal terminal, and the probe's grounding lead to the ground point, then switch the X1/X10 switch on the probe to X10, and the Input coupling switch to either AC, or DC, then find the little adjustment tool that came with your Scope probe, next, find the little adjustment hole on your Scope probe, it'll either be on the probe itself, as in my case, or it'll be on the plug that connects to the Scope's input, then you use the adjustment tool to adjust the probe's compensation till you get a nicely-shaped square-wave displayed on the Scope's screen, you might see the following on the Screen:




Although it looks like we are seeing a square wave signal being displayed, notice how the top and bottom of the signal aren't quite straight?, this indicates that the probe is under-compensated, we need to adjust the compensation so that the top and bottom are straight and the corners are sharp 90 degree angles.


This is what you should be seeing once the X10 compensation on the probe has been properly set:




Once you have the X10 compensation for one probe properly set, do the same for the other probe, if you're using one probe with a single-trace Oscilloscope, you're finished with setting the compensation and your Scope is ready for use.


You might also see this on your Oscilloscope's screen:



Notice how the top and bottom of the square wave signal are bent inwards?, this indicates that the probe is over-compensated and it needs to be adjusted so that the top and bottom are straight.