DCinDC's Oscilloscope Training Thread

[DCinDC]

Please don’t post in this thread.

If you have questions, want a clarification, caught a mistake etc, please post in the DCinDC's OScope Training FEEDBACK thread.

- If a point needing clarification is brought up, I’ll add it to this thread.

- If there is a mistake I’ll correct it in this thread.

- If additional functions on a scope need to be explained, I’ll post them in this thread.

- If you have any comment at all, please post it in the feedback thread.


The intent is for this thread to be a static training point. People can refer to a given post in this thread. If there are intermingled replies, then the post count will change, and throw off pointers.

If you post in this thread I will ask you to delete it and post in the feedback thread.

Please don’t post in this thread.

 

[DCinDC]

WOW!
Lots of people on here have scopes and don't know how to use them!

Hopefully we will fix that!

The first posts will explain what an oscope does, what the controls are on an oscope, and what they do.

We WILL move on to how to practically use your scope to troubleshoot your audio gear. No need to ask.

We WILL cover what you need to look for when buying an oscope and how good of an oscope do you need. No need to ask.

We WILL cover subjects that are brought up in the feedback thread.

I hope you all enjoy and learn! :wave:

 

[DCinDC]

How to use an Oscilloscope

Intro:

An oscilloscope displays Voltage vs Time on a visual screen.

It does not measure voltage vs frequency; that is done by a spectrum analyzer. Get this distinction.

Scopes display voltage vs time instantaneously. The time it takes for the trace to travel across the screen is what allows you to view waveforms. Remember, whatever is on the left of the screen happened before whatever is on the right of the screen. At very high speed.


A scope doesn't measure anything, you do.

An oscilloscope is a precision display device, but operator interpretation of the display is critical to get exacting measurements.

Once you learn how to correctly set up your oscilloscope, you will be able to take measurements directly from the screen.

Modern scopes have many controls, and can appear daunting, but once you realize that you only need to concentrate on a few basic controls, it’s much less complicated.

 

[DCinDC]

Controls

All scopes have similar controls, since they all basically do the same thing. Some have extra features, but it's all related to the same principle; displaying voltage vs time on the screen.

The screen is what you will spend time looking at, so let’s start there.

The screen of the scope is covered with a 'graticule' which is a transparent cover with a graduated grid, typically divided into 1 cm squares. It is these squares or "Divisions" that the controls refer to when marked /Div.

Some scopes have an “Illumination” control for a light source that illuminates the graticule markings. Handy in darker rooms.


Intensity
This sets the brightness of the trace. ALWAYS turn down the intensity when not actively looking at a waveform. If not you could burn the face of the CRT's phosphor and have a permanent 'ghost' image on the screen.

Focus
This sets the focus of the trace. You should only need to adjust this once a session, if at all. Just make the trace look crisp with sharp edges.

Vertical position
Typically this control is set with no input so that the sweep goes across the vertical center of the screen. Line it up behind the center graticule line.

Horizontal position
This control is set so that the sweep reaches from one side of the screen to the other.
You can also use it to shift the display over when counting cycles of a waveform.

* These next two are the heart of the scope, and what you will be adjusting the most:
Vertical gain
The vertical amplifier gain control sets exactly how much voltage causes vertical deflection of a given distance, typically 1 CM.

In other words, volts per centimeter, vertically on the screen. This control typically ranges from 1 mV to 1 V per centimeter.

Horizontal timebase
The horizontal sweep timebase control sets how fast the trace travels across the screen. Clockwise rotation makes the sweep faster. This control typically ranges from 1 Second/CM all the way up to 1 uSec per CM.

If you turn the timebase control far counterclockwise you will be able to watch the trace travel across the screen as just a dot moving left to right. As you turn it clockwise, it goes faster, and due to the persistence of the screen, and your eye's inability to see really fast moving items, it appears as a line on the screen.

Setting of the Horizontal/Timebase control is critical to your ability to see the waveform properly. If set too slow, you cannot see high frequency waves, as their individual cycles. Instead it appears as a wide bar across the screen. Anytime you see a wide bar on the screen, turn the timebase control clockwise until you can clearly see a few cycles of the waveform.


Cal knobs
These knobs are typically in the center of the vertical gain and horizontal sweep controls. Sometimes they labeled VAR for variable.
For almost all instances, we want these turned to their detents, so that the gain or timebase selection can be read directly from the markings. When turned away from their detents they vary the setting of the main control. But the display is no longer Cal’d, and you cannot take measurements accurately.

 

[DCinDC]

Measuring voltage:
AC voltages displayed on an oscope screen are shown as peak to peak, unlike a multimeter, which measures average voltage. There are mathematical conversions you need to make between the two.

DC voltages are shown as their absolute voltage. No conversion is necessary.

To measure DC voltage;
First, let’s assume that you centered the trace with no input to the scope, on the middle line of the graticule. In other words, centered top to bottom on the screen.

Then connect your probe to the DC voltage to measure. The line will move up on the screen for positive voltages, and down for negative voltages. Ground is the same as the middle of the screen.

Now count the number of CMs on the graticule that the line moved. Then multiply by the V/CM vertical gain control’s setting.


EG: 1 cm times 1 mV = 1 mV

 

[DCinDC]

Measuring frequency:
A screen trace that shows just one cycle of an AC waveform allows you to measure frequency.

To measure frequency;
Count the number of horizontal cm on the screen for one complete cycle. If you adjusted the timebase control to display just one cycle on the screen, and you have 10 cm horizontally, then 1 cycle times 10 cm = 10. Then multiply by the timebase setting. This is the period of the waveform. Then get the recipricol (divide 1 by the period). That is the frequency.

EG: 10 cm times 1 uSec = 10 uSec (the period of the cycle), then divide 1 by 10 uSec
(.oooo1 uSec) = 100,000 cycles per second = 100 kHz.

For exacting frequency measurements, a frequency counter is better, but knowing the frequency of what you are looking at, verifies you are looking at the right waveform.

 

[DCinDC]

It's time for a pic for us visual males!

That's a Tek 465, one of the finest scopes ever made. Mostly due to a GREAT CRT and enough features to be useful but not overwhelmingly complicated.

I posted this pic cause it shows about one cycle of a sine wave on the screen. From left to right, it goes negative, then positive, then returns to the same voltage level as when it started. One cycle.

In a perfect classroom the waveform woulda started at the zero centerline, and gone positive from there, but I can't complain.

We could change the point on the cycle where the scope starts displaying it by twiddling the level triggering control, but we'll get to that later.

Judging by where the timebase switch is, (looks like the .1 mS setting) that's about a 120 Hz sine wave.

 

[DCinDC]

Measuring AC Voltage

Above I said that conversions must be done to the peak to peak voltage shown on the scope's screen to arrive at an average voltage rating, such as would be referred to in a service manual or from a multimeter. I want to clarify a few things:

The actual signal hasn't changed. You measure it with a scope and get 1 volt peak to peak. You measure it with a multimeter and less. This is because the multimeter measures in a way that is analogous to heat created with that waveform, vs DC voltage. The scope shows the actual waveform and doesn't infer anything about it.

The conversions are simple. If you care to, read up here:
http://en.wikipedia.org/wiki/Amplitude#Peak-to-peak_amplitude

http://www.bcae1.com/voltages.htm

Great reading at the second link there.

You should be exposed to this stuff and even know it, but I'm not gonna teach it to you. Here's the Navy's presentation on the subject:
http://www.tpub.com/neets/book2/1f.htm

That's measuring AC voltage.

For audio purposes, which are never a sine wave (unless testing) peak to peak is how we measure, because the signal is never repetitive or constant.

 

[DCinDC]

Buying a scope:

I could write a whole 'nother thread on this. I have posted in a few scope purchasing threads with my thoughts before. Here they are again.

- If you can afford a NOICE scope and want one, buy one. Don't let me stop you.

- If you can't, pay attention!

Scopes are pretty easy to build and calibrate. Used things cost less.

I use a component tester very often, that uses a simple oscope for it's display. That cheap scope which cost all of $15 off of ebay gets FAR more use than my Tektronix 2213, which is not really a very expensive scope either.

The big scopes with Mil Spec designations like AN/USM-425 are likely gonna cost you more than a comparable off brand scope.

Hitachi, B&K,etc. There's plenty of other brands with nowhere near the brand desirability of Tektronix. Save yourself some money!

I've used the best scopes ever available while in the service. I'm completely happy with my crappy cheap ones.

NEVER buy a tube scope. They are heavy, and they break. If you disagree, then you have the time and skills to fix it I assume. Or are masochistic.

As a hobbyist, I think you should pay $100 or less for a working scope and then get on with learning how to use it. There are other big expenditures you will need to make; soldering stations, signal generators, workbenches, and PARTS.

Don't worry so much about cal on a used scope. DO make sure it works, has a bright trace that can be well focused, and the switchgear is not broken. You can fix the rest of the problems it might have, or work around them.

Probes:
are expensive. Try to get a decent one with your scope. Bought seperately they can be quite expensive. If you have a Tektronix scope with the Volts/Div light that moves when a X10 probe is used, try to get the probe with the pin that contacts the ring around the BNC input connector, that enables this feature.

Edit: a reader pointed out to me that new probes can be had cheap on ebay. Like 12$ a pair shipped. I bought some and they are nice.

 

[DCinDC]

Triggering:

Somewhere on your scope is a Triggering switch, a polarity switch, and a level control. Sometimes the level control has zero at it's center with neg CCW and pos CW.

The "Trigger" refers to when the trace will start across the screen IN REFERENCE to the waveform being measured.

The switch determines the triggering mode. It usually has Line, AC, DC etc settings. Line means that the sweep is triggered by the AC line. So basically it's going to have a trigger and a trace at all times, because the AC line is supplying the triggering at a 60 Hz (USA) rate.

DC means it triggers on a DC level change, and AC means it triggers on the AC component of the waveform being measured.

For audio, and most other general purpose use, just put it on AC. (not to be confused with LINE which is the AC Line).

The level control is very handy.

In the pic in post #7 above we see a sine wave on the screen. But it starts at the positive peak and goes down from there.

So obviously the level control is not in the center, but at the max positive position. So the sweep starts when the waveform hits it's max positive peak. If you blow up that pic, you can see the level control at max CW. It is the outer knob above the BNC connector in the lower right hand corner of the scope. and the polarity switch is in the middle of that knob, leaning over to the positive (CW) side.

If we adjust the Level control to about the middle of it's rotation, and we have the polarity switch on Positive, then the displayed sine wave would start on the left hand side of the screen at the zero ref line, head up, then down, then back up to the ref line (zero).

<insert pic>

 

[DCinDC]

Axis:

X is across the screen, horizontally.
Y is up and down on the screen, vertically.
Z is brightness of the trace. Towards you and away from you THROUGH the screen, shown as bright or dark parts of the trace. Away would appear dark, towards would be brighter. This is sometimes used to highlight important events within the waveform of digital signals. Not all scopes have trace modulation inputs (Z axis)

But now you know....

 

[DCinDC]

How it works reference:

If you want to know how a scope works inside this is a great link. I won't bother to repeat it.

Wiki oscope link

 

[DCinDC]

Awful pic we can use as a start to pointing things out.....