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Using Scopes

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Using Scopes

Post by Terrykc » Thu Jul 25, 2013 11:39 am

A point to remember about scope traces is that they are a bit like pictures of the scale of an AVO. Absolutely meaningless to the rest of us unless we are told which range the AVO is set to!

In the case of a scope, we need the details of both scales, horizontal and vertical as well as the clearest possible picture. This usually means filling the screen as far as possible with meaningful data.

Try this.

    Start off with all variable controls at their calibrated positions and then

  • For line measurements or to display video at line rate, set the timebase to 10µS/Div. This equals 100µS across the width of the screen and displays one full line (405) or one and a half lines (625).
  • For field rate measurements, set the timebase to 2mS/div (20mS total or one field).
  • Set the input sensitivity for the largest amplitude display which fits entirely between the top and bottom lines of the graticule, adjusting the vertical position, if necessary.
  • Adjust trigger controls as required for a stable synchronised trace.
  • Record timebase and sensitivity settings (and if a ÷10 probe is used) for all trace photographs.

Use these settings as a starting point for all future measurements.

 
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Using Scopes

Post by Terrykc » Thu Jul 25, 2013 8:46 pm

Furthermore there are various ways the controls can be laid out and combined but there are several basic settings:

  • Trigger slope +ve/-ve,
  • Trigger Level
  • Some form of Trigger Filter for optimum triggering from high or low frequency signals.
For example. Some scopes include a TV Sync separator as although, paradoxically, it may not always be the best setting on some waveforms!

Internal/External Trigger - usually including the option to select an internal mains lock - and, for internal lock on multi-beam scopes, which channel to trigger from.

You need to play with these to get used to how the controls and switches interact.

The most misunderstood setting is the Trigger Level - particular if you are familiar with TV where you expect synchronising to be automatic! The trouble is the scope doesn't know anything about the waveforms it sees whereas a TV is designed to work with only two precisely designed ones ...

This is a video signal;

Trig1.png
Trig1.png (9.47 KiB) Viewed 3222 times

The red line shows the Trigger Level. Obviously the scope is going to trigger randomly and repeatedly on the next bit of video which crosses the threshold. If the level was completely above or below the waveform, the timebase wouldn't trigger at all, of course.

What we need to do is set the Trigger Level here:

Trig2.png
Trig2.png (3.63 KiB) Viewed 3222 times

Now a locked trace should appear but what happens if the signal is now inverted ...?

Trig3.png
Trig3.png (3.66 KiB) Viewed 3222 times


The trouble is that there isn't any helpful red line on your scope to show you where the correct trigger level is - you have to learn how to find it for yourself!

The easiest way to learn is to play around until you are happy - it should become automatic after a while! For example those that have them, connect your scope to the video output of the Aurora, so that you know what signal you should see. With the trace locked showing one line, note the difference when you change the slope setting from +ve to -ve then change the timebase to display a complete field. Note the difference between the line and field trigger settings.

It is a great shame to think that members may have such an excellent diagnostic tool available to them but it just languishes in a corner gathering dust for lack of knowledge on how to use it properly.

Perhaps a practical 'how to use an oscilloscope' session would be a good idea at all these meet-ups!

 
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Re: Using Scopes

Post by Katie Bush » Thu Jul 25, 2013 9:49 pm

Terrykc wrote:
It is a great shame to think that you have such an excellent diagnostic tool available to you but it just languishes in a corner gathering dust for lack of knowledge on how to use it.

Perhaps a practical 'how to use an oscilloscope' session would be a good idea at all these meet-ups!



That could help a lot of us... :aad

I tried to find a night class at the various and many colleges in my area.. Not one of them could help.

Basket weaving, Home DIY plumbing/electrics/etc, computers for beginners, Chemistry, Physics, Biology, and so forth - you name it, they had a course, but something useful like how to fly an oscilloscope... Not a chance. :bbd

I would have thought that would have been incorporated in the Physics course? Nope. :ccb

So... Who here has the pilot's licence with the most hours on it?

Marion

 
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Re: Using Scopes

Post by CTV » Thu Jul 25, 2013 10:02 pm

Terrykc wrote:Chris, how are you adjusting the trigger?


I've been using the level on the trigger

scope.jpg

 
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Re: Using Scopes

Post by Marconi_MPT4 » Thu Jul 25, 2013 11:31 pm

oldticktock wrote:
Terrykc wrote:Chris, how are you adjusting the trigger?


I've been using the level on the trigger

Chris, how is the trigger Holdoff control set? Put simply this control allows a delay time to be set before the timebase can re trigger after the first event. Usually it is set to minimum i.e. no delay but being able to hold off the next trigger is very useful when trying to lock onto complex waveforms such as for example RS232 signal packets and composite video etc.

Getting a Tek scope to trigger properly on some waveforms has always been an art in itself!

Rich

 
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Re: Using Scopes

Post by CTV » Fri Jul 26, 2013 7:19 am

ppppenguin wrote:I think we need to split some of this into a new thread on using a scope.


Done

 
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Re: Using Scopes

Post by CTV » Fri Jul 26, 2013 7:53 am

In case members are not aware there's a great 3 hour video on Scopes for Dopes by Alan Wolke (Tek) available via the main website http://www.radios-tv.co.uk/?q=node/32 The hand-out that accompanied the lecture is also in the Technical Library.

Below scope locked to the video output of the Aurora. Looks better/clearer than it photographs.

Aurora.jpg

 
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Re: Using Scopes

Post by Terrykc » Fri Jul 26, 2013 2:48 pm

ppppenguin wrote:That's how it should look though I reckon you've got the trigger polarity as +ve so you can't see the sync pulse at the left edge of the trace ...

The trigger polarity or slope may need further explanation for clarity. It relates to the direction of the waveform that passes through the trigger level and causes the timebase to trigger. Signals of the opposite polarity are ignored.

Note the direction of the slope (square waves are never exactly square as it takes a finite time to change level, resulting in a - usually - imperceptible slope during the transition) of the sync pulses here:

Trig2a.png
Trig2a.png (4.29 KiB) Viewed 3126 times

Trig2c.png
Trig2c.png (9.62 KiB) Viewed 3126 times

One line of a 405-line signal takes just under 100µS, so will look like this if -ve slope is selected:

Trig2d.png
Trig2d.png (4.21 KiB) Viewed 3126 times

and like this with +ve slope:

Trig2e.png
Trig2e.png (4.19 KiB) Viewed 3126 times

Usually, of course, there won't be a colour burst!

Note the missing edge at the left hand side of the screen. This is because the timebase takes a finite time to trigger, by which time the rapid signal transition has ended although some very sophisticated scopes include a short memory so that the signal at the trigger point itself can be displayed.

 
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Re: Using Scopes

Post by Refugee » Fri Jul 26, 2013 11:00 pm

I have given this a little thought.
For a first time owner of a scope I would start by demonstrating a basic sine wave and a 9V battery.
V/CM can be demonstrated with the battery by setting the Y amp to o.1V/CM with a X10 probe connected and DC coupling set. Move the trace to the bottom of the screen and dab the battery on and the trace should rise by just over 9 divisions. Try again with the Y amp set to 0.5V/CM and it should rise by just under two divisions.
Then set the input coupling to AC and connect the probe to either the output of a small isolating transformer or better still an audio sine wave generator.
Set the V/CM to give 4 or 5 divisions of amplitude and set the trigger control to mid position. Set the timebase to display one or two cycles of sine wave.
You should be able to "shift" the sine wave with the trigger level control. A square wave or sync pulse will not do this. Now use the shift control to move the trace to the right so that the start of the trace can be seen and try the trigger again and watch the start of the trace and note that if you try to move the trace so that the start is at the peak it will loose lock as you turn the trigger level beyond this point on both the positive and negative peaks.
The trigger polarity switch will change the start of the trace from rising to falling so that it shifts one complete half cycle according to the setting.
As a guide 50Hz sine wave will display best at 5MS per DIV and 1KHz at 0.2MS per DIV and so on.

 
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Re: Using Scopes

Post by Terrykc » Sat Jul 27, 2013 11:33 am

Refugee's post highlights a very important and much overlooked point. Most, if not all, folk who turn up on here with a high grade scope and no driving lessons want to use it to diagnose a complicated fault on a complex piece of equipment.

This is equivalent to giving someone who has never driven a high performance car and sending them off for their first driving lesson to a busy Motorway where nothing, except for articulated lorries, travels slower than 75 mph! Furthermore, their only access to advice/instruction is via text messages! (To give this analogy some semblance of credibility, we will assume a very clever phone with speech to text and text to speech facilities is used for the text function ...)

Refugee's sequence of directions do appear to be an excellent guide for a novice to learn the basic operation of a number of features on any available scope. What we need now are some guinea pigs - any volunteers? -to go through this sequence and comment on them. With any corrections for clarity, etc. and possibly reformatted into a list of bullet points for ease of reference, it could form a very useful permanent reference.

Note that there is no reference to timebase speed until the very end. This hints at a follow-up guide which could demonstrate the use of different speeds for various applications after the basics have been absorbed ...

 
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Re: Using Scopes - Probe Adjustment

Post by Terrykc » Sat Jul 27, 2013 1:37 pm

The use of ÷10 (or 10:1) probes has been mentioned but what many users will not realise is that a probe must be correctly adjusted before use. Failure to do so can render it worse that useless in many cases - the real danger being that the user won't actually realise what is going on.

Why use a probe?

In the figure below, the equivalent circuit of a typical scope is shown. It will look like a resistor of 1M in parallel with a capacitance of about 30pF. Many applications incorporate high impedance, high frequency circuitry which makes the loading effect of any test equipment very important.

A high impedance probe

At first glance, adding a 9M resistor in series with the input will increase the impedance to 10M and create a simple ÷10 probe. The problem is that this only works at DC ...!

It is the input capacitance that is the killer, progressively shunting the input signal as the frequency increases. To make the probe work it must be both a resistive and capacitive potential divider, so that the loading on the external circuit is constant at all frequencies.

The answer seems simple enough: just add a suitable capacitor across the 9M resistor to create a 10:1 capacitive divider - but it isn't that simple ...

The 30pF scope input capacitance is a typical value which will depend on a lot of things, including the physical construction of the scope, which will obviously vary from one instrument to the next. There is also the capacitance of the screened cable between the scope and the probe to be considered (which is actually in parallel with the input).

This is resolved by making the probe capacitor variable, so that it can be set to the precise value needed to match the scope it is connected to. The net result of this is to make the input impedance of the combination, as seen from the probe tip, to be 10M in parallel with ~3pF.

Scope_input.png


Probe Adjustment

Look for a calibration output on your scope and connect your probe to it. Adjust the controls to display 2 - 3 cycles on the screen about 4 or 5 divisions high.

The adjustment will usually be via a screw head accessible through a hole in the probe housing. Adjust this carefully to display a perfect square wave on the screen as in (a), below.

If the probe capacitance is low, the scope input will shunt the higher frequencies as at (b).

If it is too high, the high frequencies will be accentuated, causing the edges to overshoot (c).

Stop when you have achieved the best possible result.

It is worth bearing in mind that, if you have a dual input scope, the two inputs may vary slightly. Assuming you have two probes, adjust one probe using input A and the other using input B. Then, if necessary, mark the probes clearly to ensure that each probe is only used with the input for which it has been correctly set.

Square_wave.png
Square_wave.png (9.26 KiB) Viewed 3074 times

 
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Re: Using Scopes

Post by Terrykc » Sat Jul 27, 2013 1:54 pm

ppppenguin wrote:... How do you think I know that a trailing earth lead will often find somewhere that will do damage :aa

Ah! Do you mean that you use the probes that don't have clip-on earth leads ...?

The problem with a thread like this is that it is very easy to jump so far ahead that the novice, at whom it is aimed, is left bewildered.

Perhaps you should start an 'Advanced' thread ...? :bba

 
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Re: Using Scopes

Post by Terrykc » Sun Jul 28, 2013 4:18 pm

Whilst the concept of setting the vertical sensitivity to a level which allows the signal to be finely examined should be self explanatory, setting the timebase speed to display the wanted waveform or, more correctly, the wanted portion of the waveform, seems to cause considerable confusion.

Almost always, this results in only a small portion of the wanted waveform being displayed. This is particularly so if the focus of attention shifts from one part of the set to another.

To analyse line problems, for example, a setting of 10µS/Div is suitable, especially for 405-line work as the total screen width of 10 divisions occupies 100µS, almost exactly equal to one line period.

If moving on to the field timebase, what happens if the scope speed settings are unchanged?

Assuming the trigger settings are adjusted to correctly lock to field rate, only the first 100µS of the field will be shown. However, one field occupies 20mS - 200 times as long! As a result, 99.5% of the information is thrown away and the result is useless. Note that, as field rate and mains frequency are the same, this applies equally to fault finding hum problems.

The timebase is calibrated in terms of time but most people are more comfortable with the concept of frequency - which is the reciprocal of time. Furthermore, the calibration refers to one division, rather than screen width, which adds further confusion.

Perhaps this table will help. It shows the timebase settings in time/division in the first column. The second indicates the frequency of a displayed signal if one complete cycle is displayed per division but the third column if likely to be of greater assistance as it shows the frequency if one complete cycle occupies the entire screen width.

Thus we know that 405-line horizontal frequency is ~10kHz. Looking for this value in the third column, then referring back to the first column shows that a timebase speed of 10µS/Div will neatly fit one line across the screen.

For mains or field examinations, the frequency is 50Hz. The table shows that 2mS/Div will be an ideal setting.

Time/Div1 Div = 1 cycle of10 Div = 1 cycle of
0.05 µS20 MHz2 MHz
0.1 µS10 MHz1 MHz
0.2 µS5 MHz500 kHz
0.5 µS2 MHz200 kHz
1 µS1 MHz100 kHz
2 µS500 kHz50 kHz
5 µS200 kHz20 kHz
10 µS100 kHz10 kHz
20 µS50 kHz5 kHz
50 µS20 kHz2 kHz
0.1 mS10 kHz1 kHz
0.2 mS5 kHz500 Hz
0.5 mS2 kHz200 Hz
1 mS1 kHz100 Hz
2 mS500 Hz50 Hz
5 mS200 Hz20 Hz
10 mS100 Hz10 Hz
20 mS50 Hz5 Hz
50 mS20 Hz2 Hz
0.1 Sec10 Hz1 Hz
0.2 Sec5 Hz0.5 Hz
0.5 Sec2 Hz0.2 Hz
1 Sec1 Hz0.1Hz

 
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Re: Using Scopes

Post by Red to black » Sun Jul 28, 2013 5:09 pm

A very useful post Terry ^^^ :aad

Although I have been driving a scope for more years than I care to remember, putting it into words that other people can easily understand is no mean feat!
You have put it far more succinctly than I ever could.

I mainly only used two timebase settings (line and field) the vast majority of the time when in 'TV/VCR repair mode', only really venturing outside of those settings when faced with setting up CD players.

Having said that my scope (Hameg) is relatively simple to use, but I did not really need anything more exotic or sophisticated for day to day TV/VCR repair use. :aaq

I think a lot of us forget some people on the forum are relatively new to such equipment(compared to us 'oldies') and have acquired far more sophisticated equipment that has fallen dramatically in price in real terms, than I could realistically afford at the time.

Still a very useful Thread. :aad

 
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Re: Using Scopes

Post by sideband » Sun Jul 28, 2013 6:39 pm

Red to black wrote:I think a lot of us forget some people on the forum are relatively new to such equipment(compared to us 'oldies') and have acquired far more sophisticated equipment that has fallen dramatically in price in real terms, than I could realistically afford at the time.

Still a very useful Thread. :aad


I think that is a very true satement! I have a simple analogue scope that I got from Philips probably 30 years ago when they were upgrading all the old testgear. For normal TV work it was more than adequate and still is and will display any TV waveform I'm likely to need. At the time, such a scope would probably have been around £100 on the second-hand market.....way above what I could pay. Now of course you can pick up really good storage scopes for about the same or even less from ebay and such places.


Rich


PS. I've made this a 'sticky'


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