Digital oscilloscope for high school use

[rsk]

My new school has a Gwinstek oscilloscope rather than the old fashioned electron tube type.

It's not the first time I've found one of these in a school and so far, I've never been able to make them usable or useful for high school students.

Has anyone managed to use one successfully to display eg sine waveforms from a signal generator, or to demonstrate the difference between dc from a cell and ac from a powerpack?

I have always found that the signal looks very noisy, so much so that the noise swamps the trace I'm trying to see.

Am I doing something fundamentally wrong or are these just not suitable for the purposes for which I'm trying to use them? The most advanced thing we'd do with it would probably be to watch capacitors charging and discharging - this is for MS/HS use.

This is the model we have https://www.tme.eu/en/details/gds-1052-u/digital-oscilloscopes/gw-instek/ - I think the very similar looking one I had in another school might have been a different brand but causing me the same problems.

 

[berkeman]

It seems like it should work fine. How old is it? Was it purchased new or used? If used, can you talk to the person who had it before to see if they had similar problems?

Do you see the same noise on both channels? Have you calibrated your 'scope probes? What signal sources do you have available?

I didn't read the manual, but is there a Bandwidth Limiting (BWL) feature? If so, you could try turning that on to see how much it improves the noise issue.

Can you probe a 1Vpp sine wave and post a picture of what you see?

 

[rsk]

Thanks for replying.

I will have a look for the probes. At present it comes with a cable which has two crocs at the end. I did have a look at the manual (online) but then didn't ahve time to set it up again. The one I saw in another school did come with the probe type attachment and it's almost certain that this one did too. It'll be stashed in a storeroom somewhere. I'll have a look.
I also watched a few YT videos yesterday and they talk about the black terminal on the probe being always connected to earth. I'm not sure how that works with what I'm trying to do.

What we do at school tends to be:
Connecting to a microphone to observe sound waves
Connecting to a signal generator which is also connected to a speaker to observe sound waves
Connecting to the terminals of a battery and to the ac output of a power pack to show the difference between ac and dc
Connecting in a capacitor circuit (with R only, no inductance in our syllabus so no LCR) to view the charging/discharging traces.
Connected to a coil, then dropping a magnet through the coil to see the induced trace.
(More rarely) using it with a transformer type set up to show the phase difference in the primary and the induced traces

Previously i've tried it with signals from the signal generators - this week I was attempting to see the ac/dc traces.

To answer the other questions, I imagine it was purchased new but it's often the case that fancy physics kit is ordered and then never used, sometimes because there isn't a physics teacher to use it, sometimes because there isn't time to spend on the setting up. I am new in this school so this is my first atempt to use this particular model, but I've had similar issues in the past with another one. The students here tell me their previous teacher wasn't keen on practical work so it's possible it hasn't ever been used.

I suspect it's just my ignorance but i don't think these are particularly designed for school use and the manuals aren't designed as dummies' guides for high school teachers.

When I posted initially I was thinking there might be other HS teachers using these who could help but I suppose it's far more likely that some of you in Further Ed / Universities might be familiar with them and can give me some pointers

 

[Gordianus]

Can you post a drawing that shows how the signal generator was connected to the oscilloscope? If possible use different colors.

 

[vela]

I also watched a few YT videos yesterday and they talk about the black terminal on the probe being always connected to earth.

When you mentioned this quirk, I looked at the picture @berkeman posted more carefully because the scopes we have at one of my schools do the same thing. I think they're the same model. I'll look more carefully at exactly which model we have when I go to campus tonight.

I'm not sure what the thinking behind shorting the ground connection between the two channels was, but it pretty much renders the second channel useless in most scenarios. You definitely don't want to connect the grounds to different nodes your circuits.

I found reading the manual, which is very brief, was helpful, mainly to learn about how to reset the scope completely and to have it automatically choose settings to display a signal.

 

[DaveE]

I'm not sure what the thinking behind shorting the ground connection between the two channels was, but it pretty much renders the second channel useless in most scenarios.

This is by far the most common configuration in multichannel oscilloscopes. It's normal.

If you need isolated probes, that's usually a separate box, and isn't simple to do well. Those probes tend to be expensive if they are any good.

 

[berkeman]

This is by far the most common configuration in multichannel oscilloscopes. It's normal.

I agree. The only 'scope that I've ever used with two floating channels was a Tek battery power 'scope with 2 special floating BNC inputs.

I once knew a fairly newbie EE who blew up a probe on his personal oscilloscope at home when he tried to probe the AC Mains output of one his wall outlets. He didn't realize that the outer shield ring connection for the probes is hard-tied to Earth ground. Boom!

 

[vela]

This is by far the most common configuration in multichannel oscilloscopes. It's normal.

I guess it's been so long since I used oscilloscopes regularly that I had forgotten that.

 

[Gordianus]

The second channel Is far from being useless. It's just a matter of finding a common node for both signals.

 

[berkeman]

Many/most signal generators will have a grounded BNC output connector, BTW.

 

[DaveE]

BTW, I'll just add that it's a good thing. This configuration is the best, easiest way to getting low noise, high quality measurements.

There are few things in the analog EE world as important or complicated as probing circuits with instruments. Lots of App notes have been written about this; if you look it's all out there.

 

[DaveE]

He didn't realize that the outer shield ring connection for the probes is hard-tied to Earth ground. Boom!

If y'all think the solution to this is to cut off the ground pin on your scope power cord, we need to talk... Hopefully before you collect crappy data, destroy your lab equipment, or kill yourself (maybe all at the same time if you're talented).

 

[Vanadium 50]

EE who blew up a probe on his personal oscilloscope

So, when I was a postdoc, one of the students was tasked with determining how much HF ripple was on one of the multi-kV supplies. Rather than find the blocking cap, he plugged the HV supply straight into one of the inputs.

<bzzt>
"Hey, this channel's dead!"
<bzzt>
"Hey, this channel's dead too! What kind of knucklehead leaves a scope out with two dead channels?"
<bzzt>
"Hey, channel 3 is dead too...."

I think he's a dean or something somewhere now.

 

[Vanadium 50]

A few years later, we had a break in an HV cable, and to fund where, I needed to send a signal down it and look at the reflection. Obviously after disconnecting it and draining any residual charge. So, I needed to make a cable to do this - BNC on one end, and SHV on the other.

The student I was working with wanted to know why we couldn't just get a pre-made cable like this. The answer is, of course, that the whole system is set up to keep us from doing exactly what we were doing. By the same token, I destroyed the cable immediately after we we were done with it. "What if we need it again?" "Then we have the 30 minutes it takes to make a new cable to think really, really hard if this is what we want to do."

The moral of this story is that for HS students you want cheap and robust. They will destroy the scopes. The best you can do is slow them down and to mitigate the impact.

 

[rsk]

Thank you all for your replies. It's 0710 here and I've just got to school so will make time to read them properly later.

I'm old, and used to the old electron tube scopes which connected via a coaxial cable with two plugs on the other end, and have never had to think about grounding them. The probes which come with these have a red clip and a black needle if I remember rightly (as above, I haven't found it yet) - can I not use these in the same way? I think I've had multimeters in the past with similar probes and we just ignored those and used our standard school connecting cables.

So, for @Gordianus who asked about a picture, all I did was connect the co-ax end to the schope and the two crocs at the other end to the output of the signal generator. I tried similar when connecting them to the battery and to a power pack - so I haven't even thought about grounding. I'm getting this all wrong, aren't I?

PS I'm sad to see the demise of the old style scopes and TV tubes, there was good physics in there!

 

[rsk]

It's working it's working it's working!!!!!!!!!!!!!!!!!!!!!

More later, Y11 are coming in 2 minutes

 

[Gordianus]

I've seen too many noise measurements due to broken cables or bad connectors. I'd check them first.
As for the ground (third leg of the mains plug), IT MUST ALWAYS BE FULLY FUNCTIONAL. My apologies for the caps in the previous sentence.

 

[rsk]

An update

First, practising without children present: to display the difference between the dc output from a battery and the ac output (plus rectified dc) from the powerpack - autoset gave a nice clean ac trace and also displayed the battery trace nicely. (Deeply embarrassed that I didn't just try autoset to begin with)

With children present: worked fine again but then when I tried reversing the battery connections, "what do you think will happen, kids?" the trace showed positive as before. Re-pressing autoset gave me a nice sine wave again - from a battery -and then a vertical line(?). I clearly have some way to go in understanding how this thing works. By the time I got to showing them the "dc" output from the powerpack, they assumed that the funny looking definitely-not-a-horizontal-line trace was the oscilloscope playing up, rather than the nature of the dc output.

Regarding the cables. I think the cable I have is far older than the oscilloscope and probably belonged to either the signal generator or an older oscilloscope. I would imagine that a 2 channel oscilliscope would come with two cables but can't find them anywhere. Our lovely lab tech is going to see if he can get some locally. On a positive, I confirm that the plug has 3 pins (we do find here that sometimes the plug has 3 pins but is used with a 2-pin adaptor.....)

 

[tech99]

My new school has a Gwinstek oscilloscope rather than the old fashioned electron tube type.

It's not the first time I've found one of these in a school and so far, I've never been able to make them usable or useful for high school students.

Has anyone managed to use one successfully to display eg sine waveforms from a signal generator, or to demonstrate the difference between dc from a cell and ac from a powerpack?

I have always found that the signal looks very noisy, so much so that the noise swamps the trace I'm trying to see.

Am I doing something fundamentally wrong or are these just not suitable for the purposes for which I'm trying to use them? The most advanced thing we'd do with it would probably be to watch capacitors charging and discharging - this is for MS/HS use.

This is the model we have https://www.tme.eu/en/details/gds-1052-u/digital-oscilloscopes/gw-instek/ - I think the very similar looking one I had in another school might have been a different brand but causing me the same problems.

I use a traditional CRO because I like to explain about the electron beam as part of the lesson, and show the effect of a magnet. It is many years since I tried a digital scope in a classroom and I found a slight processing delay which meant that the image did not relate to the sound being displayed.

 

[berkeman]

With children present: worked fine again but then when I tried reversing the battery connections, "what do you think will happen, kids?" the trace showed positive as before. Re-pressing autoset gave me a nice sine wave again - from a battery -and then a vertical line(?).

If you can link to a video of you doing this we may be able to help find your error(s). I would also discourage you using Autoset on an oscilloscope -- it's much better to learn enough about the controls to be able to just set all the configurations for the measurement you want to make.

One of the things I pride myself on is being able to set all of the dials/knobs/buttons on a curve tracer before turning on the trace. Many folks turn on the trace and then fiddle with all of the adjustments trying to get the trace they want. If you understand how the instrument works, you should be able to set everything up before first turning on the trace/display, IMO...

https://w140.com/tekwiki/wiki/577

 

[DaveE]

I would also discourage you using Autoset on an oscilloscope -- it's much better to learn enough about the controls to be able to just set all the configurations for the measurement you want to make.

Yep. I would say go ahead with autoset first. But then you'll have to figure out what it did and fix what you don't like. You'll probably find out eventually that it doesn't add much value. This is especially true if you are making a series of measurements. Autoset has a habit of changing all of the stuff you didn't want to change.

 

[rsk]

I use a traditional CRO because I like to explain about the electron beam as part of the lesson, and show the effect of a magnet. It is many years since I tried a digital scope in a classroom and I found a slight processing delay which meant that the image did not relate to the sound being displayed.

Oh, I'd love to have an old CRO one - but unfortunately I need to make the most of what I have.

If you can link to a video of you doing this we may be able to help find your error(s). I would also discourage you using Autoset on an oscilloscope -- it's much better to learn enough about the controls to be able to just set all the configurations for the measurement you want to make.

One of the things I pride myself on is being able to set all of the dials/knobs/buttons on a curve tracer before turning on the trace. Many folks turn on the trace and then fiddle with all of the adjustments trying to get the trace they want. If you understand how the instrument works, you should be able to set everything up before first turning on the trace/display, IMO...

View attachment 334482
https://w140.com/tekwiki/wiki/577

Thanks and yes - I'd love to be able to get to this stage. It's a question of finding the time though.

Yep. I would say go ahead with autoset first. But then you'll have to figure out what it did and fix what you don't like. You'll probably find out eventually that it doesn't add much value. This is especially true if you are making a series of measurements. Autoset has a habit of changing all of the stuff you didn't want to change.

I have vague recollections of attempting to display sound waves though a digital oscilloscope at a previous school and finding that the time base was automatically reset each time so that the changing frequency wasn't obvious enough to show students (who, at that stage, aren't looking at reading the scales).However, given my frustrations with the thing last week, I feel like we've made huge strides and I'd like to find time this week to look at it with capacitor charge/discharge and also with the magnet through a coil thing - both of this are current topics with some of my teaching groups. (This will mean playing with the trigger setting, I think???)

 

[DaveE]

This will mean playing with the trigger setting, I think?

yes

 

[rsk]

Look!

 

[berkeman]

Nice!

A couple comments: You are displaying Ch2 (blue) which is where you are connecting your input signal, but you are triggering on Ch1 (yellow). That is shown in the status menu on the right of the display. Also, the trigger point (for Ch1) is set slightly above the waveform (see the little yellow triangle on the right edge of the graticule). You should switch your trigger source to Ch2 and adjust the level to put the little blue trigger level triangle in about the middle of the waveform.

Keep it up!

 

[berkeman]

Also, I don't know if you've used it yet, but the "Measure" button in the upper control block will let you make time or voltage measurements, as well as some other kinds of measurements. Maybe check it out if you haven't already.

 

[rsk]

Nice!

A couple comments: You are displaying Ch2 (blue) which is where you are connecting your input signal, but you are triggering on Ch1 (yellow). That is shown in the status menu on the right of the display. Also, the trigger point (for Ch1) is set slightly above the waveform (see the little yellow triangle on the right edge of the graticule). You should switch your trigger source to Ch2 and adjust the level to put the little blue trigger level triangle in about the middle of the waveform.

Keep it up!

I know! I tried to set the trigger and lost it completely so switched to Channel 2.

But.....I had taken the little blue triangle to be the centre line, that's how I centre my trace.
I might not get time today but will have another look at the trigger thing.

Also, I don't know if you've used it yet, but the "Measure" button in the upper control block will let you make time or voltage measurements, as well as some other kinds of measurements. Maybe check it out if you haven't already.

I haven't tried that yet - I've been looking at the numbers on the screen - so I'll have a look later this week.

I did (of course) try the run / stop button to freeze that trace once I got it.I''m still hoping to find somewhere a 2nd cable, would be nice to display both R and C voltages together

 

[berkeman]

I know! I tried to set the trigger and lost it completely so switched to Channel 2.

One other tip -- To use the trigger level setting function, you need the channel's coupling to be "DC", not "AC". At least in my experience that is true. So if you were having trouble getting the trigger to work reliably on Ch1, check the coupling for that channel. In fact, on my 'scopes, they won't even show the little trigger level triangle when the channel coupling is set to "AC".

 

[rsk]

Now that I have a better idea of what I'm doing (or at least, a bit more confidence) I'll have another good look at the manual and help videos. I might be in a better position now to relate what they are doing to what I want to do.

 

[rsk]

Good news, today while looking through a box of (apparently unused) Vernier sensors we have, I found two oscilloscope cables! They are the type with the probe but will do the job nicely.

But, while looking through those Vernier sensors (photogates, light sensors, pd, pH etc) we have found that we can't connect them to the interfaces (GoLink and LabQuest) they came with. They look as though the connectors are on backwards.

It's been a long week and it might be that I'm missing something here, but have a look at these - they don't fit, do they?

 

[berkeman]

, I found two oscilloscope cables! They are the type with the probe but will do the job nicely.

Your picture is not of oscilloscope probes...

 

[rsk]

No, these are the Vernier connectors

 

[berkeman]

One other subtlety that you may already know about -- When you use a straight cable (BNC or BNC to clip leads), you set the Coupling for that channel to 1:1, and when you use a 'scope probe, you set it to 10:1 (unless it auto-senses the probe).

Also, if you are coupling a 50 Ohm output signal generator via coax into your 'scope, set the input impedance to 50 Ohms for that 'scope channel. If you are using a 10:1 'scope probe, you will set the input impedance to 1MegOhm.

 

[rsk]

One other subtlety that you may already know about -- When you use a straight cable (BNC or BNC to clip leads), you set the Coupling for that channel to 1:1, and when you use a 'scope probe, you set it to 10:1 (unless it auto-senses the probe).

Also, if you are coupling a 50 Ohm output signal generator via coax into your 'scope, set the input impedance to 50 Ohms for that 'scope channel. If you are using a 10:1 'scope probe, you will set the input impedance to 1MegOhm.

Thanks, I saw mention of the x10 in the probe on one of the videos I watched but had forgotten it.

The signal generator and the scope both have coaxial connections so that will mean either a bit of fiddling or a different cable.

 

[DaveE]

I found two oscilloscope cables!

Congratulations! I guess. After decades of using oscilloscopes, I have no idea what an "oscilloscope cable" is.

have a look at these - they don't fit, do they?

What??? Are you asking us if things that you are actually holding in your hand fit together? Or, fit what?
Electronics isn't an entirely trial and error exercise. Some reading and/or research is nearly always required. Hint: mechanical fit isn't always the only consideration.

Anyway, what's your question here? I can't figure out your post at all.