Finally got me a 'scope - Have some questions

[(*steve*)]

I had a brief look for a manual and couldn't find one. I think it's well worth searching out the manual -- even if you have to pay for it.

You got that for a great price, I hope it makes up for the disappointment of the previous one.

 

[KrisBlueNZ]

What's the story with your scope Steve? Did you fix it?

 

[(*steve*)]

What's the story with your scope Steve? Did you fix it?

Yep, it's all happy and healthy now,

 

[Supercap2F]

Well, I spent some more time looking for the manual... And I couldn't even find one for sale.

When I power up the 'scope in a different room and try to adjust the probe compensation, there is some noise on the signal (but not as much as before, with the leader).

Here's some photos:
Regular trace when not adjusting probe compensation.

Trace when trying to adjust probe compensation.
Earlier you guys said it might be something to do with the ground pin... Is there a way to test that? Also, we have about 2 computers, a laser printer, and a battery backup running in the room near by (sometimes we have 4 computers running, but when I took the pics there were only 2 running). Do you think that could be messing up the ground?

Thanks!
Dan

 

[morphingstar]

Re screwdriver used for probe: Did you use a metallic screwdriver or plastic non-magnetic non-conducting? Your floor carpet might induce static. Ground your hands at the scope chassis / metal case.
I did not see that you took these precautions, hat is why I am asking.

 

[KrisBlueNZ]

Well, I spent some more time looking for the manual... And I couldn't even find one for sale.

Can you set up a permanent filter/notification on eBay? So if one comes up in future, you'll hear about it?

It looks like the scope is picking up mains-frequency and other interference when you adjust the compensation trimmer. As morphingstar said, make sure you use a completely plastic adjustment tool.

 

[Supercap2F]

Well I don't have a plastic adjustment tool.. So I rapped some electrical tape around the metal screw driver I was using, and it took most of the noise away. Then I placed my hand on top of the scope so I would be grounded and the rest of the noise disappeared! So was the carpet the problem after all?

Can you set up a permanent filter/notification on eBay? So if one comes up in future, you'll hear about it?

Great Idea Kris! I'll have to do that.
Dan

 

[KrisBlueNZ]

Then I placed my hand on top of the scope so I would be grounded and the rest of the noise disappeared! So was the carpet the problem after all?

Not necessarily. The scope responds to signals that it measures relative to its 0V rail, which is connected to its case, and to the earth pin on its power plug. That is then supposed to be connected to the soil through the mains earth and the earthing stake at the house, and the house itself (which is at least partly conductive) should be at roughly the same potential.

But if there's a break in the earthing of the scope, and the scope's case and 0V rail is coupled to the mains voltage and/or some interference source through capacitance and/or leakage resistance, the scope will "see" interference and hum on things that are actually properly earthed. If you connected the probe to a wire out your window to a stake driven into the ground, it would show that hum and noise.

So you should check the earthing system of your house, and check for continuity from the scope case to the earth pin on its power plug.

It's quite possible that the scope is alright and there's some other problem. In either case though, I think you should investigate. It could be an indication of a potential or future danger that could cause an electric shock or a fire.

 

[Supercap2F]

Not necessarily. The scope responds to signals that it measures relative to its 0V rail, which is connected to its case, and to the earth pin on its power plug. That is then supposed to be connected to the soil through the mains earth and the earthing stake at the house, and the house itself (which is at least partly conductive) should be at roughly the same potential.

But if there's a break in the earthing of the scope, and the scope's case and 0V rail is coupled to the mains voltage and/or some interference source through capacitance and/or leakage resistance, the scope will "see" interference and hum on things that are actually properly earthed. If you connected the probe to a wire out your window to a stake driven into the ground, it would show that hum and noise.

OK, that makes since.

So you should check the earthing system of your house,

How would I do that? Is there some sort of equipment I would need?

and check for continuity from the scope case to the earth pin on its power plug.

When I did that I got about 0.3 ohms. Is that what's expected?
Thanks!
Dan

 

[KrisBlueNZ]

Yes, 0.3Ω is pretty much a direct connection.

I'm not sure what to suggest regarding checking your home earthing system. I guess, start by finding the earth stake and inspecting it visually. Then get a long piece of wire and attach one end to the stake, take the other end to the room with the mains socket, and check resistance from the earth pin on the mains socket to the wire. (You might want to check for voltage between them first, in case there's some high voltage that might damage the meter on ohms range.)

If the socket earth is properly earthed, stand on your carpet in bare feet, and measure AC voltage between your body and the mains socket earth pin.

Use your scope to look at the interference by just holding the probe end. Adjust the timebase and try to find out the frequencies of the signals it picks up. Some will be at 60 Hz I guess. There may be some at 120 Hz too. And it looks like there's something at a higher frequency. Try to identify the source(s). If there are any appliances nearby, try turning them off, unplugging them, or pulling the fuse or popping the circuit breaker for them.

Otherwise, get an electrician to do a proper check. Also you could try to find a forum used by electricians (rather than electronics people) and ask there.

Edit: You could also pop the cover off the mains socket (open the circuit breaker first) and check for spring contacts that are bent too wide so they don't make good firm contact with the pins, and/or dirty or corroded. Usually you can just clean them, sand them, and bend them back, but you might want to replace the socket.

 

[(*steve*)]

Damn, it didn't occur to me that you had a screwdriver attached to the probe when you got those readings.

So three lessons learned:

1. Get a sparkie to check the power outlets
2. Don't use a metal screwdriver as an adjustment tool
3. If you must use a metal screwdriver, make a small adjustment, then take it away to see the effect

 

[Supercap2F]

Yes, 0.3Ω is pretty much a direct connection.

So does that mean the scopes just peachy?

I'm not sure what to suggest regarding checking your home earthing system. I guess, start by finding the earth stake and inspecting it visually. Then get a long piece of wire and attach one end to the stake, take the other end to the room with the mains socket, and check resistance from the earth pin on the mains socket to the wire. (You might want to check for voltage between them first, in case there's some high voltage that might damage the meter on ohms range.)

Well I don't know where our ground rod is, and my Dad does not either... So I guess I can't do those checks.

If the socket earth is properly earthed, stand on your carpet in bare feet, and measure AC voltage between your body and the mains socket earth pin.

Would it be OK to do that without knowing if it is earthed right?

Use your scope to look at the interference by just holding the probe end. Adjust the timebase and try to find out the frequencies of the signals it picks up. Some will be at 60 Hz I guess. There may be some at 120 Hz too. And it looks like there's something at a higher frequency. Try to identify the source(s). If there are any appliances nearby, try turning them off, unplugging them, or pulling the fuse or popping the circuit breaker for them.

When I did that I saw a 60Hz sine wave with a greater then 1MHz sine wave superimposed on it.

Otherwise, get an electrician to do a proper check. Also you could try to find a forum used by electricians (rather than electronics people) and ask there.

Aggh! I cringe at the thought of spending all that money on a service call. I think I will ask on a electricians forum!

Thanks guys, for helping me get set up with a 'scope!
Dan

 

[KrisBlueNZ]

So does that mean the scopes just peachy?

Yep.

Well I don't know where our ground rod is, and my Dad does not either... So I guess I can't do those checks.

It might be a good idea to find out. It's important for the safety of everyone in the house. Over here, they're usually just outside an outside wall, or possibly under the house somewhere near the switchboard, if you have a crawl space.

(re measuring voltage between your body, standing on the carpet, and the wall socket ground) Would it be OK to do that without knowing if it is earthed right?

Yes, that's safe, if you have a digital multimeter with high input resistance (they're usually 10 MΩ). That will limit any current that might flow.

When I did that I saw a 60Hz sine wave with a greater then 1MHz sine wave superimposed on it.

Are you near any AM radio transmitters?
This is getting outside my experience.

Aggh! I cringe at the thought of spending all that money on a service call. I think I will ask on a electricians forum!

Good idea!

 

[Supercap2F]

I was thinking on this and why would am I only seeing that noise on the signal when I'm adjusting the probe compensation? If there was something on the ground wire, wouldn't it appear on all the waveforms I'm measuring? Regardless if I was adjusting the probe compensation or not.

It might be a good idea to find out. It's important for the safety of everyone in the house. Over here, they're usually just outside an outside wall, or possibly under the house somewhere near the switchboard, if you have a crawl space.

OK, I will do some research and ask around and see if I can't find out.

Yes, that's safe, if you have a digital multimeter with high input resistance (they're usually 10 MΩ). That will limit any current that might flow.

Well I checked, and my meter does have a 10MΩ input resistance. So I measured it and got about 1VAC.

Are you near any AM radio transmitters?
This is getting outside my experience.

There's a broadcasting station about a mile away... I don't know if they do AM or FM or both though.
Dan

 

[(*steve*)]

I was thinking on this and why would am I only seeing that noise on the signal when I'm adjusting the probe compensation? If there was something on the ground wire, wouldn't it appear on all the waveforms I'm measuring? Regardless if I was adjusting the probe compensation or not.

This is conjecture, but I would expect that the side of the capacitor connected to the adjustment screw is likely to be grounded. If the ground is floating, any signal injected here will not be be shunted to ground.

 

[kpatz]

The noise when you touch the calibration trimmer with your metal screwdriver is just RFI/EMI your body is transmitting into the probe via the metal screwdriver. A lot of it is 60Hz from the electrical wires in your room. Higher frequencies could be from electronic devices like TVs, computers, fluorescent light ballasts, etc. Humans make good antennas, which is why this happens, and why you can make your TV rabbit ears work better or worse by moving around in the room or holding one of the antennas.

 

[(*steve*)]

The question is not so much why there's noise, but why there's more noise when the scope is used in one room. Of course there is the possibility that there's more ambient noise in that room but I think that has been discounted.

 

[kpatz]

Noise can vary from location to location depending on where live electrical wires are relative to your scope and body, presence of electronics such as compact fluorescent lights or computers, etc. In some spots two electrical lines could augment 60Hz noise, and in other spots cancel one another out reducing the noise. An interesting test would be to hold the scope probe in your hands touching the tip so the scope displays the noise, then move about the room (within the limits of the probe cable's length of course) and see how the noise increases or decreases depending on where you are.

The quality of the ground (earth) could be different between the two locations/outlets as well. If the scope is set to a low V/div (high sensitivity) it won't take much of a difference in noise levels to make a bigger difference on the screen.

 

[KrisBlueNZ]

Good idea kpatz. Also, try standing on something that's a good insulator - for example a big block of expanded polystyrene.

You need to understand that the scope measures the signal at the probe tip RELATIVE to its 0V rail, which is connected to its chassis, and to the earth pin on the mains plug (which you've already verified). (It's also connectd to the earth clip on the probe.) So if you touch the probe tip and get an indication on the scope, this indication will consist of signals your body is picking up that are relative to earth, AND signals that are present on the scope's chassis if it's not perfectly earthed.

For example if there's an appliance that is injecting noise into the earth conductor in that room (the appliance need not be in the same room, because mains sockets are daisy-chained), the scope will show that noise, even if the probe tip is perfectly earthed.

This is why I suggested earlier that you should look for all appliances in the house that use significant amounts of power and/or could generate interference and turn them OFF individually to see what effect it has on the scope display.

You really just need to understand what the scope does, then try any experiments you can think of to try to determine the nature and the source of the noise components.

 

[Supercap2F]

I finally got some time to do some tests!

This is conjecture, but I would expect that the side of the capacitor connected to the adjustment screw is likely to be grounded. If the ground is floating, any signal injected here will not be be shunted to ground.

That makes since. So if the adjustment screw is not grounded then all that noise would show up when I try to adjust the probe comp. So I measured the resistance between the adjustment screw and the ground clip, and I got a open circuit. Do you think that may have been the problem after all?

An interesting test would be to hold the scope probe in your hands touching the tip so the scope displays the noise, then move about the room (within the limits of the probe cable's length of course) and see how the noise increases or decreases depending on where you are.

Unfortunately, when I did that I couldn't see any difference in the noise level.

You need to understand that the scope measures the signal at the probe tip RELATIVE to its 0V rail, which is connected to its chassis, and to the earth pin on the mains plug (which you've already verified). (It's also connectd to the earth clip on the probe.) So if you touch the probe tip and get an indication on the scope, this indication will consist of signals your body is picking up that are relative to earth, AND signals that are present on the scope's chassis if it's not perfectly earthed.

OK, I understand.

For example if there's an appliance that is injecting noise into the earth conductor in that room (the appliance need not be in the same room, because mains sockets are daisy-chained), the scope will show that noise, even if the probe tip is perfectly earthed.

This is why I suggested earlier that you should look for all appliances in the house that use significant amounts of power and/or could generate interference and turn them OFF individually to see what effect it has on the scope display.

I went around the house unplugging stuff and then looking at the 'scope, but I couldn't get anything to affect it. Not even the microwave.

You really just need to understand what the scope does, then try any experiments you can think of to try to determine the nature and the source of the noise components.

I think that I have a good nohow of how the 'scope works... But then again, I have been wrong a lot before.

I really appreciate all you guys helping me out!
Dan