Author Topic: Scope separate cahnnels and grounds  (Read 2504 times)

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Offline igendel

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Scope separate cahnnels and grounds
« on: March 05, 2015, 11:37:52 am »
Hi all,

I apologize if this has been answered here before but I couldn't find a relevant post (or the correct keywords).

I have an old CRT scope (HP 54501A) without a proper manual, and from the forum we're in you know that I don't know too much about scopes in general. I want to use this scope to look simultaneously at two separate, battery-powered circuits (with RF transmitter and receiver modules). I have passive probes with ground clips. My questions are:

1) Does the scope assume a common ground for all channels? Or does it use only the delta between the ground clip and the probe tip for each probe independently?

2) Probably related to the previous question - if I do probe the two circuits at once, will the probe somehow short the two circuit grounds? Can this damage the scope itself? Or interfere with the results?

Any information or not-too-technical reference will be welcomed  :)

Thanks!
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Offline MatthewEveritt

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Re: Scope separate cahnnels and grounds
« Reply #1 on: March 05, 2015, 11:44:57 am »
The ground of the different channels will all be connected together, and to mains earth, but for battery powered circuits that's fine.

You do 'short' the grounds together, but that's not a problem unless there's some other connection  between the two circuits. There's nothing to drive a potential difference between the grounds, so no current will flow.

If you draw out the combined circuit you'll see that the wire between the two grounds doesn't compete a circuit.
 

Offline igendel

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Re: Scope separate cahnnels and grounds
« Reply #2 on: March 05, 2015, 12:12:42 pm »
The ground of the different channels will all be connected together, and to mains earth, but for battery powered circuits that's fine.

I see, thanks. One more thing though - can this have some effect on the RF modules? e.g. changing antenna sensitivity, making the signal cleaner/noisier etc.
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Offline aveekbh

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Re: Scope separate cahnnels and grounds
« Reply #3 on: March 05, 2015, 03:18:00 pm »
One more thing though - can this have some effect on the RF modules? e.g. changing antenna sensitivity, making the signal cleaner/noisier etc.
Depends on the frequency, what you are probing and how the circuits are set up. The two probes and the scope provide an alternate/leakage path for the signals. However, I assume that the transmitter and receiver are close to each other when you are looking at them on the scope, so I don't think this leakage path will have any major impact on the signal quality.

Can you tell us what you are probing?
 

Offline igendel

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Re: Scope separate cahnnels and grounds
« Reply #4 on: March 05, 2015, 04:56:37 pm »
Can you tell us what you are probing?

I thought to get a glimpse into the real-world behavior of those cheap, brand-less "RF link kits" (315/433MHz). So for starters, I guess I'll have some plain MCU with a transmitter on one side (probing the MCU output pin), and a plain receiver on the other (probing its output).

I know about the AGC of these modules, training signal etc. - I just wanted to make sure the measurement itself will not mess up what I'm trying to measure  :)
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Offline AG6QR

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Re: Scope separate cahnnels and grounds
« Reply #5 on: March 05, 2015, 07:29:10 pm »
You've gotten the correct answer that scope grounds are generally shared, but rather than trust us, I recommend you try connecting a continuity tester, or DMM set to ohms, or similar device, between your two ground clips, while both probes are connected to the scope.  You can easily prove to yourself whether or not your grounds are connected.  (But if your experiment shows them to be isolated, be very suspicious.  Double and triple check EVERYTHING.)

For isolated battery powered devices, that normally won't be an issue, but it can be a serious issue when trying to probe two signals in a single device, where the two signals are not isolated from one another.
 

Offline igendel

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Re: Scope separate cahnnels and grounds
« Reply #6 on: March 05, 2015, 08:27:42 pm »
but rather than trust us, I recommend you try connecting a continuity tester

That does make sense  :D

(But if your experiment shows them to be isolated, be very suspicious.  Double and triple check EVERYTHING.)

 ;D

I was afraid I'd ruin something expensive with reckless testing... but you are absolutely right - even with the scope turned off, the resistance between the probe ground clips is very small, probably less than my Cheap-O DMMs' own error.

This leaves me with the question of if and how this shared ground influences the radio transmission. But to be honest, as aveekbh suggested it's probably negligible for my setup and needs. So thanks for the information and advice, I'll let you know if I find something interesting  :)
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Offline aveekbh

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Re: Scope separate cahnnels and grounds
« Reply #7 on: March 06, 2015, 06:21:08 pm »
I thought to get a glimpse into the real-world behavior of those cheap, brand-less "RF link kits" (315/433MHz). So for starters, I guess I'll have some plain MCU with a transmitter on one side (probing the MCU output pin), and a plain receiver on the other (probing its output).
I don't think probing the digital I/O signals will have much of an effect on the RF performance. If you probe the RF output directly, it will definitely affect the performance - although I doubt it would be observable at close range (high RX power).
 


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