Oscilloscope users: about ground leads

DarrenF

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So every probe I've seen has little 6" long ground wires on the probe, with an alligator clip on the end. Well, IMHO, this isn't very useful unless you happen to be working on an old Atari PCB (which has large test lugs all over the PCB). My issue is two-fold: the leads are too short, and alligator clips suck.

As a result, I typically find myself attaching the alligator clip to a jumper wire with mini-clips, and using the mini-clip to attach to the GND pin of some random TTL IC. Even this is an issue, because of the length of the alligator clip, I'm worried about it shorting on something, so I have to hold it in the palm on my hand all the time.

So I'm thinking about cutting of the alligator clips and soldering on another foot of wire terminating with a mini clip. AFAIK, the only reason the ground leads are so short is to minimize noise at very high frequencies... not really a concern for me.

Any other issues with my idea? How do you guys deal with the cheezy little ground leads?
 
There's a nice technical writeup by tektronix (here). High frequency is not the only problem. The longer wire would act like an antenna and introduce noise at any frequency (often 60hz).

Sure if you're doing coarse work a poor ground is fine, but they make it very short because you "should" be grounding at or near the point you're probing for the best possible signal integrity.
 
Your scope most likely has a binding post ground input. Buy or make a banana plug to alligator clip cable. Connect from the binding post to a convienent ground on the board.
 
Depending on the board I usually find a cap leg thats going to ground, and thats usually chunky enough to attach the clip to.

I agree though, those short leads suck!
 
So every probe I've seen has little 6" long ground wires on the probe, with an alligator clip on the end. Well, IMHO, this isn't very useful unless you happen to be working on an old Atari PCB (which has large test lugs all over the PCB). My issue is two-fold: the leads are too short, and alligator clips suck.

As a result, I typically find myself attaching the alligator clip to a jumper wire with mini-clips, and using the mini-clip to attach to the GND pin of some random TTL IC. Even this is an issue, because of the length of the alligator clip, I'm worried about it shorting on something, so I have to hold it in the palm on my hand all the time.

So I'm thinking about cutting of the alligator clips and soldering on another foot of wire terminating with a mini clip. AFAIK, the only reason the ground leads are so short is to minimize noise at very high frequencies... not really a concern for me.

Any other issues with my idea? How do you guys deal with the cheezy little ground leads?

I do this also but I put some black tape on the connection of the two clips.

I think if you use braided cable for a ground that will eliminate most of the 'noise' on the input line of the scope. Braided cable like the outer shield of a tv coax cable or something similar. There is a reason radio and communications use that.
 
I've actually been looking to do the opposite. I agree that the original ground leads are short and annoying, but there have been a lot of times that even that length, and especially when I extend it, that I think I have signal integrity issues, when it's actually my ground lead. So I've been thinking of picking up something like: http://www.caltestelectronics.com/ctitem/37-5mm-probe-accessories/CT3668 .

They'd probably be convenient with Atari old boards, since there's no solder mask... I'm not sure how easy they'd be to work with in other everyday situations though.

DogP
 
I've actually been looking to do the opposite. I agree that the original ground leads are short and annoying, but there have been a lot of times that even that length, and especially when I extend it, that I think I have signal integrity issues, when it's actually my ground lead. So I've been thinking of picking up something like: http://www.caltestelectronics.com/ctitem/37-5mm-probe-accessories/CT3668 .
Note they also make stuff like

http://www.caltestelectronics.com/ctitem/35-35mm-probe-accessories/CT3661
http://www.caltestelectronics.com/ctitem/144-clips/CT3659

That makes it much easier to ground to a nearby IC leg.
 
If you are just checking coarse low frequency stuff like logic levels, a longer ground is fine. I use a banana jack with coiled wire and an alligator clip.

For higher frequency work, the shorter lead is a must to see the trigger pulses and higher order bits and carryouts from counters.

90% - 95% of the time the longer ground wire is fine.

Regarding braided or stranded wire - again this helps for higher frequency work. At higher frequencies only the outer layer of the strand of wire conducts instantaneously - this is called the "skin effect". That's why stranded wire is better than solid wire for a given gauge. All of those wires in parallel give better instantaneous conduction.
 
Thanks for the feedback, everyone. On second thought, the length isn't really the bigger problem, IMO, but the useless alligator clip.

I think I'll cut of my alligator clip, and put on a short (couple inch) entension with something like one of these on the end:
150x150-5501.Jpg
 
Why hack your o-scope leads? My scope leads have clips on the end. I just clip the o-scope ground to a convenient lug/wire, then clip my o-scope lead to an old digital multi-meter lead. It's insulated and has a long probe to poke around. Never had a problem w/ground or noise. Plus this keeps any metal parts away from your board. Save your o-scope leads and use an old meter lead.
 

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Here's a pic of my lead setup. As you can see the o-scope ground and lead are kept out of the way and the long multi-meter lead gives plenty of length to poke around the entire board. Never had a problem w/ this setup.
 

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Here's a pic of my lead setup. As you can see the o-scope ground and lead are kept out of the way and the long multi-meter lead gives plenty of length to poke around the entire board. Never had a problem w/ this setup.

The problem with that method is that multimeter probes are terrible oscilloscope probes. They're fine for low frequency signals and low speed edges, but sharp edges will ring like crazy. There's a reason oscilloscopes don't just come with multimeter probes.

I used to use a Fluke Scopemeter a lot (portable oscilloscope with multimeter functions and form factor)... it used banana connectors so you could use multimeter probes, but the oscilloscope probes were almost a necessity when trying to use it as a scope. Don't forget that even if it's a low "speed" signal (i.e. 100KHz), if the edge rate of the signal is high (like the square waves of digital logic), low performance probes will greatly degrade the measurement.

If you are working with really low frequency stuff and want to use multimeter probes, you could just use a BNC to banana adapter, like: https://www.emtesco.com/index.php/ht311.html , then use a banana to alligator for the GND, and a multimeter probe for your probe. But the performance will be poor.

DogP
 
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