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To reduce the loop area and noisepick up, the GND lead of the scope probe is very next to the tip in the below probe:

enter image description here

I don’t have such probes and my scope probe GND leads are much longer alligator clips.

Does the type of probe in the photo have a name or is it homemade? And when is it used?

user1245
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    That particular one looks home-made, but when I last purchased a 'scope probe it had one of those included in the package of accessories. Not sure what it's officially called though ... – brhans Feb 05 '19 at 18:30
  • Do you know how it is made from a typical alligator lead one? – user1245 Feb 05 '19 at 18:31
  • You don't make it out of an alligator lead clip - they're completely separate accessories. This one wraps around the barrel near the tip, while the alligator lead usually attaches much further up on the probe. If you want to make one, find some stiff wire, wind it into a coil of the same diameter as the barrel of your probe, and cut the end just long enough to reach the same length as the tip. – brhans Feb 05 '19 at 18:37
  • Are they used for high freq measurements? – user1245 Feb 05 '19 at 18:39
  • Typically yes.. – brhans Feb 05 '19 at 18:43
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    High frequency or low level. If I'm looking at microvolt level stuff, the wire on the alligator clip will pick up enough powerline hum to obscure the real signal. – JRE Feb 05 '19 at 19:37
  • Most scope probes have a screw-on or snap-on extension with a clip. Remove that, and they have a pin with an outer metal barrel, which is the scope ground. Your probe should have that -- you only need to make the wire dingus that extends that barrel out as a pin. Some scope probe kits come with that wire dingus pre-made, some don't. – TimWescott Feb 05 '19 at 19:48
  • @TimWescott Why do you think it is attached in inductor form but not straight rigid wire? Does it have a purpose? – user1245 Feb 05 '19 at 19:55
  • The coil portion of the ground thing is a spring intended to make good contact to the ground sleeve of the probe - the coil turns should be shorted by the ground sleeve, so there should be no inductor effect. – Peter Bennett Feb 05 '19 at 20:25

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This is not a "probe", this is called "ground accessory" to some old active probes like Tektronix P5100 etc. The shown is a hand-made accessory, but there were ground tips made of springy wire and officially sold by oscilloscope companies. An example of a smaller size ground contact,

enter image description here

or for Agilent probes (circled in red:

enter image description here

The purpose of these short probe leads is to decrease inductance of leads (ground in particular) and gain few dozens of MHz to overall probe's bandwidth. And to reduce unwanted parasitic interference from nearby signals. Here is the detail:

enter image description here

Ale..chenski
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  • Their purpose is to decrease inductance yet they are exactly in the shape of an inductor. Im confused:(( – user1245 Feb 05 '19 at 19:59
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    No, the coiled part is supposed to lay tight on the ground barrel of the probe tip, so it serves the mechanical purpose only. The inductance comes only from the relatively short straight tip. – Ale..chenski Feb 05 '19 at 20:05
  • I see the coiled part will be tightly joint to the ground barrel that explains my confusion. – user1245 Feb 05 '19 at 20:07
  • You say "old active probes" and "there were", but these are still provided with every scope probe I've ever been involved in purchasing. Granted, none of those were active probes, but you'd think if they provide them with passive probes, they'd definitely provide them with the substantially more expensive active ones. – Hearth Feb 06 '19 at 02:56
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    @Hearth, yes, these spirals are still supplied with low-end sub-GHz probes. Better probes have pogo-pin accessories right from the active tip, or the ground attachment is done with gold-plated bronze wraps, and the best one have ground and signal soldered to the points of interest, and use differential mode or switchable "tri-mode", https://www.tek.com/datasheet/trimode%28tm%29-probe-family. – Ale..chenski Feb 06 '19 at 03:22
  • @Ale..chenski I suppose the lab I work in has never had cause to use any probes quite that fancy. They're not exactly something you buy on a whim, after all. – Hearth Feb 06 '19 at 04:23
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    Oh man, I have always wondered what spare part the spring is for, turns out it was never a spare part to begin with! – crasic Feb 06 '19 at 07:53
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I've used such, either from Tektronix P6201 900MHz active FET probe (a 1970s era beauty) accessory kit, or ones I made from solder-wick.

With the probe's 10X attenuator slide on, the capacitance was 1pF. With 2cm of solderwick producing about 20nanoHenry inductance, the input LC resonance was near 1,000MHz and life was very good for examining what was really occurring on the PCBs.

analogsystemsrf
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