Hf compensated ground cable? (!)  - Page 1
Forum section: electronics / testgear

author: IAmBack
author_rank: Regular Contributor
timestamp: November 16, 2021, 02:48:09 pm
content: What it is? What is it made from (unobtalium core with dark matter shield?):https://www.pmk.de/en/products/2_5_mm_tastkoepfeListed among other accessories for passive scope probes (BTW, this is manufacturer of probes for LeCroy's scope's - at least some of them).My idea is that this cable is made of litz wire, maybe with some kind of shielding. Any info appreciated.Best.

author: TimFox
author_rank: Super Contributor
timestamp: November 16, 2021, 04:28:59 pm
content: A quick reading of the English datasheet seems to indicate that they have multiple auxiliary connection options to ensure a very short ground connection near the probe tip.The photo shows the probe without any of these options mounted.A long ground wire to an alligator clip, which our software engineers always lost anyway, is problematic for probing fast signals.

author: KevinA
author_rank: Regular Contributor
timestamp: November 16, 2021, 05:45:02 pm
content: Look at the Tektronix P6139B as an example. As well as the normal crock clip ground lead, it is supplied with push on sprung ground leads only mm long for fast signals.

author: IAmBack
author_rank: Regular Contributor
timestamp: November 16, 2021, 06:03:09 pm
content: Quote from: KevinA on November 16, 2021, 05:45:02 pmBut this one is 22cm long...

author: IAmBack
author_rank: Regular Contributor
timestamp: November 16, 2021, 06:04:03 pm
content: Quote from: KevinA on November 16, 2021, 05:45:02 pmThis one is 22cm long...

author: TimFox
author_rank: Super Contributor
timestamp: November 16, 2021, 06:08:57 pm
content: The relevant length for the probe ground connection is from the device under test to the barrel of the probe (around the probe point), not the entire cable back to the oscilloscope.

author: alm
author_rank: Super Contributor
timestamp: November 16, 2021, 11:15:35 pm
content: I would expect that the HF compensated ground lead will be resistive to spoil the Q of the inductor represented by the ground lead, so any ringing is damped. That's the only sensible way I can imagine to reduce the effect of ground lead inductance through just the lead (as opposed to lowering the input capacitance).

author: Geek459
author_rank: Contributor
timestamp: November 16, 2021, 11:41:42 pm
content: Have you look at the end of page "find ut more"?

author: MarkL
author_rank: Unknown
timestamp: November 17, 2021, 02:53:00 pm
content: I have a couple of these HF compensated ground leads that came in a probe accessories kit from Keysight. (PMK makes many of the passive probes and accessories for Keysight, LeCroy, R&S, etc.)The HF compensated ground lead is a piece of RG174/U coax (as printed on the coax). The center conductor and shield are soldered together at the alligator clip. At the C-clip end there is a small bulge and which I believe is a resistor connecting the center conductor to the clip. The lead measures 68 ohms from alligator to C-clip.Attached is a scope trace with a regular 9" ground lead, and then with the compensated one, which is also about 9".It works to reduce ringing, but the coax makes it a bit bulky and stiff to use.

author: IAmBack
author_rank: Regular Contributor
timestamp: November 17, 2021, 08:54:19 pm
content: Quote from: MarkL on November 17, 2021, 02:53:00 pmThank You for explanation. I expected this kind of design, and it is possible to replicate it according to Your description. Thanks (once more).