Scope Probe Modificaiton? - Page 1
Forum section: electronics / testgear

author: CraigD73
author_rank: Contributor
timestamp: April 20, 2018, 06:22:52 am
content: I hate the probes that came with my Rigol DS4024, the clip grabber is junk. Found some old new stock Tektronix P6053C 250Mhzprobes on Ebay. While not 500Mhz like the Rigol, they should be better mechanically and fast enough for general use.So here is my issue -- they are 2M long, would like to shorten them to 1.5M Just wondering if anyone has ever attempted to repair or shorten a Tektronix probe. Best I can tell the compensation network resistor values may change slightly, but the larger issue is reattaching the coaxial cable to the BNC/compensation module after shortening. Anyone know the coax is attached, crimped, threaded ferrule etc. and if it's possible to get reattached after cutting?Craig

author: Performa01
author_rank: Super Contributor
timestamp: April 20, 2018, 10:11:56 am
content: Quote from: CraigD73 on April 20, 2018, 06:22:52 amThe bandwidth rating of a probe is far less important than the input capacitance. In fact, it tells next to nothing about the ultimate performance of the probe/scope combination. It is perfectly possible that a 250MHz probe yields better bandwidth and flatness than a 500MHz probe.That said, the probes supplied with any decent scope usually perform best because they are matched with the scope frontend.Quote from: CraigD73 on April 20, 2018, 06:22:52 amIt is not only the resistance, but also the capacitance. The probe lead is an integral part of the probe and its compensation circuitry. Shortening it will significantly change the characteristics and the result might not be desirable. Of course it depends on how the unmodified probe performs in combination with the scope in question.Below is the simulation result for a probe with 2m cable length perfectly matched to a 1MOhm//18pF input using a low frequency compensation capacitor value of 11.5pF (red).Then the cable is shortened to 1.5m, everything else unchanged (green).Finally the low frequency compensation capacitor value re-adjusted to 9.2pF (blue).Probe x10_75_25 02aIn practice you’ll hardly get a flat initial frequency response like shown in the graph, but the relative change should be representative, i.e. something similar to the +3dB peak at 150MHz shown above can be expected.

author: CraigD73
author_rank: Contributor
timestamp: April 21, 2018, 11:15:49 pm
content: I picked up the probes yesterday, still heat sealed in the envelopes. They have the 3.5 mm compact style tips and came with all the accessories. Haven't done a full band width test yet but a rise time comparison between the Rigol supplied probes (500Mhz RP3500A) and P6053C is almost identical using the scope's calibrator output and I had no issue compensating it to the scope. The P6053C's input capacity is 12.5pF vs the Rigol's 13pf so it seems to me they are fairly equivalent for most everyday use. Will have to do a full sweep test when I get time.It really doesn't look possible to shorten the cable looks like it may be crimped to the compensation box fitting. The Tek data does list the components used in the compensation circuit for the shorter cable options of the probe. For now I think I will just use them at their 6' length.Will have to be on the lookout for an active probe.

author: tggzzz
author_rank: Super Contributor
timestamp: April 22, 2018, 08:06:19 am
content: Quote from: CraigD73 on April 21, 2018, 11:15:49 pmAnd whatisthe risetime? You are looking for tr=0.35/bandwidth.The cal output is there to adjust thelowfrequency response of theprobe.QuoteVery sensible: *10 "high" impedance probes are surprisingly subtle pieces of engineering. If you are interested in why I wrote ' "high" ', read the references inhttps://entertaininghacks.wordpress.com/library-2/scope-probe-reference-material/QuoteThey are expensive and relatively electrically fragile. Be sure you understandwhyyou need that type of probe vs the other types of probe. See the reference above.

author: vk6zgo
author_rank: Super Contributor
timestamp: April 28, 2018, 07:55:51 am
content: If by "clip grabber" you mean the cone shaped bit with the spring loaded clip, the Tektronix ones fit most other probes, so maybe all you need to do is pull them off the Tek probes & slide them onto the Rigol ones.

author: nanofrog
author_rank: Super Contributor
timestamp: April 28, 2018, 08:27:50 am
content: FWIW, I'm accustomed to that accessory being called a Witch Hat.