oscilloscope probe cable? - Page 1
Forum section: electronics / testgear

author: freda
author_rank: Regular Contributor
timestamp: October 18, 2022, 10:34:06 pm
content: would i be right to assume oscilloscope probe cable is made of RG174 ?tablehttps://web.archive.org/web/20120511024343/http://belden.com/resourcecenter/tools/cablefinder/upload/06-3_15.pdfRG174 jacket OD 0.11inchi actually measure closer to 0.12inso seems about right.... ?

author: alm
author_rank: Super Contributor
timestamp: October 18, 2022, 10:59:10 pm
content: No, passive high-impedance scope probes use resistive coax with generally some sort of Nichrome center conductor, which I don't think is generally available from the cable manufacturers. See page 15 ofTektronix Oscilloscope Probe Circuits, a publication that while old is probably the most thorough technical description of oscilloscope probes. If you read the pages leading up to it, it explains why ordinary coax wouldn't perform well at all, because the oscilloscope input is far from 50 Ohm. Or seethis video.If you were building a probe designed to work into a 50 Ohm input, like what is generally calleda resistive divider probe, or an active probe driving 50 Ohms, then you would use regular 50 Ohm coax like RG-174 or even thinner.

author: bdunham7
author_rank: Super Contributor
timestamp: October 18, 2022, 11:03:14 pm
content: If it is a probe designed to be used with a 50R input, then perhaps. If it is a 10X or 100X probe for a 1M input, then it should be a specialty resistive cable designed for low group delay dispersion rather than low amplitude loss. If it is a 1X only probe, then I don't know and it likely doesn't matter very much.

author: freda
author_rank: Regular Contributor
timestamp: October 18, 2022, 11:08:21 pm
content: thanks for the reading material. looks good.yeah i was thinking about making my own higher voltage probe, so what cable to use came to mind.might be good to just get a cheap probe off aliexpres or something and cut that.

author: alm
author_rank: Super Contributor
timestamp: October 18, 2022, 11:40:51 pm
content: What kind of bandwidth are you aiming for? You will likely need to modify the compensation box on the scope side. Finding an older, bigger probe with a larger compensation box might be easier to modify.Nichrome wire does not solder well at all. It's normally crimped instead of soldered.

author: freda
author_rank: Regular Contributor
timestamp: October 19, 2022, 12:05:46 am
content: just a few MHz would be more than ample. i'm aware needing compensation for the divider.could be a interesting diy.....but i see you can buy 100:1 probe for cheap anyway, still i wonder where they put the divider?hopefully in the probe and not at the box with the compensator at the BNC end.hmm should i diy? have all the bits for making a divider (<1kV). might do both.

author: alm
author_rank: Super Contributor
timestamp: October 19, 2022, 12:25:47 am
content: At a few MHz the cable may not be so critical. But without a low-pass filter, the higher frequency components will still show up on the scope and might confuse you.The divider will be at the probe to keep the reactance in parallel with the divider down. Any reactance (capacitance or inductance) after the divider gets divided by the division ratio. So if the cable has say 60 pF of capacitance, and the divider is behind the cable, the probe has an input capacitance of at least 60 pF. While if there is a 10x divider in front of the 60 pF cable, then the contribution to the input capacitance is only about 6 pF. See figures 2-15B and 2-16A of the Tek document which show the 9 MOhm series resistor with cap in parallel at the probe end.Figure 2-4 shows a very rough schematic of what a 100x probe might look like, with a 9.9 MOhm resistor in series and a 111 kOhm resistor in parallel with the 1 MOhm scope input to form 100 kOhm, a /100 divider. The advantage of this that even with the probe unplugged, the voltage at the BNC connector is still roughly 1/100th of the input voltage, as opposed of the alternative design of a 99 MOhm series resistor which would present a much higher voltage when the probe is disconnected, or the scope is set to AC coupling.

author: tautech
author_rank: Super Contributor
timestamp: October 19, 2022, 12:27:39 am
content: Quote from: freda on October 19, 2022, 12:05:46 amWhy would you ?Better to buy a 100x probe with known ratings for safetys sake than cobble something together that could expose you to lethal voltages.

author: tggzzz
author_rank: Super Contributor
timestamp: October 19, 2022, 12:32:55 am
content: Do you also need isolation? Don't forget that with almost all scopes the case - and therefore scope probe shield - is directly connected to the protective mains earth.Never ever disconnect the protective mains earth. Isolation transformers aren't a complete solution either. FFI seehttps://entertaininghacks.wordpress.com/library-2/scope-probe-reference-material/

author: noisyee
author_rank: Contributor
timestamp: October 19, 2022, 01:09:17 am
content: Quote from: freda on October 19, 2022, 12:05:46 amFor just a few MHz, it doesn't matter if the coaxial is 50Ω or a special high insertion loss one.Usually the upper resistor of the divider is directly after the probe tip to minimize shunt capacitance seeing form the tip. The lower resistor is your scope input impedance.Sure you need compensation because the upper resistor is 99MΩ for 100:1. Typical shunt capacitance for a 1m coaxial is about 100pF, which turns out to be a tens of Hz LPF.Such kind of HV passive probe is not difficult to DIY. But for safety reason, you should buy a decent one with safety certification if you are really serious.

author: bdunham7
author_rank: Super Contributor
timestamp: October 19, 2022, 01:56:11 am
content: Quote from: freda on October 19, 2022, 12:05:46 amThere are different varieties, but the 'cheap' ones like the P2301C commonly found online simply use a 99M resistor close to the probe tip and the 1M scope input becomes the bottom of the divider. So they are x100 and 100M total input impedance, plus capacitance of course.

author: tggzzz
author_rank: Super Contributor
timestamp: October 19, 2022, 07:56:59 am
content: Quote from: bdunham7 on October 19, 2022, 01:56:11 amAnd, as David Hess has to frequently point out, if you use those with AC coupling then theentireDC voltage appears across the input coupling capacitor. What could possibly go wrong?!

author: freda
author_rank: Regular Contributor
timestamp: October 19, 2022, 08:56:22 am
content: Quotethere always has to be someone with the safety trolling......haven't you read the memo forum is for enthusiasts and professionals.cobbling something out of what you have is the best of fun mostlyI'll give a plug for this utubhttps://youtu.be/Rl8I4PO66Uwalthough just 5x10M for the divider, I have alot more VR25 (1.6kV breakdown) types to use....

author: freda
author_rank: Regular Contributor
timestamp: October 19, 2022, 09:03:25 am
content: Quoteyou mean....wtf? maybe it is better to diy......

author: tautech
author_rank: Super Contributor
timestamp: October 19, 2022, 09:07:53 am
content: Quote from: freda on October 19, 2022, 08:56:22 amYou might be surprised how little we buy these for:I looked everywhere for a HV 100x probe that I felt properly screamed at you to be careful when working at elevated voltages and these were the only bright red ones I found.Call me a safety troll if you must but I'm still alive and sleep well knowing our customers will be safe too.

author: tggzzz
author_rank: Super Contributor
timestamp: October 19, 2022, 09:25:57 am
content: Compare and contrast the tone and knowledge displayed in the two adjacent posts copied below. Myemphasisadded for clarity.Quote from: freda on October 19, 2022, 08:56:22 amandQuote from: freda on October 19, 2022, 09:03:25 amIf the UUT and scope warrant it, I strongly suggest spending money on a *100 HV differential probe. It will be cheaper than either replacing a scope/UUT, or your relatives commuting to the hospital to see you.

author: nctnico
author_rank: Super Contributor
timestamp: October 19, 2022, 10:53:36 am
content: Quote from: tggzzz on October 19, 2022, 07:56:59 amNot much. Nowadays those coupling capacitors are typically MLCCs which are extremely tolerant to overvoltages. There are some appnotes on that. If the capacitor arcs over, there still is the 99M Ohm resistor in series. Where things get iffy is at high frequencies where the series capacitance of the probe starts to become low impedance.So for higher frequencies, getting a differential probe is a better choice indeed.

author: Mechatrommer
author_rank: Super Contributor
timestamp: October 19, 2022, 12:46:48 pm
content: Quote from: freda on October 19, 2022, 12:05:46 amall probes i know have the divider at the probe tip.. diy is good experience to understand how its done and especially its tips impedance and effect on circuit under probed... you can try with normal (non-resistive) coax first and try to cobble things to get flatter respond, ymmv..https://how-to.fandom.com/wiki/How_to_make_a_100X_oscilloscope_probe

author: bdunham7
author_rank: Super Contributor
timestamp: October 19, 2022, 01:37:21 pm
content: Quote from: tggzzz on October 19, 2022, 07:56:59 amTrue for certain scope/probe combinations, but mostexpensiveprobes work the same way. So either you get fairly rare x100/10M probes, use a scope that doesn't have the AC coupling up front or just remember to use DC coupling when measuring things with a large DC offset. Many well-regarded bench DMMs have the exact same issue in that their AC-coupling input capacitors are only rated to 400V and thus will blow up under the same conditions.

author: bdunham7
author_rank: Super Contributor
timestamp: October 19, 2022, 04:44:24 pm
content: Quote from: nctnico on October 19, 2022, 10:53:36 amI'm not sure why single-ended vs differential or even active vs passive really matters. The total load, both capacitive and resistive, on the DUT would seem to me to be the primary issue. And, as we're discussing here, if you have a small HF signal on a large DC bias, then 100M resistive seems better than the typical lower resistances of differential probes. If you have high voltages at high frequencies, then your solution may be more expensive--and I wouldn't assume that inexpensive differential probes solve this problem as they have to be derated too.

author: tooki
author_rank: Super Contributor
timestamp: October 19, 2022, 06:10:37 pm
content: Quote from: tautech on October 19, 2022, 09:07:53 amAnother brand to look at is Testec: they’re made in Germany, but very reasonably priced. (Their basic 1/10x probes are under US$20.) Look here and select “HV up to 2.5kV”:https://www.testec.de/en/products/product-categories/standard-probes/One is bright red, too.And this is where they have x1000 probes from 8kV to 39kV:https://www.testec.de/en/products/product-categories/highvoltage-probes/

author: jonpaul
author_rank: Super Contributor
timestamp: October 19, 2022, 06:43:37 pm
content: Bonjour,1/ Best scope probes are Tektronix, see ebay or groups.io tekscopes or tekscopes22/ Ref on probes: Tektronix Circuit Concepts ...Oscilloscope Probe Circuits bookhttps://w140.com/tekwiki/images/6/62/062-1146-00.pdf3/ RG/174/U is NOT used in most probes4/ You can DIY a X10 Zo probe for use on 50 Ohm inpurs for HF measurents, 450 Ohm 1 % >>50 Ohm cable (RG174/U OK!) >>BNC>>50 Ohm scope input5/ HV probe not need for 400V or less, use genuine TEK 10X probes NOT chinese.6/ Use X100, X1000 HV probe for 1 kV...50 kVBest is TEK P6015ABon soiree,Jon

author: BillyO
author_rank: Super Contributor
timestamp: October 19, 2022, 07:12:46 pm
content: Quote from: freda on October 19, 2022, 08:56:22 amWARNING:Old, still living, guy story hereRecently a friend of mine was troubleshooting his dishwasher. He needed to connect something in the appliance (a wax motor) to his meter and free up his hands to actuate a switch to see if the detergent was being released. He had a great idea. He'd make some nice long banana to alligator clip cables and used some really nice looking banana plugs with gold plated lanterns that he had left over from a speaker project (which on the package said they were insulated). Well, he had everything hooked up and was making tests but found himself in an awkward position, so reached down to move the meter out of his way. Unfortunately for him, those fancy audio oriented banana plus were not insulated at all. The shells were anodized aluminum. As he reached for the meter, the palm of his hand near his wrist contacted both plugs. The resulting convulsion caused his other arm to strike a sharp edge of the dishwasher frame causing a deep 4" gash in his forearm. Luckily his wife was home and was able to bandage him up and rush him to hospital to have 7 stiches to close the wound.Fun indeed.

author: freda
author_rank: Regular Contributor
timestamp: October 20, 2022, 02:43:23 am
content: Quote from: BillyO on October 19, 2022, 07:12:46 pmi mean blah blah, don't we all have such stupid stories.....I remember when in my primary school years, watching dad attempting to replace themulti selector switch on a washing machine and getting it wrong. Using a 240v lamp he wired upand the house fuse as a diagnostic, yeah i know! WTF! And dad and mum having an almighty blue, I think mum was pissed off with the fuse blowing times interrupting General Hospital or Days of Lives, she was so addicted to them shows.Its funny how one can have such vivid recollections of things when we so young 8or9ys or so. Nothing bad happened except the argument escalation, some days later a proper repairman came around, clearly remember itneeded another new switch, i guess dad's fucking around destroyed an otherwise new switch.

author: BillyO
author_rank: Super Contributor
timestamp: October 20, 2022, 12:34:46 pm
content: Quote from: freda on October 20, 2022, 02:43:23 amWell, you do you. That's the main thing.