Oscilloscope Probe Extensions. - Page 1
Forum section: electronics / beginners

author: JJ_023
author_rank: Frequent Contributor
timestamp: November 29, 2023, 02:34:28 am
content: Based on a member suggestion I read several books on oscilloscope probes. One of which was "Oscilloscope Probe Circuits"https://w140.com/tekwiki/images/6/62/062-1146-00.pdfInformative read for sure.After reading I was curious to see if there are any oscilloscope probe extensions commercially available. I didn't do a super exhaustive search but I did search and could not find any. If there are any commercially available, I would love to see the links for them, although I assume the pricing for them is astronomical.Also from my recent readings I understand that you just can't take a BNC cable and use it as an extension on its own as it would affect the capacitance as well as the Impedance.So I wanted to explore what it would take for me to extend my measurement capabilities with a standard namebrand oscilloscope to about 12 feet away from the scope.Any comments or suggestions for achieving this are welcomed. I'm sure I will generate follow-up questions and learn in the process.For those of you that do, how do you extend your oscilloscope probes?

author: joeqsmith
author_rank: Super Contributor
timestamp: November 29, 2023, 03:31:21 am
content: Depending what you need, I have an old Tektronix P6006. You could get these with a 12' cable.https://w140.com/tekwiki/wiki/P6006Looks like there are a few on eBay that are complete.https://www.ebay.com/itm/334897952689

author: macboy
author_rank: Super Contributor
timestamp: November 29, 2023, 03:42:50 am
content: Basically, you can't.For low-Z passive probes, or active probes like FET or differential ones, you can usually literally just extend them with a piece of 50 ohm coax. You might lose a little bandwidth. But for high-Z passive probes, the BNC connector on the compensation box needs to be at the scope input, and the probe tip needs to be on the signal. From tip to scope input circuitry, the whole thing is precisely engineered and very sensitive to disturbances.Tek did make some high-Z passive probes with a choice of cable length, and you could even replace a cable for a longer one designed specifically for that probe. The longer cable still wasn't very long, 2 meters was likely about all you got. Certainly not 12 feet or 4 meters (edit: except as mentioned in the previous reply). I have a Tek power supply for using their active probes on a scope without the TekProbe interface. It connects to the scope with a normal 50 ohm coax BNC cable. You could use a 12 foot cable there.

author: JJ_023
author_rank: Frequent Contributor
timestamp: November 29, 2023, 03:47:29 am
content: Quote from: joeqsmith on November 29, 2023, 03:31:21 amThanks for that. I see that they make a 12 foot version.The eBay listing doesn't Specify that it is 12 feet though.

author: joeqsmith
author_rank: Super Contributor
timestamp: November 29, 2023, 03:54:00 am
content: Using 50ohm coax is certainly an option. If you wanted to spend some cash, you could purchase a 1M to 50ohm buffer. See link:https://www.eevblog.com/forum/projects/high-z-wideband-buffer-amp-for-50-ohm-cro-inputs/msg947988/#msg947988

author: JJ_023
author_rank: Frequent Contributor
timestamp: November 29, 2023, 03:55:00 am
content: Quote from: macboy on November 29, 2023, 03:42:50 amThank you for that. Like I said I am aware that you just can't use a coaxial cable based on what I learned from reading.There are some really smart people on this forum that come up with some really interesting designs. I'm sure this may not be a very common need but a need nonetheless. There has to be a way to compensate for the extra length and modify an existing probe.

author: JJ_023
author_rank: Frequent Contributor
timestamp: November 29, 2023, 04:04:59 am
content: Quote from: joeqsmith on November 29, 2023, 03:54:00 amThat is an interesting post. The Amazon link has no price. Thank you for sharing.

author: joeqsmith
author_rank: Super Contributor
timestamp: November 29, 2023, 02:01:44 pm
content: I think they were selling that buffer for $500. You can find used buffers on ebay. From that thread, depending on your requirements it is certainly possible to build one from scratch.

author: JJ_023
author_rank: Frequent Contributor
timestamp: November 29, 2023, 03:51:54 pm
content: Quote from: joeqsmith on November 29, 2023, 02:01:44 pmI appreciate the information. I have saved that particular thread for future use. At the current time I don't think this is something I would be interested in doing.What I am trying to explore is maybe figuring out how to balance out the extension. If they made 12 foot probes that means it's possible. If it's possible then it's just a matter of ascertaining that knowledge. I don't think the components from my readings indicate that they would be difficult to obtain.

author: bdunham7
author_rank: Super Contributor
timestamp: November 29, 2023, 05:11:46 pm
content: Quote from: JJ_023 on November 29, 2023, 03:55:00 amThe issue with your question is that you haven't sufficiently stated all the requirements and the solution to the problem will depend on those unstated parameters--amplitude, attenuation factor and bandwidth. A universal, passive extension for existing 10X probes that allows them to retain their specified characteristics would be pretty difficult. If you absolutely need to use a 10X probe at 100MHz BW from 12 feet away, the buffer solution will work up to a certain amplitude, typically ~3.5V at the buffer input, or 35V at the 10X probe tip. The simplest build-it-yourself solution I've seen is:https://www.edn.com/high-impedance-fet-probe-extends-rf-spectrum-analyzers-usable-range/or you can look for the HP 41802A. I have the latter, but have never used it other than to test it. You'll need a scope with a 50R input to make that all work properly at high frequencies.If you are just probing at audio frequencies, you don't need any solution at all since a 1X probe will work with a coax extension or you can just skip the probe entirely and use a BNC-to-alligator clip adapter.If you need to work up to 10 MHz or so and you don't have very small amplitudes, then an inexpensive HV differential probe would be the easiest solution. They typically may have a 50 ohm-ish output impedance, but they can still drive a high-impedance scope input just fine and this is what they usually are intended to do. At lower frequencies, the 50R coax simply represents additional capacitance that would be too much for a typical 10X probe to compensate for, but the differential probe will drive it without any issues.Simply adapting an existing 10X probe would entail replacing the cord with a longer one and then modifying the compensation network. You won't ever get the same performance as the original probe since longer cables always involve some losses. Probemaster makes versions of their 4900-series probes with 10-foot cables, but the BW is lower and the load capacitance is higher than the equivalent probe with a shorter cable. I suppose they could make one with a 12-foot cable with even worse specs. In theory, an extension made from the same fine, lossy cable as the probe would be a start but you'd still need to modify the compensation network and possibly the probe end to balance out the capacitance change.

author: joeqsmith
author_rank: Super Contributor
timestamp: November 29, 2023, 05:32:53 pm
content: Quote from: JJ_023 on November 29, 2023, 03:51:54 pmIt is certainly possible, depending on your requirements. 20ish years ago I was able to work out a deal with ProbeMaster where I purchased some of their raw oscilloscope probe components for a custom on-off project I was working on. You may want to try giving them a call. Maybe just have them send you 25' of cable and a couple of probes to experiment with. I'm sure it wouldn't be cheap and I wouldn't be surprised if they declined. Doesn't hurt to ask.

author: JJ_023
author_rank: Frequent Contributor
timestamp: November 30, 2023, 03:57:17 am
content: Quote from: bdunham7 on November 29, 2023, 05:11:46 pmThank you for such a detailed explanation. You are correct I didn't state enough information. I tried to generalize the approach to explore the idea so that maybe others would benefit if there was a solution. As far as the requirements I would have for this would primarily revolve around power supplies, CNC control related applications, the other one being automotive. Some of these require extra length for access.I reviewed the link you sent and it was definitely an interesting read. Thank you.The HP 41802A It's probably more than I would like to spend on the solution. The pricing I saw was around $1500.The differential probe is a great idea. I have a few MicSig differential probes already (1 of the models is DP10007)Again thank you for such a detailed explanation and the ideas.

author: JJ_023
author_rank: Frequent Contributor
timestamp: November 30, 2023, 03:58:44 am
content: Quote from: joeqsmith on November 29, 2023, 05:32:53 pmThat is an excellent idea. Even if the solution is overpriced maybe I can get an education from them just by having a conversation on the subject matter. Thank you.

author: jonpaul
author_rank: Super Contributor
timestamp: December 01, 2023, 11:16:51 am
content: TEK probes are avail in various legnths. Most other probe vendors have them as well.Except for X1 probes X10, X100, X1000 must be used directly on a std scope 1M 10..33 pF BNCJon

author: tszaboo
author_rank: Super Contributor
timestamp: December 01, 2023, 11:48:30 am
content: Quote from: JJ_023 on November 29, 2023, 02:34:28 amI have my own Z0 probe designed, 1:20, that in theory can be used with any practical length coax. I haven't tried more than 1m yet though.It depends of course on what you are measuring.

author: JJ_023
author_rank: Frequent Contributor
timestamp: December 01, 2023, 04:00:37 pm
content: Quote from: jonpaul on December 01, 2023, 11:16:51 amI've only seen 12 foot probes from TEK. As my original post I don't think I would need longer than that. I also could not find any other manufacturers that make anything close to that length. If you know of any other manufacturers I would appreciate the information.Thank you.

author: JJ_023
author_rank: Frequent Contributor
timestamp: December 01, 2023, 04:01:36 pm
content: Quote from: tszaboo on December 01, 2023, 11:48:30 amPrimarily revolve around power supplies, CNC control related applications, the other one being automotive.

author: bdunham7
author_rank: Super Contributor
timestamp: December 01, 2023, 04:56:42 pm
content: Quote from: JJ_023 on December 01, 2023, 04:01:36 pmOne of the many issues here is that when you are testing something that far away in an industrial environment, the ground potentials at two different points may not be indentical and you can end up with some pretty interesting ground loop currents. So long probes may not be a magic solution or even a very good one in some cases. Every solution is going to have specific issues that you'll have to consider. This topic is actually closely related to your other thread on floating scopes, so I did a few bench experiments to demonstrate some issues you'll run into.I took a Siglent AWG (SDG2042X) and set it up to output a 1 MHz 2Vp-psquare wave with a 2V DC offset from each channel, but then changed the phase on CH2 by 180 degrees. That produces a 4Vp-pdifferential square wave with a 2V common-mode DC offset. Nice and safe, 50R (to ground) output impedance with short protection and so on, so we can try floating scopes and whatever with no fatalities anticipated. Then I tested a few different ways of looking at that signal, both with and without a 12-foot BNC extension.The first method was to use a Micsig DP10013 HV differential probe and some fairly clunky BNC-to-banana adapters with a direct connection, then via my 12-foot BNC cable extension which was really 2 6-foot cables with a coupler. The next was an Siglent SDS1104X-E powered by an APC SMT1500 (LF sine wave) UPS unplugged and floating, using a 1X probe connected to the BNC outputs as carefully as possible with a short ground lead. Again, first I tried it directly connected and then through the 12-foot extension. Then I tried it again with the probe set to 10X, with and without the extension. Last I tried the so-called "poor man's differential probe" using two probes without ground leads and the math function (CH1 - CH2) on the scope. I did not try extensions with the dual probes.Edit: I also added the case of a Tek TPS2024 200MHz isolated channel scope using a single 10X probe so you can compare floating a conventional scope to an actual isolated model.

author: JJ_023
author_rank: Frequent Contributor
timestamp: December 02, 2023, 01:46:13 am
content: Quote from: bdunham7 on December 01, 2023, 04:56:42 pmThis is absolutely awesome of you to do this. It is greatly appreciated. In my opinion only the 1st 3 are worth anything. Would you agree with that?Even before starting these 2 threads I knew that an extension and a USP would not be doable, this substantiates my guess.I have a few questions for you.Why did you decide to choose 4 V with the phase shift instead of just 2 V?Do you think the results would be different with a Micsig DP1007? Seems to be better suited for lower voltages.Do you think that if you had not coupled 2 pieces of wire the results would have been better?Do you think adding a small amount of capacitance to a 12 foot lead would improve the results you got?If the voltages were higher, say towards mains would the result be better with a deferential probe since it's designed and performs better with higher voltages?I think you did a great job. This gives me a lot of insight. If you would be so kind to Answer my questions it would be greatly appreciated.Again thank you so much.

author: jonpaul
author_rank: Super Contributor
timestamp: December 02, 2023, 10:53:19 am
content: Suggest to first read and learn:Excellent (1980s!0 Tektronix Circuit Concepts series bookshttps://w140.com/tekwiki/index.php/Concepts_SeriesOscilloscope Probe Circuitshttps://w140.com/tekwiki/images/6/62/062-1146-00.pdfProbe measurementshttps://w140.com/tekwiki/images/1/19/062-1120-00.pdfENJOY!Jon

author: bdunham7
author_rank: Super Contributor
timestamp: December 02, 2023, 05:36:07 pm
content: Quote from: JJ_023 on December 02, 2023, 01:46:13 amI picked the signal details to eliminate low-level noise from the equation and to try and provide an example that differentiates between the options. I didn't do any experimentation, it just takes a little experience to have some idea where certain issues start showing up. It wouldn't make much difference if I chose different values within the same order of magnitude.The DP10007 would only improve on noise, which isn't an issue here. Also, it's input capacitance is slightly higher, so the first example might look slightly different. And without testing it, I don't know if it is better, worse or the same as far as driving the output capacitance of the extended cable.I doubt the coupling of the cable was a significant factor in this case and the main issue is driving the additional capacitance, so adding capacitance would make things even worse.The voltage is high enough so that noise isn't an issue, so going to higher voltages wouldn't change much unless the output amplifier of the DP had problems driving that capacitance at higher levels. Your higher voltages are unlikely to be clean 1MHz square waves, so it isn't practical for me to test this. Using these setups on actual mains or mains-derived voltages may cause significant other noise problems.

author: JJ_023
author_rank: Frequent Contributor
timestamp: December 03, 2023, 01:28:02 am
content: Quote from: jonpaul on December 02, 2023, 10:53:19 amThank you for the recommendations. Somebody already recommended the 2 books on probes from a previous thread I started. I read both of them. They are good recommendations. I'll have to rummage through the wiki.Thank you.

author: JJ_023
author_rank: Frequent Contributor
timestamp: December 03, 2023, 01:34:39 am
content: Quote from: bdunham7 on December 02, 2023, 05:36:07 pmThank you for such an elaborate explanation.I just recently read a paper on lowering capacitance in longer cables. I would like to share it with other people on the forum. If anybody has any opinions on it it would be interesting to hear.https://www.ncbi.nlm.nih.gov/pmc/articles/PMC10574904/