Help a clueless Old n00b: how to make scope probes? - Page 1
Forum section: electronics / beginners

author: audiotubes
author_rank: Regular Contributor
timestamp: July 12, 2021, 09:33:43 pm
content: Hi Guys,Many, many years ago I was fairly competent in using discrete components and some linear ICs, did some building of kits and also designed and built my own power supplies. Decades later and after having been away from things, I find the changes more than this old guy can keep up with. And I have forgotten almost everything I knew. I'm trying to get back into it since I have had to part with some of my hobbies due to life changes and now electronics would be somewhat more feasible. Sorry for the boring intro but hopefully it sets the stage.I am into hifi and have been all my life. Recently I got some gear and would like to troubleshoot some odd behavior it has. Specifically, a CD recorder does not lock on the signal from the AES/EBU output of one device I have, but does lock on the AES/EBU output of another device. I thought if I could see the signals, specifically the clock portion of the signal, I might be able to understand why. I have a new scope and was surprised to find it has 50 ohm inputs rather than 75 ohm, but anyway AES/EBU is supposed to be 110 ohms. So I understand I'll need to make up some kind of a probe set that has a way to match the impedance to the scope input. And I understand I need some way to trim the probes to calibrate them. I used to make up all kinds of cables for amateur radio but now I can't even figure out what kind of crimper I need and what BNC connectors work with what cable stock. I figured to buy a couple of preterminated 50 ohm cables and chop off one end to get started. I have XLR connectors on hand.Would some patient, bored person care to give me some idea how I can do a good job with this? I am sure there are plenty of issues I have no idea are even relevant.Thank you.

author: Benta
author_rank: Super Contributor
timestamp: July 12, 2021, 10:03:15 pm
content: Welcome back to electronicsFirst, didn't your new 'scope come with probes? If not, buy some. Don't even go near making them yourself, it's an effort that will never pay off.Second: 'scopes generally have a 1 Mohm input impedance (plus a few pF capacitance). The connectors are 50-ohm BNC, but that's of no consequence.Then you're up and running. Trimming the probes is a later exercise after you've unpacked them.

author: bdunham7
author_rank: Super Contributor
timestamp: July 12, 2021, 10:10:53 pm
content: What scope model? Oscilloscopes typically have either 1 megohm or 50 ohm impedances, or both. A 75R scope would be quite unusual.You don't need to match the impedance of the AES/EBU signal at your scope probe, typically you would tap into it with a high impedance probe so as to not disturb the circuit. The complication with AES/EBU is that it is a differentially driven twisted pair, so you either need a differential probe or two high-impedance probes and a math function on the scope. Let us know exactly what you have and maybe we can figure out how to use that.

author: audiotubes
author_rank: Regular Contributor
timestamp: July 13, 2021, 06:37:51 am
content: Quote from: Benta on July 12, 2021, 10:03:15 pmHello, thank youYes, the scope came with probes. I'm sorry, I see I didn't express myself well. I didn't mean to make general purpose probes. I'll probably continue to use the wrong words, unintentionally.What I want to do is make a harness so I can connect the device under test to the scope. I don't want to use clips or probe tips. I want to make a set of special purpose probes that have an XLR connector at the device end (AES/EBU) and two 50 ohm BNC connectors at the scope end.I'd like to make a few sets of these for common test scenarios. Some others would be RCA phono plug to BNC for testing coaxial S/PDIF, and BNC to BNC for testing 50 ohm WCLK.

author: audiotubes
author_rank: Regular Contributor
timestamp: July 13, 2021, 06:43:17 am
content: Quote from: bdunham7 on July 12, 2021, 10:10:53 pmHi and thanks. Oh, I didn't know that. I thought all the scopes had 75 ohm BNC connectors as standard. I bought a Siglent 3045 last year after reading a lot on this forum and I liked it, so I bought a Siglent 2104X a few weeks ago after reading here on scopes. It is more scope than I am ever likely to use, but they didn't have the 2 channel model I wanted and the bonus pack was included at a sale price.Thank you for the comments regarding impedance matching. Yes, ideally I want to make up a specific Y cable that goes from XLR on the AES/EBU side to two BNC connectors so I can run AES/EBU output from various devices right into the scope. I did not think about measuring the devices in operation and tapping off them while in use with some kind of breakout box...I guess that would be better. But from a simplicity standpoint, I meant to run the scope off the device outputs.My first priority is measuing the AES/EBU outputs of two DDCs. But I want to make similar probe sets for coaxial S/PDIF and WCLK.

author: audiotubes
author_rank: Regular Contributor
timestamp: July 14, 2021, 03:28:23 pm
content: Did I say the wrong thing?

author: RoGeorge
author_rank: Super Contributor
timestamp: July 14, 2021, 03:47:25 pm
content: All OK, just that your reply rather looks like a chit-chat than as a question.Usually the things goes like this:- I did this, and that, I expected to see X, but Y happened instead. Why Y and not X? or,- I want to achieve C, the steps I know is A, B, C but I don't know how to do step B, How do I get to B?Thinks like that. More like a point and shoot question and answers.Do not expect others to ask you questions about what it didn't work or about what you don't know, or about your projects or your life, in general.You said you wanted to do some cables, OK. But you didn't ask anything, you just told what are your plans. Great.So, what's stopping you to do that, or what do you need to find out about it? <--- Don't expect such questions here. It's your job to say what you want, what you did already, and what didn't work as expected, and it's your job to ask the question about what you don't know regarding the topic, here regarding making the cables.

author: bdunham7
author_rank: Super Contributor
timestamp: July 14, 2021, 04:13:12 pm
content: Quote from: audiotubes on July 14, 2021, 03:28:23 pmNo, it's just a relatively arcane subject and most conversations around here are asynchronous due to everyone being in different time zones.The S/PDIF would just be an RCA-to-BNC adapter followed by a 75R pass-thru terminator, scope input set to 1M. Those are all commonly available parts and I keep them around with a few dozen others to be mixed and matched as needed. I wouldn't see the point to making anything for that.For the XLR-connected AES signal, the signal is fully differential and not ground referenced, although putting a ground at the center of the receiver termination may not be problem. I think the best idea is to make an XLR cable that can be tapped into and then use either a high-impedance differential probe or two 10X 10M probes and the MATH (SUBTRACT) function of your scope. This way you can see what is actually going on with an actual receiver with its complex termination characteristics at the end.If you want a quick way to measure without a receiver, your choices are 1) simply take the 2 signal wires from the XLR cable and connect them to the centers of two BNC connectors, then connect the shield lead to the shells of the BNC connectors via a capacitor (see the attached diagram) and then use the 50R input setting of the scope (this gives a 'close enough" termination of 100R) or 2) terminate the 2 signal wires with a 110R resistor and then connect them the same way as in #1 and use the 1M input setting or 3) terminate with a 110R resistor and use a differential probe.If you are asking how to physically construct these, ask again as that is a separate question. I would start with a ready-made molded coax-BNC cable and an AES XLR-style cable, cut them in half and then wire everything up in a small box. There's probably many ways that will work and I'd start with what you have in hand.

author: audiotubes
author_rank: Regular Contributor
timestamp: July 14, 2021, 08:19:20 pm
content: Thanks, this is the kind of info I was looking for!It is difficult for me to get to the back panels of some of my hifi gear because I have to take apart the shelving to do so. My DDCs are sitting on the top shelf, so I can easily unplug the existing cables and would like to measure directly off them.I don't understand the diagram exactly though. Are there transformers at both ends? Or is that just a picture of what the scope and device under test look like (RLC networks)?For option 2) in your post, are you saying run a 100 ohm resistor in series with each of the hot and cold leads? And do I bypass each of hot and cold to ground with the .1 uF cap?I see I didn't say it clearly but yeah, my question should have been:If I wire a 110 Ohm AES/EBU connector to two 50 ohm BNC connectors, is that good enough to measure the AES/EBU output of a device? I just didn't want to appear challenging or clueful, so I tried to give some background, that I'm really lost today, and ask an open ended question about how to accomplish this.I make my own AES/EBU cables, I haven't made a coaxial cable in a long time. So I plan to buy a couple of 50 ohm coax cables and chop off the ends and wire in the AES/EBU cable as you said in 1) and I tried to say above.One of the two DDCs under test is rated at 5V out at 50 ohms and my scope has a label saying (I think, since I don't have it in front of me) <5V @ 50 ohms.What happens if I apply too much voltage? Does the scope get damaged?

author: bdunham7
author_rank: Super Contributor
timestamp: July 15, 2021, 01:00:27 am
content: Quote from: audiotubes on July 14, 2021, 08:19:20 pmChop the diagram in half. The left part is the transmitter, the right is the receiver. The 'ground' on the right would be the BNC shells of the scope. You need to replicate the receiver as best you can with your scope termination. Imagine the big triangle is your scope and eliminate the transformer--and remember that you need to use two scope channels because this is a differential signal. The most correct way would be the 110 ohm resistor across the two signal wires, just as shown, followed by a direct connection to the scope, using the 1M inputs. There are no 'hot' and 'cold' leads, this is a balanced signal. And the bypass to ground is for the shield, the third lead. Bypassing the signal lines to ground with a capacitor will short out the signal.I suggested using the two 50R inputs of the scope as an alternative to the 110R physical resistor. This would put a second ground connection right at the middle of the termination resistor, which is not the way it normally would be connected AFAIK, but may be OK with a balanced, isolated signal. As for your scope inputs, I think they're rated for 5VRMSand should be OK, but if you are nervous you can use external pass-thru terrminators. I do not know if the 50R internal terminators on this scope will automatically disconnect in case of an overload. They should, and do on some other scopes, but I have not found this information in any manual.

author: james_s
author_rank: Super Contributor
timestamp: July 15, 2021, 01:05:34 am
content: Pretty much the only place I've seen 75 ohms is TV and video, outside of that 50 ohms is the norm for all kinds of test equipment and most radio gear, I've never seen or even heard of a scope with 75 ohm inputs. 50 and 75 ohm BNC connectors are very similar looking to each other.You can buy a set of decent 100MHz scope probes for under $20 shipped these days, there's no point in trying to build one unless you need something special.

author: audiotubes
author_rank: Regular Contributor
timestamp: July 15, 2021, 06:18:56 pm
content: Quote from: james_s on July 15, 2021, 01:05:34 amAs far as I know, everything in pro audio aside from AES/EBU and balanced analog is 75 ohm coax. I have some TASCAM, Mutec, and Mytek gear and everything is specced for 75 ohm including word clock, AES3/ID, and S/PDIF RAW (DSD).https://www.mutec-net.com/downloads/images/MC-1.1plus_Back_CMYK.jpghttps://www.mutec-net.com/downloads/images/MC-3_plus_USB_alu_Back_CMYK.jpghttps://www.markertek.com/productImage/alternate-3/hi-res/TCM-DA-3000.JPG

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 15, 2021, 08:49:40 pm
content: 110 ohms differential, implies 55 ohms from each side to ground. Assuming it's ground referenced. That is, put two 55R's in series and ground the center tap. Probe one side or the other and there you have it.If you don't need the comm channel functional (e.g. the transmitter just runs open loop and it's not bidirectional and trying to negotiate anything with the receiver), then you can make exactly such a line terminator, and make one of the 55Rs a series/parallel equivalent with the scope's input impedance (set to 50R terminated, or with an external tee and terminator, or passthru terminator). You may want to make a 20dB attenuator, to help isolate the scope from the signal; this way you don't have to worry much about terminating the unused channel, nor if transients get into the line, and potentially causing trouble for the scope. (Unlikely for most scopes, but a typical spec has lower input voltage/power limits for example, so it can show up in some tests like this.)You can furthermore make it symmetrical, so the pair of 55R terminators are each going to connectors, then use baluns to transform the CM and DM modes to get a better view of signal fidelity, and interference or EMI. (This might be best done directly, as with transformers wired as in the diagram, and with a center tap added to pick up the CM.)Somewhat more generally, you might simply consider the low-Z probe: a 50 ohm cable into a terminated load (the scope), with a 450 ohm resistor in series to it. This loads the circuit somewhat (55 ohms in parallel with 500 is what?) but is very unreactive, so can give good results -- with very little effort required. Do make sure the ground-return path is very solid: use a ground plane, and keep connections short.Tim

author: Faringdon
author_rank: Super Contributor
timestamp: July 17, 2021, 07:59:29 pm
content: Heres a great video on scope probes