spdif to oscilloscope - Page 1
Forum section: electronics / beginners

author: metebalci
author_rank: Frequent Contributor
timestamp: December 28, 2021, 06:45:03 pm
content: What is the best way to interface an spdif (coax) signal to oscilloscope ? I have 1M and 50ohm input options on my scope. I believe spdif is 75 ohm. There are bnc-rca adapters but I dont understand (I dont think) they have inline termination on rca side. I am interested in both a quick no need to be very correct but enough to see the signal solution and also a correct long term solution that I can use multiple times with different devices.

author: magic
author_rank: Super Contributor
timestamp: December 28, 2021, 06:51:49 pm
content: I have one of those adapters, there is nothing in it.I suppose you could put a BNC tee on the scope and a 75Ω BNC terminator plug.Good enough for tens of MHz. Ancient Ethernet networks ran happily on such hardware (50Ω in that case).

author: metebalci
author_rank: Frequent Contributor
timestamp: December 28, 2021, 08:29:14 pm
content: OK so it is like terminating 75ohm and reading it at 50ohm input ?I just saw something like a matching adapter, 75ohm bnc f and 50ohm bnc m, and then I can use a bnc-rca cable I guess. It says for the matching adapter dc-2ghz, it looks like a good solution.

author: bdunham7
author_rank: Super Contributor
timestamp: December 28, 2021, 08:44:50 pm
content: Are you concerned about the terminating resistance or the characteristic impedance of the BNC connectors themselves? If the latter, don't worry about as that section is very short. As far as terminating resistors, the appropriate solution is an RCA-BNC adapter to a F-M-F BNC tee with a 75R terminator on the far leg. However, if you just use an RCA-BNC adapter straight into the scope at 1M impedance, it will probably work OK anyway because all of the reflections will be absorbed at the source.

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: December 28, 2021, 09:14:19 pm
content: You can use 75 ohm leads for the 50 ohm line. You will most likely not be able to even see the difference.If you have 75 ohm lines with BNCs use themThe 50 ohm inputs on your scope will give you good reading of the voltage of the input signal. If the signal is coming from a 50 Ohm output, like a signal generator. The 50 ohm input can be thought of as a "50 Ohm Load" for the other device being sampled. The megOhm input will still give you a trace but it will be bigger and not accurate in height and maybe not good at seeing something like a square wave correctly.Any attempt at "correcting" the 75 to 50 Ohm mismatch will most likely cause more trouble than it is worth.RCA phono jacks are really 50 ohm connectors.Wally

author: metebalci
author_rank: Frequent Contributor
timestamp: December 28, 2021, 09:17:17 pm
content: Terminating resistance. Thanks for the description, just to make everything explicit in the connections:Spdif output is rcaRca to bnc f adapter75 ohm bnc cable m-m to bnc tee f75 ohm bnc terminator m to bnc tee f50 ohm bnc tee f-m-f (I guess 50 ohm refers to m connector going to scope ?)Bnc tee m to scope 50ohm inputIs that correct ?

author: bdunham7
author_rank: Super Contributor
timestamp: December 28, 2021, 09:37:27 pm
content: Quote from: metebalci on December 28, 2021, 09:17:17 pmNo, the scope should be high-impedance (1M). The 75R terminator on the tee near the scope is adequate termination. The 50R characteristic impedance of the BNC tee itself is not an issue, you an ignore it.

author: metebalci
author_rank: Frequent Contributor
timestamp: December 28, 2021, 10:30:29 pm
content: OK, it is like terminating the spdif line with 75ohm and using 1:1 probe but in an easier better way with tee and terminator, got it.

author: metebalci
author_rank: Frequent Contributor
timestamp: December 28, 2021, 10:37:02 pm
content: Quote from: Wallace Gasiewicz on December 28, 2021, 09:14:19 pmSo if I understand right you are saying I can directly connect spdif (75 ohm) out to 50ohm scope in without losing much. So a single rca-bnc 75 ohm cable would be good enough ?Maybe I should add I am not interested in amplitude measurement or rise fall times, I assume they are fine, I am interested in serial decoding and jitter measurements.

author: magic
author_rank: Super Contributor
timestamp: December 28, 2021, 11:00:36 pm
content: If the transmitter includes proper source termination of 75Ω (it theoretically should) then you can have impedance mismatch at the receiving end of the cable and no data distortion will occur, only wrong amplitude.If the cable is not required due to physical distance between devices, you could simply hoop up a standard 10x probe straight to the output jack and sidestep all that drama. Output amplitude will be twice that seen by a properly terminated receiver.

author: metebalci
author_rank: Frequent Contributor
timestamp: December 28, 2021, 11:13:21 pm
content: I need my hands free and need a stable connection so I prefer not using the probes. OK, so I think I will try a few alternatives mentioned here.

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: December 29, 2021, 03:27:04 pm
content: Yes, in most cases you can directly connect a 75 ohm signal to a 50 ohm input and not notice the difference. As long as the output from the device is not more than several volts. I am not familiar with the voltage of spdf but I can put output of ten volts into my scope without a problem.Do not put the output from something like a transmitter into a 50 ohm scope input.Think of it as a very small (in Watts) load. Should say something on the front of the scope input as to max.The output of a device is not necessarily correct unless terminated in it's correct impedance. If you are sampling something that is already terminated (going into something else) that is when you use the scope probes with high input impedance. That is most of the time you are using the scope.If you have a function generator with a 50 ohm output connect it to the scope and see how this works. Switch back and forth from 50 ohm to high impedance input on the scope and see how it affects a square wave. If you can do this (if you have a function generator) you will quickly grasp the situationWally

author: metebalci
author_rank: Frequent Contributor
timestamp: December 30, 2021, 05:30:54 pm
content: OK so I tried 3 alternatives:1) spdif out -> rca-bnc (75ohm) cable -> 50 ohm input : Vpk-pk = 435mV2) spdif out -> rca-bnc (75ohm) cable -> 75ohm-50ohm impedance matching pad -> 50 ohm input : Vpk-pk = 240mV3) spdif out -> rca-bnc (75ohm) cable -> bnc T (50 ohm) F-M-F, other end F is 75 ohm terminated, M goes to scope -> 1M input: Vpk-pk = 508mVI dont know if anything would be different with a bnc T 75 ohm, I dont have one yet so I cant try. Is it more correct to have a bnc T 75 ohms because spdif cable and termination is 75 ohm ? but the scope connector is 50 ohm.If I am looking to correct spec, it says SPDIF is (0.5+-1) Vpk-pk when terminated with 75 ohms. So (3) above verifies this. (1) is a bit less than 500mV, and is it because of reflection (~0.2) between 75 ohm to 50 ohm ?The impedance matching pad has VSWR 1.1 at 50ohm, what is the calculation here to reach 240 mV ?Can you enlighten me how (1) and (2) can have these values ?

author: bdunham7
author_rank: Super Contributor
timestamp: December 30, 2021, 06:15:16 pm
content: Quote from: metebalci on December 30, 2021, 05:30:54 pmThe characteristic impedance of the BNC Tee is going to have virtually no effect here. Your values all look pretty much as expected, IMO. Option 3 gives you the 'correct' result.QuoteSo forget reflections and VSWR, instead consider the source to be a 1-volt battery with a 75R resistor in series. Now ignore the characteristic impedances of the coax and connectors, consider them all as a pair of wires with infinite resistance between them. Consider the terminator plug, the scope input and your matching pad to simply be resistors (not sure what the numbers are for the pad) and calculate the result using Ohms law only. IOW, just analyze it as a DC circuit. See how close you get to your measured results.

author: metebalci
author_rank: Frequent Contributor
timestamp: December 30, 2021, 06:40:39 pm
content: Quote from: bdunham7 on December 30, 2021, 06:15:16 pmgot it, thanksI got the feeding spdif to oscilloscope part but stucked at differential manchester now. My plan was to see/decode the signal and measure jitter, but I guess I cant use data jitter analysis on the scope directly now because it cant decode differential manchester, so cannot recover the clock, I also cant say the data rate I think because the symbols do not always have the same width. I thought it would be simple to do this

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: December 30, 2021, 06:59:36 pm
content: There is a "loss" in any matching pad. I assume your matching pad has about a 3 db loss.I don't know which pad you are using.Perhaps you could look up the specs of you particular pad.The 50 ohm inputs on a scope are never exact either.IMHO, Those BNC "T" s are never quite right, no matter what the listed impedance, but they are so short that we often neglect any problem. But you have achieved an acceptable reading. The load in this case is your scope input impedance in Parallel with the 75 ohm load, not just the 75 ohm load.A 75 ohm load on a 50 ohm output will give you a 1.5 to one SWR, at least that is what I remember.It is exactly the same the other way around, from 75 ohm source to 50 ohm load. SWR is still 1.5 to one. Really quite small.Anyway, good job so far. I think the scope on 50 ohm input gives you a good look at a terminated output for your signal.Since S PDIF is a digital slow signal, the slope of the waveform is probably important. Use whatever setting gives you a good look at the waveform.