Is measuring square wave on spdif cable possible? - diyAudio
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Old 8th February 2014, 08:24 AM   #1
Paul S is offline Paul S  United Kingdom
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Default Is measuring square wave on spdif cable possible?

Hi all,

I'm after some advice if possible from you regarding testing the 'accuracy' shall we say of a digital interconnect. I have produced several, with careful attention to spacing etc. They have improved sonically each time, and the latest version is better than I have ever heard as far as macro dynamics, detail and space is concerned, really very good. Which is great, but i do want to see how it performs on a scope. My thinking is to connect to a square wave generator, set at 5.6khz / 0.5v, and observe the sure wave on screen. But, the scope's input impedance are mostly 1Mohm, with some also offering 50ohm. Also the output impedance of the generator is not 75 ohm either. So any reading will be arbitrary. I would also like to do the ams for 110ohm AES cable, and analog IC's too.

So, is this a non starter, or are there work arounds that can get me where I can see an accurate result? I currently do not own a scope, but if this is possible somehow I will purchase used or new perhaps a 10-20Mghz 2 ch device. But, it's the impedances that are halting this so far.

Any constructive advice would be very very welcome..

Thanks in advance,
Paul.

Last edited by Paul S; 8th February 2014 at 08:53 AM. Reason: grammar
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Old 8th February 2014, 08:47 AM   #2
peufeu is offline peufeu  France
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Yes you can use a scope.

However what matters is the waveform at the receiver's pins. The cable is only a part of that, the transmitter and receiver circuits are equally important. They may not be correctly matched, perhaps there are some not quite optimal transformers, etc. Using 10x probe on the receiver pins will provide more information than just testing the cable.

You can get used 100 MHz Tek scopes on ebay for low prices. If you do a lot of DIY stuff it is very useful. If you don't, find a buddy in your area who has one.
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Old 8th February 2014, 11:37 AM   #3
Paul S is offline Paul S  United Kingdom
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Many thanks for the reply, good to hear and confirms my thoughts.

Can you expand upon using a 10x probe for me? I'm keen to see whether the cable maintains the square wave as it should most of all, over varying length's also.

Do Tek scopes seem to be a good choice for this sort of thing, they seem to be often recommended above most others. There are several currently available, along with LeCroy and HP..
Thanks again, all help is appreciated very much..
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Old 8th February 2014, 04:12 PM   #4
peufeu is offline peufeu  France
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The scope probe and cable will disturb what you are measuring. Using the probe in 1X mode is like directly connecting the scope cable : its impedance is 50 ohm (causing a mismatch), it has non-negligible capacitance, and there will be reflections at the scope end. In 10x mode the signal is attenuated 10x and probe loading effects are also 10x less. Google high frequency oscilloscope probing and the like, you'll find nice articles.

Tek and the other brands you quote are good. I got a used 100 Mhz Tek 465 off ebay for like 100 €, had it calibrated and by a specialized shop (they also changed some bad old caps) which cost me another 100€, this thing works like new...

Its cost new was $1725.... in 1973 dollars... you could buy a car with that...

Or if you want digital modern gear, Rigol DS1052, pretty cheap too. Probably wont last 40 years though.

Last edited by peufeu; 8th February 2014 at 04:18 PM.
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Old 8th February 2014, 05:03 PM   #5
Art M is offline Art M  United States
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Scope input impedance in the Normal Probe Mode is 1 Meg Ohm not 50 Ohms. Scopes usually have a 50 Ohm termination setting also for use with 50 Ohm source impedance.
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Old 8th February 2014, 06:05 PM   #6
Paul S is offline Paul S  United Kingdom
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This is great help, many thanks guys..
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Old 8th February 2014, 07:23 PM   #7
peufeu is offline peufeu  France
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Quote:
Originally Posted by Art M View Post
Scope input impedance in the Normal Probe Mode is 1 Meg Ohm.
cable 120pF
scope 30pF

total 150pF => 1kOhm @ 1 MHz

150pF on a very slow 20 ns risetime square wave ... draws almost 40mA...

1x probe is useless at high source impedances and/or moderately high frequencies...

Last edited by peufeu; 8th February 2014 at 07:31 PM.
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Old 9th February 2014, 04:26 PM   #8
DF96 is offline DF96  England
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If your cables are 'improving sonically' each time then they must be tampering with the signal, hence they are not good 75R transmission lines. All you need is a competent 75R coaxial cable. Factories make them.

To measure, terminate the line with a 75R resistor and put a 10:1 scope probe on that. You will need to go much higher than 5.6kHz to see anything useful.
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Old 9th February 2014, 04:33 PM   #9
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Quote:
Originally Posted by DF96 View Post
If your cables are 'improving sonically' each time then they must be tampering with the signal, hence they are not good 75R transmission lines. All you need is a competent 75R coaxial cable. Factories make them.

To measure, terminate the line with a 75R resistor and put a 10:1 scope probe on that. You will need to go much higher than 5.6kHz to see anything useful.
There is an other possibility: if the DAC is very bad in jitter rejection and timing, changes in cabling can actually be audible.

Jna
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Old 9th February 2014, 08:11 PM   #10
Paul S is offline Paul S  United Kingdom
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Df96, can you expand on recommended f to see what's what? Initial coax trials were most likely not making 75 ohm, the latest are clearly far better, so are most likely tampering less with the signal, and now I'd like to see how well the signal is preserved. I have tried various off the shelf coax made by Belden etc, and they do not sound as good, by some margin. Another reason for wanting some back up science. Both the transport and DAC use RCA terminals, so not ideal.

Jan, (jna?), the DAC has no filter at all, hence the differences are audible, and sonically is among the best sounding digital I have heard..
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