I am running out the door.

Posted by John Marks on March 4, 2016 at 13:04:59

In Reply to: RE: What I have been told is... posted by John Elison on March 3, 2016 at 19:40:28:

Sorry for the brevity.

I am not an engineer, but...

1. Resistance is vis a vis DC; impedance is vis a vis AC, but both are measured in Ohms. The "characteristic impedance" of a coaxial cable is primarily a function of the perpendicular measurement between center conductor and outer shield. NOT of the running length, which would be the case of a length of lamp cord when resistance is measured.

2. There are several factors that determine cable impedance but the physical problem with RCA jacks is that between the diameter of the hole for the pin on the RCA plug and the outer diameter of the jack, there is not enough room to fit in enough dialectric material.

Yes, there are small-diameter coax cables but their center conductor is much thinner so there is less need for dialectric material.

BNC connectors are configured differently and can create a closer match.

3. To ESTIMATE cable impedence, you need a HF laboratory frequency generator, an SWR bridge and meter, some math, and a long enough length of cable to get a semi-accurate result.

I link to Wiki.

These issues are far more often of concern in amateur radio than in audio:

http://www.eham.net/ehamforum/smf/index.php?topic=20623.0;wap2

Also somewhat relevant or interesting is:
https://en.wikipedia.org/wiki/Time-domain_reflectometer

Which may be able to detect the unavoidable reflections between an 110-Ohm cable and the XLR plugs, which have their own mismatch issues.

Unless you are dealing with a special instrumentation cable, almost all coax cables are 75-Ohm.

Most coax cables are labeled along their length with country of origin and characteristic impedance and maker's name or logo.

Hope this helps.

john

SWR measuring (Open in New Window)

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Follow Ups Full Thread

Topic - AES/EBU digital 110 ohm characteristic impedance - Duster 18:40:57 03/3/16 (19)

RE: AES/EBU digital 110 ohm characteristic impedance - Thorsten 19:38:09 03/9/16 (3)

RE: AES/EBU digital 110 ohm characteristic impedance - noway 06:48:19 03/24/16 (1)

RE: AES/EBU digital 110 ohm characteristic impedance - Thorsten 06:57:23 03/25/16 (0)

RE: AES/EBU digital 110 ohm characteristic impedance - 91derlust 12:57:29 03/21/16 (0)

RE: "...one might think that a rather strict tolerance is required..." - genungo 00:51:05 03/7/16 (0)

What I have been told is... - John Marks 19:05:25 03/3/16 (13)

Let's be careful going forward... - Chris Garrett 22:11:52 03/6/16 (0)

RE: What I have been told is... - PAR 16:26:47 03/4/16 (1)

I agree with many of your points--especially what happens behind the back panels. - John Marks 16:40:26 03/4/16 (0)

RE: What I have been told is... - John Elison 19:40:28 03/3/16 (9)

I am running out the door. - John Marks 03/4/1613:04:59 03/4/16 (8)

Thanks! I think I found a better link, though.... - John Elison 13:57:07 03/4/16 (7)

RE: Thanks! I think I found a better link, though.... - flood2 18:57:32 03/6/16 (5)

RE: Thanks! I think I found a better link, though.... - John Elison 19:50:26 03/6/16 (4)

You can measure this with a Time Domain Reflectometer - Tony Lauck 10:42:40 03/7/16 (2)

RE: You can measure this with a Time Domain Reflectometer - John Elison 22:18:08 03/7/16 (1)

RE: You can measure this with a Time Domain Reflectometer - flood2 14:59:42 03/8/16 (0)

You're welcome.. - flood2 20:25:19 03/6/16 (0)

That's what the math looks like! - John Marks 16:31:35 03/4/16 (0)

Follow Ups

I am running out the door. - John Marks 03/4/1613:04:59 03/4/16 (8)

Thanks! I think I found a better link, though.... - John Elison 13:57:07 03/4/16 (7)

RE: Thanks! I think I found a better link, though.... - flood2 18:57:32 03/6/16 (5)

RE: Thanks! I think I found a better link, though.... - John Elison 19:50:26 03/6/16 (4)

You can measure this with a Time Domain Reflectometer - Tony Lauck 10:42:40 03/7/16 (2)

RE: You can measure this with a Time Domain Reflectometer - John Elison 22:18:08 03/7/16 (1)

RE: You can measure this with a Time Domain Reflectometer - flood2 14:59:42 03/8/16 (0)

You're welcome.. - flood2 20:25:19 03/6/16 (0)

That's what the math looks like! - John Marks 16:31:35 03/4/16 (0)

I am running out the door.

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