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Old 9th June 2014, 06:36 PM   #1
Harpy is offline Harpy  United States
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Default Non Powered USB Cable

Any thoughts on making a non powered USB cable? I use battery power on my convertor and it does not use any USB power. I would like to drop the 5 volt leads and just go with Data + and Data - with either a 0 volt return wire or shielding.
I was thinking of twisting 22 awg or higher wire for the data leads, maybe of silver wire with Teflon insulation. Any thoughts? If I kept it short could I go without the shielding?

Last edited by Harpy; 9th June 2014 at 06:40 PM.
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Old 9th June 2014, 08:15 PM   #2
dmills is offline dmills  United Kingdom
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You really need to hit the correct charicteristic impedance for USB to work well, spec is 90 ohms +-15%, and you need to control the common mode range so you will need some sort of 0V connection.

USB is fastish digital, and needs to be treated as such, trying to make it work with a lashup will only result in heartache.

Regards, Dan.
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Old 9th June 2014, 08:57 PM   #3
Harpy is offline Harpy  United States
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Thanks, Dan. I will consider purchasing one instead.

Last edited by Harpy; 9th June 2014 at 09:02 PM.
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Old 9th June 2014, 09:11 PM   #4
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I designed a USB mixer that was self powered and simply didn't connected the 5 volts on the USB socket.
I would never make my own cable as they are so cheap to buy.
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Old 9th June 2014, 09:59 PM   #5
Harpy is offline Harpy  United States
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What determines the 90 ohms? Wouldn't that be set by the internal comments or does the twisting of the data cables effect that too? The transport has a jumper on the PCB the lets you switch between USB Ground and Chassis Ground that would go to an earth ground. I was sure what that might be used for.

Thanks Jim
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Old 9th June 2014, 11:12 PM   #6
dmills is offline dmills  United Kingdom
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The 90 ohms is the transmission line impedance of the cable (What you would see looking into the end of an infinite length of lossless cable), it is determined by conductor diameter and spacing and the dielectric constant of the insulating material.

Formulas are in any good EM theory book, "Fields and waves in communication electronics" is good for this stuff.

Regards, Dan.
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Old 10th June 2014, 10:41 AM   #7
DF96 is offline DF96  England
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Quote:
Originally Posted by dmills
"Fields and waves in communication electronics" is good for this stuff.
An excellent textbook, but not exactly beginner's material! Someone like Krauss may be more accessible.
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Old 10th June 2014, 04:10 PM   #8
Harpy is offline Harpy  United States
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Ok, Thanks guys. The information has really been helpful. I don't want to start a cable debate, but do most of high end audio USB cables meet the 90 ohm transmission spec? I would think that and shielding would be important in determining the quality of the cable.
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Old 11th June 2014, 10:05 AM   #9
DF96 is offline DF96  England
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All genuine USB cables meet the 90ohm spec. Some 'high end audio USB cables' might not be genuine USB cables; they are not designed for data integrity but for currency extraction!
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Old 11th June 2014, 11:33 AM   #10
phofman is offline phofman  Czech Republic
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Quote:
Originally Posted by DF96 View Post
All genuine USB cables meet the 90ohm spec. Some 'high end audio USB cables' might not be genuine USB cables; they are not designed for data integrity but for currency extraction!
Especially those hand-wired for greater audiophile pleasure/extraction :-)
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