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qmacpoint
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cables/wires for custom USBs

Fri Jul 12, 2019 9:21 am

So I'm trying to make my own USB cables (including sleeving) but I am having issues finding the right cable to use. I used ethernet core wires to make a test cable, and the end result ended up feeling very "wire-y" and not like an actual cable. Would you know what type of cable would be best suited for this?
 
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Re: cables/wires for custom USBs

Fri Jul 12, 2019 10:11 am

You're dealing with transmission lines for RF frequencies here, so you need to pay careful attention to characteristic impedance to avoid reflections which will degrade signal integrity. Ethernet twisted pair is 100 ohm, while USB is 90 ohm. Close enough that it should work... most of the time... but not optimal.

You really want bulk cable that is specifically designed for USB, like this. Or just chop the connectors off the ends of commercial USB cables.

You will also need to pay attention to the routing of the wires/traces which connect to your cable, as any deviations from 90 ohm transmission line design rules will cut into your signal integrity as well.
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superjawes
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Re: cables/wires for custom USBs

Fri Jul 12, 2019 10:38 am

just brew it! wrote:
You're dealing with transmission lines for RF frequencies here, so you need to pay careful attention to characteristic impedance to avoid reflections which will degrade signal integrity. Ethernet twisted pair is 100 ohm, while USB is 90 ohm. Close enough that it should work... most of the time... but not optimal.

You really want bulk cable that is specifically designed for USB, like this. Or just chop the connectors off the ends of commercial USB cables.

You will also need to pay attention to the routing of the wires/traces which connect to your cable, as any deviations from 90 ohm transmission line design rules will cut into your signal integrity as well.

Y'know, I think JBI is right. Either get a roll of cable already spec'd to USB, or get a few long cables from Monoprice and re-terminate them at the lengths you need.

I was going to ask what you meant by "wire-y", but I think that solves the problem. A pre-built run of cable should have the right "feel" because it is using the correct wire gauges and stranding, and it will have the correct impedance for signal integrity.
On second thought, let's not go to TechReport. Tis a silly place.
 
qmacpoint
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Re: cables/wires for custom USBs

Fri Jul 12, 2019 10:43 am

That is literally what I needed! Thank you very much!! :)

@superjawes: "wirey" meant that I could bend the cable in any form or shape and it would stay that way... it didn't occur to me that this would happen when I tried to remove the sleeve off the ethernet cable, but welp - yeah didn't take into consideration the correct impedance when I took this initial jab at it.
 
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Re: cables/wires for custom USBs

Fri Jul 12, 2019 10:53 am

Digi-Key has been one of my primary sources for components, cables, connectors, etc. for most of my life. In pre-internet days I ordered out of their print catalog via snail mail. They were the discrete electronics components version of the Sears catalog!
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Re: cables/wires for custom USBs

Fri Jul 12, 2019 11:08 am

just brew it! wrote:
They were the discrete electronics components version of the Sears catalog!

A reference I, but few others, will grok.
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Re: cables/wires for custom USBs

Fri Jul 12, 2019 11:09 am

just brew it! wrote:
Digi-Key has been one of my primary sources for components, cables, connectors, etc. for most of my life. In pre-internet days I ordered out of their print catalog via snail mail. They were the discrete electronics components version of the Sears catalog!

Same with Mouser.
 
qmacpoint
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Re: cables/wires for custom USBs

Fri Jul 12, 2019 12:01 pm

I will definitively have these references handy for any DIY projects then!

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