USB spectrum

[audiox]

Does anyone know where to find info about spectrum of the USB 2.0 (full-speed) 12 Mbps data signal? I don't have a spectrum analyzer with suitable input to do the measurement myself. The data rate alone doesn't help much since I am not familiar with the protocoll.

Someone (not me) has placed a USB cable in the same cable pipe with mic lines. Not hard to guess what is the problem. It is impossible to change the cable route without breaking the wall. I am going to filter the mic lines first and if it doesn't help add some common mode filtering to the USB data wires. It would help a lot to know what kind of signal I am dealing with.

 

[rodabod]

Study the protocol and the regularity of frame headers, etc. to determine what dominant frequencies will be present.

Outside of that, it's probably encoded so that there will not be many series of either 1's or 0's to keep away from DC and possibly to allow polarity inversion a la AES (Bi-mark encoding?.... I can't remember).

 

[Mark Burnley]

Try starquad cable?

I've had great success with this in similar situations in "noisy" buildings.

Mark

 

[BradAvenson]

You might try some ferrites on the mic lines.
http://www.audiosystemsgroup.com/SAC0305Ferrites.pdf

 

[Andy Peters]

Does anyone know where to find info about spectrum of the USB 2.0 (full-speed) 12 Mbps data signal? I don't have a spectrum analyzer with suitable input to do the measurement myself. The data rate alone doesn't help much since I am not familiar with the protocoll.

I don't recall the edge rate (which is what matters), but do note that it uses differential signaling over a shielded twisted pair.

Someone (not me) has placed a USB cable in the same cable pipe with mic lines. Not hard to guess what is the problem. It is impossible to change the cable route without breaking the wall. I am going to filter the mic lines first and if it doesn't help add some common mode filtering to the USB data wires. It would help a lot to know what kind of signal I am dealing with.

My first question has to be -- how long is the USB cable in this pipe? USB spec says 5 meters.

You can buy a USB cable with a ferrite bead for noise suppression. Putting additional filtering in line with the two data signal lines will likely cause the thing to fail.

-a

 

[Michael Tibes]

Totally different aproach, don't know if that gets you somewhere. Aren't there USB to network adapters? If so, could this even be sent via WLan? Just an aproach to get the USB away without opening the wall. Or would either the USB1 or 2 spectrum cause less interference? Don't know what you want to connect. If it's just a keyboard and a mouse? Well, if it was then you could anyway just use a wireless. Just thinking...

Michael

 

[Svart]

The problem with USB is that it's digital.  ;D

Since you have fast edgerates you are going to have harmonics galore.  Close-in harmonics will be strong enough to mix with farther out harmonics and you'll get mix products that *could* pop up in your audio range.

You can filter out the higher band stuff but any mix spurs in the audio band are there to stay.  Besides, adding enough ferrite (to your USB cable) to start to push the harmonics down will be because you are laying your edges over on the signal you do want to keep and thus killing your USB integrity.



This is a longshot..

Maybe you can tape a better shielded usb cable to the end of the one you have and gently use the old one to pull the new one through?  You might only get a single shot at it so be prepared..

 

[Mark Burnley]

Aren't there USB to network adapters?

If you can still route a cable through here, convert USB to CAT5 and send this down an FTP (Foil Screen) CAT5 cable- I've used these foiled cables in situations with mic cables, and never had any trouble. Make sure at least one end is actually terminated though- you need the shielded CAT5 connectors (looks like a a standard plug, but with metal shield) and you need to allow the drain wire to make contact with the connector shell when you do the crimp.

It will probably have better shielding properties than a pre-made USB cable.

Extra cost for the converters, but if it helps...

Let us know what you end up doing!

Mark