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USB hub

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In Reply to: RE: USB isolation hub (Gordon, audioengr, John - input wanted) posted by John Swenson on February 22, 2008 at 13:09:17

I thought it might be curious to see this hub design (prior to my modification). It seems at least some sort of consideration went into designing the power supply. If you read the NEC 720114 datasheet, it appears that Vbus can be used just for "handshake", and the rest of the chip is powered by the +5 V supply through the EH11A regulator. In my case, it runs off a rechargeable battery. I might eventually remove D2 for the extra assurance. Thus, I did not see it relevant to install the inductor on the Vbus line. The way I have it now is in hope it alleviates some hi-frequency currents between my hi-fi and my laptop via USB ground. By the way, ALL of my power supplies are TWO pin, for better or for worse.

Follow Ups:

RE: USB hub - audioengr 11:06:03 02/23/08 (1)
The fact that there is a crystal on this board may indicate that it is probably using a PLL internally to reclock. The datasheet is not much help. One thing that you could try is to replace the crystal with a low-jitter oscillator.

Steve N.
- RE: USB hub - pryamomimo 12:40:47 02/23/08 (0)
  Thank you for your comments. Another reason I have a feeling this chip might reclock is the way it simultaneously handles USB 1.1 and 2.0 "streams" ("split transaction", "transaction translation", etc.). Do not pretend to have an in-depth understanding of USB protocol, but my guess is that it might involve reclocking.
  
  So far I find an audible improvement when powering the hub from an external battery. Will have to listen more. I will be also trying to remove the D2 diode to disconnect for sure the downstream USB bus power from the PC (in this case the only connection to the PC power bus is via an RC circuit to the Vbusm pin - second from the borrom on the left side of the chip - used ONLY for monitoring whether there is a computer attached).
  
  Will also try other things to bring the picture to a resonable ideal - only the differential USB pair running to the USB chip, and power and ground lines isolated (or filtered) as much as possible.
  
  Then might try the oscillator route...

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