USB Low Speed Only Data Filters/Chokes

[wuchy143]

Hi All,

I'm desinging a USB hub which will be going through a lot of EMI testing and have a couple questions.

From USB.org I found this design manual: https://www.usb.org/developers/docs/hs_usb_pdg_r1_0.pdf but could not find the low speed version. They are clearly just using chokes on the data lines and they even give P/N's for viable chokes. Problem is I can't find one for low speed devices. Does anyone know how different(if at all) low speed chokes are implemented?

 

[crutschow]

Why would you want chokes for the low speed devices? Doesn't you hub need to handle devices of any speed? The chokes for the low speed devices may degrade the performance of the high speed devices.

 

[wuchy143]

You are right. Though, our application is low speed only. Meaning, the hub will only be used connecting low speed devices. There may be a rare case down the line where a customer may want to hook up full speed but that will entail changing the chokes which is a simple BOM change. I need to "dial in" the EMI performance for Low speed devices only. I am wondering if there was literature for desinging low speed devices. Scouring the internet I didn't come up with much. Only for faster USB devices.