Bob Schematic - Usb Filtering


jokeman

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i've a question about the USB Host BoB Schematic.
i've already finished my BoB and it's working fine, but i also want to know, what i have done ;)

what is this USB filtering for?
and what is filtert?

i looked into the USB 1.1 Spec and there are similar schematics (figure 7-8)
but i don't know, what they are for
 
art said he couldn't get anything to work without the filtering because the signal was too noisy.

That's all I know...
 
It is a "low-pass" filter, which means that it allows low-frequency signals to go to the output and blocks high-frequency signals.

If you know a little about circuits, you probably know that a capacitor in a DC circuit will act like an open circuit. In other words, it blocks low-frequency voltages (DC can be thought of as a 0 Hz signal). But, to high-frequency signals, the capacitor acts like a short.

In the USB filter circuit, noise from ambiant sources will generally be relatively high-frequency, so they will be dissipated to ground through the capacitor. On the other hand, the low-frequency pulses of the USB signal will see the capacitor as an open circuit and will be read by the GP2X.
 
hmm... ok... this make things clear
thanks

so has this something to do with the "USB termination" scope picture in the USB Host Wiki?

and is this low-pass filter the same thing described in the USB 1.1 Spec chapter "7.1.2 Data Signal Rise and Fall"?
or is this something different?
 
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