[SI-LIST] effect of common choke on high speed differential signals
- From: "Joel Brown" <joel@xxxxxxxxxx>
- To: <si-list@xxxxxxxxxxxxx>
- Date: Wed, 5 Aug 2009 12:27:41 -0700
We have been using common mode chokes on USB and DVI interfaces.
We don't know if we need them or what effect they are having on EMI or
signal quality.
They were added because they were recommended on some application notes and
design guidelines.
The specification of one of the chokes is 90 ohm common mode impedance, 20
ohm differential impedance, 3 ohm DC resistance.
Although common chokes are supposed to have low differential impedance, it
seems the 20 ohm differential impedance would be a discontinuity when used
in a 100 ohm differential system.
Since the part is only 0.05 inches in length it seems this discontinuity
would be negligible at DVI data rate of 1.6 GBPS.
Also it seems that the 3 ohms of resistance would cause some attenuation by
acting as voltage divider in series with the termination resistance.
I have asked the manufacturer for a spice model so I can do some simulation,
but I am not sure if they will provide one.
Is there a simple spice model I can create based on the specifications? It
seems that capacitance would be one parameter that is missing.
On our next design we plan to have traces shorting across the choke so we
can test with and without it.
Thanks - Joel
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