[SI-LIST] Re: [Help] How to do termination in differential signal with series termination ?

CM signal is unwanted only when we are talking about differential
signaling right. I want to know few things about this reducing the
common mode signal by using common mode choke. How this thing works
exactly? Can we use for all frequency or any particular range.

Regards,
Kiran



-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
On Behalf Of olaney@xxxxxxxx
Sent: Friday, December 05, 2008 12:03 PM
To: yardala@xxxxxxxxx
Cc: weirsi@xxxxxxxxxx; si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: [Help] How to do termination in differential
signal with series termination ?

Series termination implies a round trip reflection, and that is exactly
what you don't want to do with a common  mode signal unless the intent
is
to more efficiently radiate it.  Instead, use a three resistor
termination at the receiving end to parallel terminate both modes, pi or
Y configuration depending on relative impedances.  You can use just two
resistors if Zcm = Zdiff/4, with each = Zcm/2 to ground.  Better yet,
suppress the CM signal at the sending end with a CM choke.  I presume
that any CM signal is unwanted, and not some sort of independent channel
as is sometimes used in telephony, or for PoE (power over ethernet).

Orin Laney

On Fri, 5 Dec 2008 14:20:09 +0800 "King Da" <yardala@xxxxxxxxx> writes:
> Dear Steve,
> Thanks for your answer, but I don't really know what your mean 
> "you've got
> your impedances backward" ? Could you explain more detail ?
> I use 100 Ohm for differential termination for diffrential signal 
> and also
> want use series termination for each line for common mode signal
> termination. How to choose the series resistor value ?
> 
> 
> On 12/5/08, steve weir <weirsi@xxxxxxxxxx> wrote:
> >
> > Parallel terminations absorb energy at the far end and is related 
> only to
> > the channel, not the driver Z.    Also, you've got your impedances 
> backward.
> >
> > Steve.
> > King Da wrote:
> >
> >> Hi,Folks,
> >> I have a differential pair which odd mode impedance is 48 Ohm and 
> even
> >> mode
> >> impedance is 108 Ohm by TDR measurement.
> >> I would like use 100 Ohm for differential termination at far end, 
> but how
> >> to
> >> choose series termination resistor for each trace at near end?
> >>
> >> I refered Bogatin's "Signal Integrity:Simplified" book that said 
> parallel
> >> termination R2 equal Zeven, So, I should use 108 Ohm subtract 20 
> Ohm of my
> >> driver's Ro is 88 Ohm for my series termination at near end, am I 
> right?
> >>
> >> The Rseries = 108 (Zeven) - 20 (Driver's Ro) = 88 Ohm, right ?
> >>
> >> Does the differential termination resistor is 100 Ohm better than 
> 91Ohm ?
> >>
> >> Thanks,
> >>
> >> Yardala
> >>
> >>
> >> 
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