[SI-LIST] : Adding ferrites...

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From: Charles Grasso (chasgra@msn.com)
Date: Thu Sep 28 2000 - 11:52:52 PDT


Ken - I agree completely. Putting what amounts to a frequecy dependant
resistor is series with a transmission line is not ( and I will use your
words) not my idea of fun either.

Ferrites - and indeed filter caps - were used in the very early days
of EMC in ITE design to deliberately kill the clock edges. We didn't
need a whole lot of bandwidth in those days nor did were we concerned
very much with Signal Integrity. Products were pretty slow and forgving.
Not so nowadays!!

----- Original Message -----
From: "Ken Cantrell" <Ken.Cantrell@srccomp.com>
To: "Charles Grasso" <chasgra@msn.com>; "KokTongTHAM"
<kt_tham@uti2000.co.jp>; <si-list@silab.eng.sun.com>
Sent: Thursday, September 28, 2000 10:27 AM
Subject: RE: [SI-LIST] : inductor

> Charles,
> I've been wondering about the same thing. The only location where I have
> used ferrites successfully is the typical slow edge rate board outputs
> (RS-232, etc.)for emissions issues. It would seem that a frequency
variable
> impedence on a transmission line (series termination)would create a new
set
> of problems, especially with assymetric rise and fall times coupled with
> component tolerance. Since the impedence has a range of values, the
> acceptance function will vary. Let's assume that Zmax for the ferrite is
> our desired Rs. As you frequency approach Zmax, the initial series
> "resistance" will be less than the desired Rs causing overshoot and
> injection of noise into the power frame via the internal clamping diodes
at
> the receiver as you excursion beyond Vcc. For each incremental increase in
> frequency the overshoot diminishes until you reach f(Zmax). For
frequencies
> > Zmax you encounter the same situation (in reverse, as f increases,
> overshoot increases). Having a moving target is not my idea of fun.
Most
> assuredly you will increase the noise pollution of the power frame since
> f(Zmax)is the only critically damped point on the curve. I haven't had a
> chance to test this in the lab, and perhaps all this potential variation
is
> a don't care in the real world. But on the surface, it doesn't sound like
a
> good idea. Let me know what you think.
> Ken
> -----Original Message-----
> From: owner-si-list@silab.eng.sun.com
> [mailto:owner-si-list@silab.eng.sun.com]On Behalf Of Charles Grasso
> Sent: Thursday, September 28, 2000 9:36 AM
> To: KokTongTHAM; si-list@silab.eng.sun.com
> Subject: Re: [SI-LIST] : inductor
>
>
> I most certainly would not recommend that. Add the correct termination
> and watch the ringing disappear! The inductor will add delay and more than
> likely exacerbate the ringing.
>
> I know that some folks add ferrites in place of series resistors and I
would
> like
> to have a discussion on that. Comments?
> ----- Original Message -----
> From: "KokTongTHAM" <kt_tham@uti2000.co.jp>
> To: <si-list@silab.eng.sun.com>
> Sent: Thursday, September 28, 2000 3:07 AM
> Subject: [SI-LIST] : inductor
>
>
> > SI Gurus,
> >
> > I have told by an EMI engineer about using inductors can reduce ringing
of
> > a transmission line effectively instead of series resistor. Can anyone
> > tell if this is true?
> > As far as I know, inductor can act as a filter to cut unwanted
harmonnics.
> > How do we chose the suitable value for the inductor?
> >
> > Thanks.
> >
> > Regards,
> > KTTHAM
> >
> >
> >
> >
> >
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