RE: [SI-LIST] : inductor

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From: Ken Cantrell (Ken.Cantrell@srccomp.com)
Date: Thu Sep 28 2000 - 10:27:24 PDT


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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