From: [email protected]
Date: Fri Sep 29 2000 - 02:55:28 PDT
Tham,
I recently worked around an old system employing a ferrite. As Mike pointed out
I found the low level negative and the high level a little higher than without
the ferrite.
The idea of the ferrite is basically to lowpass filtering the source, arriving
to an Fmax whos wavelength is still much longer than the line. This will mask
the propagations effects. You can also think of it in the time domain: your
series inductance should kill the risetime, arriving to a risetime greater than
the travelling time on the line.
The advantage on a simple source terminating resistor is that you don't care
anymore about the dissimmetry of the driver.
This in theory: it turns out that parasitics (in particular the parallel
conductance) are limiting the effect of the ferrite and you won't increase the
Tr (beyond a certain amount) increasing the ferrite value, but just diminish the
level of your incident wave (after all is a series impedance).
A parallel capacitance, as Charles was saying, works better: there are less
parasitics.
The fact is that this technique was used to recover a wrong design, and it can
work only for slow-medium clocks. I would say, till about 20 MHz for a 40-50 cm
bus link (it still depends on the Tpd...). But it is not elegant at all. I would
suggest you to design your link in a more standard way.
Good Luck,
Luca
KokTongTHAM <[email protected]> le 29/09/2000 08:48:50
Veuillez répondre à KokTongTHAM <[email protected]>
Pour : Ron Miller <[email protected]>, [email protected]
cc : [email protected] (ccc : Luca GIACOTTO/TRV/DTR/GECALSTHOM)
Objet : RE: [SI-LIST] : inductor
Dear Ken, Ron Miller, Charles and other SI gurus,
Thanks for answer my question.
First of all, I'll like to apologize to you and the rest of the SI Gurus
who have read my question regarding on using inductors on transmission
line. It was suppose to be ferrite bead instead of inductor. Sorry for the
confusion and misunderstanding.
Back to the question, how can ferrite bead reduce ringing? How it
works? What are the criteria in choosing the correct ferrite for certain
frequency? Are there any books or articles on usage of ferrite beads in
transmission lines that you know of?
Thanks
Regards,
KTTHAM
At
$B8aA0(B 11:20 00/09/28 -0700, you wrote:
>Hi
>
>I think the inductors that you are talking about are ferrite torroids
>rather than
>inducters, and they have an apparent resistive component as well as an
>inductive
>component which dampens oscillations.
>
>Ron Miller
>
>
> > -----Original Message-----
> > From: KokTongTHAM
> > Sent: Thursday, September 28, 2000 3:07 AM
> > To: [email protected]
> > 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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