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Re: Feliz Haridad

Ron Miller:

At 03:40 PM 17-12-99 -0800, Ron Miller wrote:
>Hi Pat and Rich 
>Let me help ease some of the confustion on bandwidth requirements: 
>5.    Bottom line is, specify the filter by: shape(bessel), number of poles(probably 1, 2 or  3) 
>and by the attenuation specified at the frequency.  [...]

we had a small bit of difficulty with this waaaaay back in 10BaseT days, at a
time when the proposed version was to include an external filter module
in the transmit path.

at that time the task force was asked to define many electrical characteristics 
of a link, and with that, recommend minimal characteristics of reflection, insertion 
loss, transition band slope, attenuation, and so on, for this module.  after a bit 
of work some of us found that most characteristics were relatively easy to 
realize with off-the-shelf components, but that desired return loss was not 
as easy.  

we concluded that for most characteristics you could use either 1 pole 
(which did not offer enough transition band slope) or 5 poles, but nothing 
in between offered sufficiently flat impedance variation over frequency , from
a 2-port perspective.  to be fair, we did not characterize things well enough 
to explain exactly why this was true (though if memory serves it had a lot 
to do with manufacturing variability and design margin).  it's my perception 
the committee were mostly interested in arriving at an acceptable answer, 
and this was sufficient.  module vendors wound-up offering something 
like 8 or 9 poles, and all of that was supplanted by the filterless version. 

finally, so-called Bessel-Thompson filters offer a nice tradeoff between
transition band slope and group delay distortion, hence their relatively wide 
acceptance in datacom.  i'm not certain i know what a "Bessel" filter is.

J M Wincn, Staff Engineer
Cielo Communications, Inc.
325 Interlocken Pkwy, Bldg A
Broomfield, CO 80021-3497
Voice: 303-464-2264
Fax: 	303-460-6370