Hi, Steve.  This design is source terminated, that is, it absorbs the
reflections at the source, which is 50 ohms.  The resistors only match any
residual difference between the source and the line impedance.  So if I can
get that close to zero, they go away.  I'm hoping someone will say that at
133mhz, the process variations won't matter to much for short lines (which
get a lot shorter as I eliminate the resistors - it really tightens up).


regards,
Tim Hutcheson
[EMAIL PROTECTED]




> -----Original Message-----
> From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED]]
> Sent: Monday, August 06, 2001 1:25 PM
> To: Protel EDA Forum
> Subject: Re: [PEDA] 5/5 Manufacturing Issues
>
>
> In a message dated 8/6/01 2:18:58 PM Eastern Daylight Time,
> [EMAIL PROTECTED] writes:
>
>
> > I have discovered that I might be able to remove about 200
> source resistors
> > from my 5/5 mil design if I can manufacture with exact impedance match a
> > 4-layer FR4 board.  It seems that laminates down to about 2.5 mil are
> > available but conventional wisdom is to not go below 4 mil and
> preferably
> > stay at 5 mil.  I would need to use about 3.5 mil of FR4
> material to try to
> > get a 50-ohm impedance match with my processor.  The question
> is whether or
> > not films are available in such precise thick nesses and are the results
> > reproducible enough.  I would expect that the first prototypes
> would give me
> > my baseline and I would go from there but only if the process
> is repeatable.
> > Any thoughts by anyone would be appreciated.  I have committed
> all of the
> > calculations to my calc and verified them with numerous
> examples so I would
> > be able to verify any suggestions quickly, in so far as
> calculations go...
> > ;-)  My design is for 66/100/133 mhz operation.
> >
> >
>
> I do some RF design though I wouldn't really claim to be an RF
> engineer. All
> the info I have seen and heard says that FR-4 is not a good bet
> for tightly
> controlled impedances - too much variation in thickness,
> dielectric constant,
> etc. You may need to switch to PTFE or some of the newer Rogers
> materials for
> the board. Depending on your assembly costs, etc., the more
> expensive board
> material might still be more cost-effective overall, and you'll get more
> repeatable designs. But I'm not so sure you can really eliminate
> that many
> parts - I'm having trouble visualizing a design where the use of
> controlled-impedance traces would eliminate the need for termination
> resistors. The input pins where the lines terminate are still
> high-impedance,
> and require some sort of termination to avoid reflections. On the
> other hand,
> from my perspective 133 MHz is practically DC anyway, so you
> might get away
> without them - but I wouldn't necessarily bet a board turn on that!
>
> Steve Hendrix
>
>
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