> > I have discovered that I might be able to remove about 200 source
> > from my 5/5 mil design if I can manufacture with exact impedance match a
> > 4-layer FR4 board.

I have to respond to your concerns and about controlled impedance. First of
all if the resistors you are claiming to eliminate are located at the source
as is a commmon practice,   These resistors are not  in the circuit for
impedance purposes.  I will stay away from all the math, but the resistors
are in the circuit to increase the R value of  the first time constant which
directly effects the risetime of the pulse. By slowing the leading edge of
the pulse you reduce much higher frequencies inhereent to  the fundamental
133 MHZ.

Second in order to gain "controlled impedance"  every layer must be
referenced to a plane layer. For internal layers that means signal layers
must be sandwiched between (read non split) planes.

And last, even the best materials will yeild +/- 10 percent of your target
impedance.   Variations in thickness of the pregreg and epoxies will ocurre
with any material  you choose.  The purpose of choosing a high speed
laminate is to reduce losses caused by absorbtion.   Controlled impedance
can be achieved on cardboard, if you understand the properties well enough.

I would not attempt to elimante your 200 resistors, they are in your circuit
to reduce the leading edge risetime not a controlled impedance matching.

Mike Reagan
Frederick Md

 It seems that laminates down to about 2.5 mil are
> > available but conventional wisdom is to not go below 4 mil and
> > stay at 5 mil.  I would need to use about 3.5 mil of FR4 material to try
> > get a 50-ohm impedance match with my processor.  The question is whether
> > 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
> > Any thoughts by anyone would be appreciated.  I have committed all of
> > calculations to my calc and verified them with numerous examples so I
> > be able to verify any suggestions quickly, in so far as calculations
> > ;-)  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.
> the info I have seen and heard says that FR-4 is not a good bet for
> controlled impedances - too much variation in thickness, dielectric
> etc. You may need to switch to PTFE or some of the newer Rogers materials
> the board. Depending on your assembly costs, etc., the more expensive
> 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
> and require some sort of termination to avoid reflections. On the other
> from my perspective 133 MHz is practically DC anyway, so you might get
> without them - but I wouldn't necessarily bet a board turn on that!
> Steve Hendrix

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