Mike Reagan wrote:

>
> Tim,
> 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.

Not completely true, but the slowing of the incident wave is correct.
But, these source termination resistors DO, indeed, terminate the reflection
from the far end of the line, as long as the driver stays active.  If the driver

shuts off, the line becomes unterminated.

> 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.

You can have controlled impedance on non-sandwiched signal traces.  A
signal with only one ground plane is termed "microstrip" and has different
equations than one with two ground planes, termed "stripline".  In his case,
the two ground planes lower the effective impedance for a given trace width.

> 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.

RIGHT!  But, 10% matching may well be good enough to keep reflections to a
level acceptable on his data lines.

Jon

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