No need to demonstrate your viscous pedantry. You better
appply it to many less obvious areas of RIMM design.
First, we both perfectly know we were talking about
wide-bandwidth square-wave-like signals. Then
why did you bring out the academic
example about impedance matching at a single
frequency in the first place? To mislead less
sophisticated and semi-literate individuals here,
and to make a fool of your opponent?
Therefore, your second answer must be correct:
"This is not appropriate to the much wider bandwidths used in Rambus."
Now you are trying to squirm with "limited" bandwidth.
Wrong again. Every signal in the real world has a
limited bandwidth. So what?
More:
When you make a section of a trace "narrower", what do
you think happens to the trace impedance? You are trying
to sweep the problem under the rug. No matter what you
do, the impedance mismatch problem is not going
away, it just slightly changes it's shape. Of course,
it is possible to slightly optimize the most ugly
reflections, but the trick of trimming traces just
disperses the waves to some extent. But in the whole
transmission domain the phases of the reflected waves
still may combine in some less predictable way, with
the same final effect: the loss of bit of information.
In addition, the use of instrumentation with
inadequate bandwidth only aggravates the problem.
You just cannot identify the problem if you do not
see it in a heavy-filtered oscilloscope.
Here comes the difference you so deliberately trying
to hide: yes, the DDR load is a tree, but the RIMM load
is space-time distributed, so the wave propagation
effects play critical role. For less sophisticated
reades, please take a look at the following picture
ednmag.com
in order to understand the background of the problem.
Also, as I explained in many other posts, there
is no such thing as "constant current driver",
especially if the Vcc rails are only 2.5V. So your
implication that the driver has an infinite impedance
and therefore does not affect the matching, is
one of many official Rambus lies. You mislead
people by creating a false impression that RIMM
design is perfect. It is not.
With no respect,
- Ali |
