Ali,
Then
why did you bring out the academic
example about impedance matching at a single
frequency in the first place?
Because you said that it was impossible. Its not academic, it is a widely used technique. It is also similar to the method that Rambus use for matching the loads of the memory chips to the bus.
Now you are trying to squirm with "limited" bandwidth.
Wrong again. Every signal in the real world has a
limited bandwidth. So what?
So any discussion about imperfections in a transmission line must take account of the frequency range over which it is to be used.
No matter what you
do, the impedance mismatch problem is not going
away, it just slightly changes it's shape.
No, up to a certain frequency, the mismatch becomes small enough to be negligible. The key is to ensure that frequencies above this limit are not present in the system. That is what Rambus does.
They are helped by the fact that losses in copper/epoxy glass PCBs increase with frequency which damps down any small ripples that may occur.
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.
Wave propagation effects play a critical role in both types of system.
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.
Perfectly true, there is no such thing as a perfect constant current driver. However, the Rambus data sheets specify the minimum allowable output impedance. It is sufficiently high (many times higher than the bus impedance) that it is unimportant.
You mislead
people by creating a false impression that RIMM
design is perfect. It is not.
No, I don't suggest it is perfect. It is a very elegant engineering design which achieves what it claims.
I will leave others to judge who is trying to mislead.
John |
