Signal Delay in RC Network

[[email protected]]

Hi all,


Actually, I posted this topic in sci.electronics.basics before. And
then, I found this group. And I think it is a better to post this.
(Just for interest) I'm now studying something related to RC network
delay. One of the ways Elmore's) to calculate the delay is:

-/\/\/-+-/\/\/-+-...­..-+-/\/\­/-+
R1 | R2 | | Rn |
--- --- --- ---
C1--- C2--- --- ---Cn
| | | |
___ ___ ___ ___
= = = =

time delay from left to right
= (R1*C1) + (R1+R2)*C2 + ... (R1+R2+...+Rn)*Cn equ.(1)

Now consider a 1-pi RC network below:
-/\/\/-+-/\/\/-+
R1 | R |
--- ---
C/2--- C/2---
| |
___ ___
= =

Also consider a 2-pi RC network:
-/\/\/-+-/\/\/-+-/\/­\/-+
R1 | R/2 | R/2 |
--- --- ---
C/4--- C/2--- C/4---
| | |
___ ___ ___
= = =


Interestingly, according to equ.(1), the time delay of both pi-model is
the same!! (time delay = R1*C + R*C/2)

However, some books say that the higher the order (higher pi) of the RC
network, the more accurate and practical is the model.(Of course, it
takes more time for simulation since there are more components in
higher order RC network.) How come this is more accurate and practical
provided that they have the same time delay using equ.(1)??? Is this
because equ.(1) is just approximation and not accurate enough to
describe the model??

Any comment would be very appreciated

Will