[SI-LIST] Re: Rise time calculation

Hello,
  It seems there is a fallacy in the argument.   Source termination is decided 
based on following :
1. to avoid transmission line reflections by proper termination.
2. the value is selected based on the drive capability of the driver.  If the 
driver has 20 Ohm
output impedance, we may have to place 30 Ohm for proper termination when using 
50 Ohm trace to take
care of (1).  But if the trace length is so large or the drive strength is so 
poor inspite of good
rise time at the output of the driver, which normally is the case, the series 
resistor will not be
30 Ohm. The value may have to be less than 20 Ohm.
What I feel is, this should be treated as a low pass RC combination rather than 
the equation given
below.   The equation below does not include the series termination value; the 
assumption of having
Zo in the equation leads to the fact that the series termination is the 
difference between the
characteristic impedance and the source impedance.  But that is not the case in 
practical. The
series termination will be different.    I feel that treat R as the source 
termination resistance
and C as the total capacitive load (trace + pin + load) and apply 2.2RC for 
getting the rise time
(assuming infinite current source).   Even for a slow logic like TTL, the peak 
currents are as high
as 600 mA.  Hence it can be approximated that the driver is a infinite AC 
current source.

Please comment.
Regards,
Ravi
subramanya C K wrote:

> Good Morning all,
> here are my two cents to the topic.
>
> It's been observed by practice (and also proved mathematically) that a
> series termination adds up to propagation delay and also slows down the fast
> rising edges.
> Some amount of mathematics can be used to prove that propagation delay
> introduced by series termination is double the propagation delay introduced
> by the parallel termination.
>
> Following equations might help you on this front;
>
> If Zo is the intrinsic impedance of the trace (transmission line), Ct is the
> Thevenin equivalent load capacitance and
> Tpd is the propagation delay of the trace,
>
>         then:
>                         Tpd = 0.70 X Zo X Ct, for a series terminated line and
>                         Tpd = 0.35 X Zo X Ct, for a parallel terminated line.
>
>         Some more mathematical calculations provide the following results;
>
> If Tri is the input rise time, (ie., Rise time of the source) and Tro is the
> output rise time(at the output end of  termination resistor),
>
>         then:
>
>                         Tro = (Zo X Ct) X ln [ {(Zo X Ct)/(0.18 X Tri)} X 
> [{e^{(1.2 X Tri)/(Zo X
> Ct)}-1}],
>                         for Series terminated line.
>         and
>                         Tro = {(Zo X Ct)/2} X ln [ {(Zo X Ct)/(0.36 X Tri)} X 
> [{e^{(2.4 X
> Tri)/(Zo X Ct)}-1}]
>                         for parallel terminated line.
>
> Hope this will help.
>
> Best Regards
> Subramanya C K
> Wipro Technologies, Bangalore- India
>
> -----Original Message-----
> From: si-list-bounce@xxxxxxxxxxxxx
> [mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of Jinto N.Jose
> Sent: Monday, July 30, 2001 3:12 PM
> To: SI List
> Subject: [SI-LIST] Rise time calculation
>
> Hi all,
>         How should I calculate the rise time at the receiver of a series 
> terminated
> signal ? What all parameters I need to know for doing this calculation ?
>
>   Driver         Trace of known
>    |\    Rs      impedance & length  |\
>    | \__/\/\/\__|------------------|_| \
>    | /          |------------------| | /
>    |/                                |/
>                                    Receiver
>
>      I want to calculate the rise time at the receiver input.
>
> Thanks
> Jinto
>
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