[SI-LIST] Re: Flight Time Approximation
- From: "Adeel Malik" <adeelm@xxxxxxxxxxx>
- To: <a.ingraham@xxxxxxxx>
- Date: Tue, 13 May 2003 12:58:39 +0500
Hello Andy,
Thanks for the reply. Plz excuse for the for the figure but it
was the basic one showing the driver with its output resistance connected to
the receiver with its I/P Capacitance via the transmission line.
Firstly, you have said that for a long line Td >> Trise, but how much
greater? . Can we quantify it by saying that the line can be termed as long
if its length is greater than (L/6) where L = Length of rising edge = Rise
Time of Signal / Propagation delay ?.
Secondly, the formula for computing the 10% - 90% charging time (or rise
time) for a voltage signal fed to a receiver with I/P Cap. C via a
transmission line with Impedance of Zo is 2.2 x Zo x C ( in case of series
terminated line). You have said that Td should be added to a factor
proportional to Zo x C but as a rough approx. Can you provide me with the
good approximation method for computing the total flight time from the
driver to a receiver ?.
Assume that we have a series terminated transmission line with micro-strip
configuration.
Thanks for the reply,
ADEEL MALIK,
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx
[mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of Ingraham, Andrew
Sent: Monday, May 12, 2003 9:39 PM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Flight Time Approximation
Adeel Malik,
> Consider the figure shown below or if you can't see that in your Email
> software, view the word attached file
No attachments on this list, they get removed automatically. The figure
didn't come through either.
> I want to find the max. time take by the signal to go from Low to high at
> the receiver input. This time is dependent upon the rise time of the RC
> network formed by the O/P Resistance of Driver and Input Capacitance of
> Receiver plus the propagation delay of the transmission line (whether it's
> microstrip or stripline ...).
If the transmission line is long, the receiver's input capacitance doesn't
"see" the Rout of the driver; it sees Zo, the line's characteristic
impedance. The time constant, which may be R*C for very short lines,
becomes Zo*C for a long line.
For the long line case, the driver's output resistance affects the amplitude
of the first step (and reflections). This can affect the time the waveform
crosses receiver input thresholds too.
> Now my question is that in order to find the complete flight time from the
> driver to the receiver, should we add t1 ( the propagation delay of line)
> to
> t2 (the charging time) or how we should add the delay effect of
> Transmission
> line to the charging time ?>
For a long line (Td >> Trise), it may be a good approximation to add Td and
a factor proportional to Zo*C.
But only as a rough approximation.
Regards,
Andy
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