[SI-LIST] Re: Reflection due to far end capacitor
- From: Joe Paul M <joepaulm@xxxxxxxxx>
- To: <sinnovate88@xxxxxxxxxxx>, <si-list@xxxxxxxxxxxxx>
- Date: Thu, 24 Aug 2006 15:09:00 +0530
Hope you understood it well from Steve's reply.
Else read on
We use the equation Zo=sqrt(l/(c+Cl/L) to find effective Zo , ONLY IF
we have a number of them along the trace , with a kind of equal distribution
, and spaced at a distance small enough for the wave to consider it as
distributed , not a point a load.
Eg;
A Multidrop (like cPCI ) backplane running at 33Mhz , with connectors spaced
at 0.8/ 1.0 inch.
In this case , to find effective Zo of a fully loaded backplane to some
acceptable approximation , we can apply the above equation to accommodate
the connector , stub and buffer capacitance.
In your case the wave will see the length of trace as good transmission line
, with a capacitance load at end
So when the wave reaches the end after time td, it sees an additional need
of current (capacitance sinks it till its charged), so voltage dips and this
dip travels back to source ,in another td time. That's how you see the
glitch after 2xtd time
----- Original Message -----
From: "steve weir" <weirsi@xxxxxxxxxx>
To: <sinnovate88@xxxxxxxxxxx>; <si-list@xxxxxxxxxxxxx>
Sent: Friday, August 18, 2006 3:28 PM
Subject: [SI-LIST] Re: Reflection due to far end capacitor
> Shekhar, no your conclusion would be correct only if the line were
> very short compared to the signal rise/fall time.
>
> Steve
>
> At 01:05 AM 8/18/2006, shekhar sharma wrote:
>>Hello all,
>> I am simulating a driver with 50-ohm transmission
>> line. Far end of the transmission line is terminated by 50-ohm
>> resistor. In normal simulation there is no reflections are seen.
>> When I add a capacitor at the far end, it shows a glitch at near
>> end after 2*td. It seems that transmission line is considering
>> capacitor as short for signal transition. But I think this is not
>> the real effect of capacitor on the transmission line. If load
>> capacitor at far end is Cl, it will change the caracteristic
>> impedance of the transmission line and it will become
>> Zo=sqrt(l/(c+Cl/L)). Where L is the length of the transmission line
>> and l and c are the per unit parameters.
>> Does my conclusion is correct?
>> Please shed some light on it.
>>
>> Thanks in Advance,
>>
>> Shekhar
>>
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