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Drive a length of cable , 14m of RG58 and see the steps
The E1 video amp drives 8 x 75 ohm cables. The signal has a pronounced Shark tooth shape.
The output and is probably designed to drive a circuit:
Amp ---[ 75 ohm ] ---===75ohm coax === | === 75Ohm coax === | -- [75 ohm load ]--
I relality the Amps are driving 8 lengths of un terminated 75 Ohm cable
Amp,Vout --[ Zout ] --+---=== 75ohm coax === | O/C | +---=== 75ohm coax === | O/C | +---=== 75ohm coax === | O/C | +- Vnode
So while the signal is traveling down the 75 ohm coax, it is 75 Ohm resistor.
When the signal bounces off the O/C and back, it is then High Z.
So we can itterate:
Vnode = Vout * K
where K is [ Zout ] / (Zout + Zco-ax/8)
When the signal bounces back for the next T bounce.
Vnode = (Vout-Vnode)*k +Vnode
The speed of signal is 200m / microsecond, but as it got there and back 100m /micro second.
Tbounce = length /100 microseconds.
A cable 10m long Tbounce is about 100 ns
Itterate this and you will get a CR type curve.
What is the Time constant?
For a CR network the t= CR and is when V gets to about 0.63 or = ( 1 - exp( -1 ) )
So when does the way for get to 0.63 of max. It will need more iterations the smaller K, and will be proportional to t.
If we itterate for k = 0.1 you need about 8 itterattions before Vnode gets to 63%
What is a typical Zout? 75 ohm. K = 0.11
It takes about 8 itterations to get to 63 % or the "TimeConstant"
So for a 10 metre cable it takes 8 * 100ns or 800ns.
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var Vout=10 var Vnode=0 var Zout = 25 var k = (75/8)/( 75/8 + 25 ) var opStr= "0," self.document.write( "\n" ) for( x=0 ; x<20 ; x++){ Vnode = ( Vout-Vnode )*k + Vnode opStr += Vnode +" , " self.document.write( x+ " , " + Vnode/Vout +"\n" ) } self.document.write( "\n" )
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