Sharks Teeth

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Examples CO-AX reflecting

Drive a length of cable , 14m of RG58 and see the steps

Video Amp Have Shark Tooth Waveforms

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.

 __---___   ___---___
         ---

</div>

<script>

  var Vout=10
  var Vnode=0
  var Zout = 25
  var k = (75/8)/( 75/8 + 25 ) 
  var opStr= "0,"
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\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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