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understanding the changing reflection coefficient in TDR

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yefj

Hello,in the simulation example bellow out Vs=2V rho is the definition of reflection coefficient.
gamma_g=(50-30)/(50+30)=0.25
gamma_L=(10000-30)/(10000+30)=0.25
but in plot of rho they start with -0.25 (not 0.25) then it rises to 1.6 where as in my calculations if we look at the point V_TDR
we start 2*0.25=0.5 then 0.5 is multiplied by 1 at the load and return o the V_TDR as 0.5+1.
But as you see in the simulation its acting a lot different both the VTDR plot and the rho plot are totaly different.
Where did i go wrong?
Thanks.

FvM

Super Moderator
Staff member
The simulations results are plausible but I don't understand your calculations. The initial rho value is (30-50)/(30+50) = -0.25. Your gamma_g value is the reflection coefficient seen from the transmission line into the 50 ohm source. gamma_L is wrongly calculated. The actual value is near to 1.

yefj

Hello FVM,its exactly what they do in the manual bellow.
why I have not calculted it correctly .

yefj

Hello,In this simulation example they show that a capacitor causes impedance drop in TDR.
But i can see why? maybe its somthing to do with rise time of the source?
Also using Z0 and length of the transmission line ,how do i see the amount of capacitance and inductance in that TL?
Thanks.

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FvM

Super Moderator
Staff member
its exactly what they do in the manual bellow.
why I have not calculted it correctly .
You didn't read thoroughly. The said reflection coefficent is for the wave travelling back to the load, not the initial reflection.

Thanks.

FvM

Super Moderator
Staff member
Corresponding to your overly complex test setup, you are asking a bunch of questions in one post. Try to evaluate the basic TDR phenomena in simple test setups, e.g. like this

You see that a capacitor acts like a momentary short for a pulse with infinitisemal rise time. Increasing the rise time respectively reducing the pulse bandwidth reduces the reflection depth.

LTspice files attached.

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• rc rl reflection.zip
1.6 KB · Views: 21

yefj

Hello FVM, i can try to understand your logic by saying that infinite rise time is high order harmonics so Z_c=1/jwc
and Zc=0 if w is high so reflectencoefficient gamma will be -1 in that moment.

But our impedance responce is only during thethe changes of in signal,DC has no effect.
so when 50 ohms near it will take charge?
because as i see it its always will see short in the load.
Thanks.

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