# understanding the changing reflection coefficient in TDR

Status
Not open for further replies.

#### 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. Last edited:

#### 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.

#### yefj 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.

#### Attachments

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

#### 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. Last edited:

Status
Not open for further replies.