Continue to Site

Welcome to EDAboard.com

Welcome to our site! EDAboard.com is an international Electronics Discussion Forum focused on EDA software, circuits, schematics, books, theory, papers, asic, pld, 8051, DSP, Network, RF, Analog Design, PCB, Service Manuals... and a whole lot more! To participate you need to register. Registration is free. Click here to register now.

understanding the changing reflection coefficient in TDR

Status
Not open for further replies.

yefj

Advanced Member level 2
Joined
Sep 12, 2019
Messages
649
Helped
0
Reputation
0
Reaction score
2
Trophy points
18
Activity points
3,541
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.

1615576223649.png


1615576118217.png
 

FvM

Super Moderator
Staff member
Joined
Jan 22, 2008
Messages
50,778
Helped
14,600
Reputation
29,475
Reaction score
13,677
Trophy points
1,393
Location
Bochum, Germany
Activity points
290,811
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

Advanced Member level 2
Joined
Sep 12, 2019
Messages
649
Helped
0
Reputation
0
Reaction score
2
Trophy points
18
Activity points
3,541
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.
1615618796239.png
 
Last edited:

FvM

Super Moderator
Staff member
Joined
Jan 22, 2008
Messages
50,778
Helped
14,600
Reputation
29,475
Reaction score
13,677
Trophy points
1,393
Location
Bochum, Germany
Activity points
290,811
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

Advanced Member level 2
Joined
Sep 12, 2019
Messages
649
Helped
0
Reputation
0
Reaction score
2
Trophy points
18
Activity points
3,541
Hello FVM, could you please help regarding reply 4?
Thanks.
 

FvM

Super Moderator
Staff member
Joined
Jan 22, 2008
Messages
50,778
Helped
14,600
Reputation
29,475
Reaction score
13,677
Trophy points
1,393
Location
Bochum, Germany
Activity points
290,811
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

1615632772457.png


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.

1615633207320.png


LTspice files attached.
 

Attachments

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

yefj

Advanced Member level 2
Joined
Sep 12, 2019
Messages
649
Helped
0
Reputation
0
Reaction score
2
Trophy points
18
Activity points
3,541
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.

1615634211698.png
 
Last edited:

Status
Not open for further replies.

Part and Inventory Search

Welcome to EDABoard.com

Sponsor

Top