Why 100 ohm & 25ohm are used in input ports in differential TDR simulation?

Hello,

I am using ADS 2015. I have created a differential channel.Now I want to do TDR simulation. I googled and found schematic settings below.

In the settings after step in the input 100 ohm and 25 ohm are put and similar to the output ports as well. I want to know why is 100 ohm and 25 ohm are attached? why not 50 ohm & 50 ohm?

Is there any other way to do TDR simulation for differential channel except TDR frontpanel tool?

Have you noticed the balun element in this schematic? At the differential port, we then have a 50 Ohm + 50 Ohm = 100 Ohm, at the common mode port we have50 Ohm || 50 Ohm = 25 Ohm.

In the help documentation, there are two functions "tdr_sp_gamma()" and "tdr_sp_imped()" that can be configured to perform the iDFT calculations independently from the TDR front panel tool. An even less automated method can be realized with the ts() and ***_sum() functions.

Barrett

Thanks both of you for your help. I also want to do it with another way also wxcept these 2 methods that you said. Can that be possible? I found in a literature to use two step generator of opposite step and connect with 50 ohms of each n and p channel. You can see the picture below. Now how can I get the TDR impedance for this schematic? I could not get that. Is it possible in this way?

@aguntuk,

Yes, this is an additional way to do this, especially if you are concerned with the non-causal effects that generating a mathematically calculated TDR can yield though this is not a method I am familiar with; however, reading over the seminar notes from this reference, page 10 gives the equation for rho as Vref/Vinc, which is acquired (in hardware) by probing the voltage between the 50 ohm resistor and the device under test. To normalize rho to impedance, the equation Z=(100)*(1 + rho) can be used.

Barrett

,

Thanks for your reply.

I was also digging a bit and found a problem that i could not trace the problem. Maybe you can check if you have any idea. I posted as new question in this link in the forum.

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