celebrevida
Member level 2
Thanks to members here, I learned that for a 2-port single-ended transmission line, I can obtained its characteristic impedance using:
Z0_meas = sqrt(Zopen*Zshort)
where:
Zopen is input impedance with output port open
Zshort is input impedance with output port shorted
In SPICE, I built a single-ended transmission line model and performed AC sims to obtain Zopen and Zshort to obtain Z0_meas.
Knowing that if I model a transmission line with many "transmission line unit elements" with R,L,G,C parameters per unit length, I get:
Z0_calc = sqrt((R+jwL)/(G+jwC))
My simulations to determine Z0_meas is identical to Z0_calc from 1Hz to 100GHz proving Z0_meas=sqrt(Zopen*Zshort) works!
However, how do I obtain Z0 for a differential transmission line with 4-ports?
Below is my thinking and let me know if it works or not:
What I was thinking of doing was driving the two input ports differentially. Then I would treat the two ports as though they were independent of each other. So I would do the same thing as was done for the single-ended line with Zopen and Zshort. This would give me Z0_plus=Z0_minus.
Then is it fair to say that Z0_diff = 2*Z0_plus = 2*Z0_minus?
To properly terminate a differential transmission line, I am thinking that I can:
Connect the two output ports with two resistors in series with value Z0_plus=Z0_minus. Let's call the node between the two resistors, node C:
1. Do not connect node C to anything it just connects the two resistors together.
OR
2. Connect node C to the output reference node which is ground. Since Node C is an AC virtual ground anyway, it should work the same as (1).
Let me know if my thinking on differential transmission line is correct.
Thanks again!
Z0_meas = sqrt(Zopen*Zshort)
where:
Zopen is input impedance with output port open
Zshort is input impedance with output port shorted
In SPICE, I built a single-ended transmission line model and performed AC sims to obtain Zopen and Zshort to obtain Z0_meas.
Knowing that if I model a transmission line with many "transmission line unit elements" with R,L,G,C parameters per unit length, I get:
Z0_calc = sqrt((R+jwL)/(G+jwC))
My simulations to determine Z0_meas is identical to Z0_calc from 1Hz to 100GHz proving Z0_meas=sqrt(Zopen*Zshort) works!
However, how do I obtain Z0 for a differential transmission line with 4-ports?
Below is my thinking and let me know if it works or not:
What I was thinking of doing was driving the two input ports differentially. Then I would treat the two ports as though they were independent of each other. So I would do the same thing as was done for the single-ended line with Zopen and Zshort. This would give me Z0_plus=Z0_minus.
Then is it fair to say that Z0_diff = 2*Z0_plus = 2*Z0_minus?
To properly terminate a differential transmission line, I am thinking that I can:
Connect the two output ports with two resistors in series with value Z0_plus=Z0_minus. Let's call the node between the two resistors, node C:
1. Do not connect node C to anything it just connects the two resistors together.
OR
2. Connect node C to the output reference node which is ground. Since Node C is an AC virtual ground anyway, it should work the same as (1).
Let me know if my thinking on differential transmission line is correct.
Thanks again!