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shunt capacitance question

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solis365

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okay, so I have some funky data to analyze. theres a lot more aspects to it, but mostly we are looking at Return Loss (S11) of a passive RF circuit in the (very wide) band of 1GHz to 20GHz.

I am trying to get a ballpark model made up in Ansoft Designer that exhibits most of the basic curve characteristics of the real life data, so we can get an idea of what is causing it to occur.

One part is a custom adapter. Essentially I am modeling discontinuities in impedance that we think are occurring in the adapter. One part involves extra solder on the ground of the coax cable, resulting in higher inductance.

Thus, to model this discontinuity, I think a lumped-element inductor followed by a lumped shunt (parallel) capacitor will be used, and I'll adjust the values to see if I can make it look how I need.

I understand how an inductor causes more reflection at high impedance - it goes to open at high f, so its impedance will be higher at 20GHz than at 1GHz. The high impedance mismatch will cause a large reflection.

However I am not understanding the purpose of the shunt capacitor; exactly HOW it increases return loss or causes higher reflection.

At high f it becomes a short, leaking more and more RF to ground as freq. increases. how does this INCREASE the amount of reflected power? I see it decreasing reflection as well as transmission.

anyone wanna explain this in basicish terms, about the technical level that i described the effect of series inductance in?

thanks a lot!
 
understood! i sat and thunk for a few minutes.

cant increase/decrease both transmission and reflection at the same time, as

GAMMA = 1 + T

as the frequency gets higher and higher, the capacitor gets closer and closer to a short to ground. this causes an impedance mismatch between the transmission line and the cap. the impedance mismatch causes an increasing reflection.

thus reflection goes up, and transmission goes down with the shunt cap as f increases.

anyone want to confirm that my thinking is correct, at least?
 
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