LTspice S-parameters extraction with dependant sources.

Question

Hi!

I have a huge model in a library that I created for a customer. The model is based on voltage dependent voltage sources to emulate Laplace Transforms. I did a simplification of the problem in the picture below because the problem is the same at the entire model scale. Why can't I do an S-parameter extraction in the circuit 2 if both circuits are practically the same? Am I missing something in the model of E1? Or is it just a LTspice limitation? Arr the input ports from E1 considered as a n open circuit and that's why S11 = 0dB? Thanks in advance!

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AndyI · Answer

Hi alanum21, 
 You might be missing something about E1. It correctly mirrors the voltage from V2 to the circuitry to its right. Yes, even in .AC analysis. 
 However, it presents an infinite load impedance to V2. Therefore the voltage across V2's terminals is not the same as the voltage across V1's terminals (because of V1's 50 ohm source impedance and the load on it). 
 Also, E1 presents a 0 ohm Thevenin-equivalent source impedance to L2, C2, and R2, which is quite different from the top half of the circuit. 
 Your two circuits are not "practically the same", even though they might appear similar. They are very different. 
 Indeed, the input port of E1 is an ideal open-circuit, which is why its S11 is 0dB. 
 These ideal voltage-controlled sources in SPICE/LTspice sense one voltage while not affecting it, and replicate that voltage as an ideal voltage source (Rser=0) with the specified voltage gain. Also they have perfect isolation between input and output (e.g., no common ground connection, unless you add one). Their input impedance is infinite and their output impedance is zero. And there is no "signal" going backwards through them, which would have been the only way for something at the input to know or "see" what is happening at its output. 
 Regards, 
 Andy

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LTspice S-parameters extraction with dependant sources.

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