Hi baab,
Quote:no reflection at the load = output is matched???
Nope.
You seem to be confusing the output and the load. Don't worry, everybody has this confusion when they are starting out. The output impedance is looking from the 50-ohm load back into the circuit. It is basically a parallel combination of your transistor output impedance and the inductor's impedance. The load impedance is the next circuit block. For example, in your case it is probably a mixer. In textbooks, the load impedance is set to 50-ohm, but in practice there is no such requirement.
It might help if you imagine that there is a transmission line connected between the source and the DUT, and the DUT and the load. Like so,
R
S ------tline (Z
01)------DUT------tline (Z
02)------R
LHere is the sequence of events
from the output side:
t1: DUT generates output wave on tline (Z
02)
t2: output wave reaches load. For Z
L = Z
02, R
L = 0 and therefore there is no reflection (end - this is the case for S
11 analysis), otherwise go to t3.
t3: there is some reflection at the load (Γ
L), and this travels backwards down the tline (Z
02) to the DUT output.
t4: upon reaching the DUT output, there is some transmission back to the input (S
12), and if Z
out ≠ Z
02, there is some re-reflection.
t5: so this basically goes on for ever with the signal getting smaller and smaller as it is absorbed by all the loss and load.
In reality, the reflection coefficient should be defined wrt the transmission line since reflection is only a meaningful concept when the distance traveled by the signal is long enough for it to experience a time delay. So in this case, the load reflection coefficient would be defined by the mismatch between Z
02 and R
L.
However, for spectre's SP analysis, Z
02 is actually defined by R
L. Therefore Γ
L in the simulation is by definition equal to zero. So if you were to use a 1-kohm impedance for the load port, then for S
11, the load would be 1 kohm, but for Z
11, it would be infinite. A good reference if you want to understand this properly is "Power Waves and the Scattering Matrix" (I'm just quoting that from memory, so you'll have to look around"). Its quite an old paper.
regards,
Aaron