SMF - Just Installed!
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A todo esto, he encontrado un colega de Noruega LB3HC, que ha hecho lo mismo que yo pero usando en VNA en modo S11, en vez de S21.
That's a reflection-based measurement, and has VERY poor accuracy when Z being measured is much greater than or much less than the system impedance (50 ohms).
The common measurement error is that the measured resonant frequency is much lower than the actual resonant frequency.
S21 has the greatest potential for success, but it requires a load resistance to form a voltage divider with the choke.
Two common methods used to perform impedance measurements with a
vector network analyzer are the reflection method using one port and the shunt/
series method using two ports. Figure 6 shows the relationships between the
impedance and S parameter for each of the two measuring methods. When
the impedance measured (Zx) is in the vicinity of the characteristic impedance
(Zo = 50 Ω), the reflection method provides high-sensitivity measurements
because the vector voltage ratio significantly changes with a small change in
the impedance. However, when the impedance measured is not in the same
vicinity as the characteristic impedance, the reflection method is not suitable
due to trace noise.
The two port method allows you measure impedance over a wide impedance
range using shunt/series connections. With the two port method, you can use
either S11 or S21 to perform measurements; however, S11 has a more limited
noise floor than S21, and it is highly susceptible to VSWR and other factors that
exist between the fixture connector and substrate. Because of this, generally
S21 is preferred. The S21 shunt connection is suitable for 50 Ω or lower impedances
while the S21 series connection is suitable for 50 Ω or higher impedances.
Because EMC components span a wide range of impedances depending on their
purposes, their impedances must be measured with an adequate combination of
S21 shunt/series connections.
