I have done some impedance measurements on an unknown 1 port device. I have measured its impedance at different frequencies (from 10 MHz to 600 MHz) and I have seen it on the normalized (on 50 Ohm) Impedance Smith Chart:
At Low frequencies we are at the short circuit point (left). Then, the device offers an inductive impedance, until, at about f0 = 470 MHz, we arrive at the real axis (right). It seems it arrives at the open point but it is not: if you see the following graph you see that the impedance is, obviously, purely real, but not infinite. It is equal to about 471 Ohm (absolute value).
You may see these results also in the following graphs (the first one contains real and imaginary parts of the impedance, while the second one its absolute value).
So, at f0 the devices resonates and offers a purely real impedance. Now my question is: which is the equivalent RLC circuit of this device according to this analysis?
- from the Smith Chart I may get the value of L: I should take the Imaginary Part of Zin at low frequencies (in which the device is almost purely inductive) and divide by 2*pi*f.
- from the Smith Chart I may get the value of R (it is 471 Ohm, as told before)
- from the Smith Chart I may get the value of C (at high frequencies, where the device is almost purely capacitive)
So, which is the equivalent model? A parallel RLC, a series RLC? Moreover, at f0, does the device offer a parallel resonance, or a series Resonance? I have been told that if I measure R,L,C through the Impedance Smith Chart, they are the values of the RLC series model, but I am not sure about it because I can also do my measurements on the Impedance Smith Chart for a device which is not a RLC series circuit. For instance, something like that:
Moreover, I'd say it cannot be an RLC series circuit, because at low frequency we are at short circuit point, and not open circuit point.