Measurements at high frequency are usually made using S-parameters, but what typically matters more is Rx. Assuming that a series resonator is at resonance, its Lx cancels out its Cx and only the series Rx is left:
Circuit model of a series resonator at resonance, with S-parameter measurement.
S21 is defined as:
V + 1 is called the incident wave which is really just vin ⁄ 2, as if the source were delivering power into a matched load, i.e.
V − 2 is the wave incident on the load. Again, it is just a voltage divider:
Divide the two and you get:
Equivalently:
For more details, see Characterization of Crystal Resonators.