Qcircle measurement error
Abstract
HighQ lumped and distributed networks near resonance are generally modeled as elementary three element RLC circuits. The widely used Qcircle measurement technique is based on this assumption. It is shown that this assumption can lead to errors when measuring the Qfactor of more complex resonators, particularly when heavily loaded by the external source. In the Qcircle technique, the resonator is assumed to behave as a pure series (or parallel) RLC circuit and the intercept frequencies are found experimentally at which the components of impedance satisfy the absolute value of Im(Z) = Re(Z) (unloaded Q) and absolute value of Im(Z) = Ro+Re(Z) (loaded Q). The Qfactor is then determined as the ratio of the resonant frequency to the intercept bandwidth. This relationship is exact for simple series or parallel RLC circuits, regardless of the Qfactor, but not for more complex circuits. This is shown to be due to the fact that the impedance components of the circuit vary with frequency differently from those in a pure series RLC circuit, causing the Qfactor as determined above to be in error.
 Publication:

NASA STI/Recon Technical Report N
 Pub Date:
 May 1991
 Bibcode:
 1991STIN...9122495H
 Keywords:

 Electrical Impedance;
 Error Analysis;
 Network Analysis;
 Q Factors;
 Resonant Frequencies;
 Rlc Circuits;
 Bandwidth;
 Resonance;
 Resonators;
 Electronics and Electrical Engineering