Digital Q measurement of audio-frequency inductors

A measurement technique which enables precise digital readout of the quality factor Q of audio-frequency inductors is presented. It employs a switched capacitor C
N
to obtain a negative conductance whose magnitude G
N
is directly proportional to the switching frequency f
c
. This variable negative conductance is connected across a parallel resonant circuit consisting of the inductor under test and a capacitor C. The switching frequency is slowly increased and the loss in the circuit is reduced until the tuned circuit starts oscillating at the resonant frequency f
0
. The quality factor Q of the coil at the frequency f
0
is given by Q = 2π(C/C
N
)(f
0
/f
c
). Since the frequency ratio can be determined to a high degree of precision, the accuracy of the Q measurement is governed by the precision of (C/C
N
). The instrument design and experimental results are presented.