A digital system for measurement of resonant frequency and Q factor

A new automatic computer-controlled system for accurate measurement of the resonant frequency and Q factor of radio frequency and microwave resonant circuits was developed and tested. In this system, the inflection points of the resonance curve are identified through a process of mathematical evaluation using an on-line digital computer and utilized to calculate the resonant frequency and the Q factor. The primary advantage of the system is that the results are independent of the resonant circuit characteristics at the resonant frequency and the 3-dB points, amplitude and frequency drift of the signal source, as well as detector instabilities. An experimental system operating between 1 and 2 GHz was designed, assembled, and tested with a laboratory minicomputer PDP 11/34 used for data acquisition and processing, as well as for overall system control. The measured range of Q factors is 100–1500, with estimated uncertainties of 40 ppm in the resonant frequency and 7 percent in the Q factor for a resonant circuit with a Q factor of 1000.