As an RF engineer, you likely frequently hear the term “quality factor”, or Q factor, used as shorthand figure of merit (FOM) for RF filters. In short, Q factor is expressed as the ratio of stored versus lost energy per oscillation cycle.
More specifically, Q factor generally describes specifications such as the steepness of skirts, or the selectivity, and how low the insertion loss is. Overall losses through a resonator increase as Q factor drops and will increase more rapidly with frequency for lower values of resonator Q. However, truly understanding how Q factor is determined is a bit more intricate. Let’s start by looking back to the example bandpass filter specification we showed in Part 3.