A programmable self-characterization technique is presented, whose purpose is to determine the extent of mismatches present in a variable-capacitor (varactor) array as part of an LC tank of a digitally controlled oscillator (DCO). The varactor array represents a digital-to-analog conversion function, such that mismatches in it cause distortion in the DCO's digital frequency tracking and modulation. The presented technique, relying exclusively on internal resources in the system-on-chip (SoC) and on dedicated software, is implemented in a 65nm CMOS Digital RF Processor (DRP) based transceiver, and demonstrates sufficient accuracy to allow relatively quick measurements of mismatches of a few percent.