Digital Frequency Discriminator (DFD) Improvement by LO Leakage and I/Q Imbalance Compensation

Abstract

This study presents the design of a phase correlator for a digital frequency discriminator (DFD) that operates in the 2.0-6.0 GHz frequency range. The accuracy of frequency discrimination as determined by the isolation of the correlator mixer was analyzed, and LO-RF isolation was found to have a significant effect on the frequency discrimination error by deriving various analytic equations related to the LO-RF isolation and phase performance. We propose a novel technique (phase sector compensation) to improve the accuracy of frequency discrimination. The phase sector compensation technique improved phase error by canceling the DC offset of the I and Q signals for only the frequency bands where the mixer's LO-RF isolation was below a specified limit. In the 2.0-6.0 GHz range, the phase error of the designed phase correlator was decreased from 4.57° to 4.23° (RMS), and the frequency accuracy was improved from 1.02 MHz to 0.95 MHz (RMS). In the 4.8-6.0 GHz range, the RMS phase error was improved from 5.59° to 4.12°, the frequency accuracy was improved from 1.24 MHz to 0.92 MHz, and the performance of the DFD correlator was improved by 26.3% in the frequency sector where LO-RF isolation was poor. Overall, the DFD correlator performance was improved by LO leakage compensation.