A digital approach to an FM exciter

摘要

A digital approach to an FM exciter is presented that provides several advantages over previous analog techniques. This approach, called direct digital frequency modulation (DDFM), is centered around a direct digital synthesis (DDS) chip which allows true digital generation of the FM signal. The DDS chip is a perfectly linear modulator with superior phase-noise characteristics over the current analog voltage-controlled oscillator (VCO) approach. The DDS modulator drives a low-noise phase-locked loop (PLL) which allows upconversion to any FM channel without manual tuning. The exciter demonstrates superior microphonics immunity by separating the frequency tuning from the modulation path, thus allowing wider PLL bandwidths. The exciter can accept either the existing composite analog format or can interface with future digital sources.