Linearization of direct FM frequency modulators

Voltage controlled crystal oscillator (VCXO) frequency modulators have found increasing use in recent years in portable frequency modulation (FM) equipment as an alternative to a standard crystal oscillator followed by a phase modulator. The advantages of using a direct FM VCXO over an indirect FM phase modulator are considered. A model for the crystal and frequency control element will be developed and basic circuit design considerations will be presented. Conditions necessary to minimize audio distortion generated by the VCXO modulator will be dealt with in detail.