Distortion in Varicap FM Oscillators

The performance of FM systems, particularly FDMFM, is often limited by the distortion of key components. Voltagecontrolled oscillators (VCOs), used both as modulators and demodulators, may be a major source of distortion. This paper presents a method for calculating the distortion in varicap tuned oscillators, with the effects of both series and shunt fixed capacitors included. General distortion coefficients are derived, using a Taylor series expansion for the oscillator tuning curve, which are a function of the oscillator resonant circuit only. These coefficients are then used in the computation of harmonic distortion for the sinusoidal modulation case and in the computation of noise power ratio (NPR) with Gaussian modulation. The latter is particularly useful in the design of FDM-FM systems. NPR is plotted as a function of deviation for flat FM, phase modulation, and International Radio Consultative Committee (CCIR) preemphasis characteristics.