Nonlinear distortion model for VCO-PLL FM transmission systems

The nonlinear distortion of frequency or phase modulation systems composed by a voltage-controlled oscillator (VCO) modulator and a phase-locked loop (PLL) discriminator is addressed. Volterra series nonlinear transfer functions up to third order, for an LC Colpitts type modulator and a PLL with simultaneous nonlinear phase-detector and VCO, are combined as a cascade of mildly nonlinear systems. This approach gives a fully analytical description of that complex arrangement, which allowed the derivation of theoretical linearization conditions for that FM/PM modem.