Design and implementation of a novel frequency modulation circuit using a phase-locked loop synthesizer

A phase-locked loop (PLL) is widely used in frequency synthesizers for modern communication systems. However, frequency modulation (FM) of a PLL for the modulation rates used inside the loop bandwidth, is generally difficult. In this article, a novel hut simple FM circuit of a PLL is described, based on the compensation of the amount of feedback of the PLL. The designed circuit was successfully implemented and showed a flat frequency deviation as expected in the design. In addition, a novel method for the measurement of the FM modulation index with a spectrum analyzer is suggested and used to verify the proposed circuit.