A Novel Low-Distortion Wide-Band FM Generator

A new technique for generating a wide-band frequencymodulated sinusoidal carrier is presented. The frequency-modulated square-wave output of a voltage-controlled oscillator (VCO) is first filtered through a constant-
Q
switched capacitor-type tracking bandpass filter (BPF) whose switched sinusoidal FM output is then passed through an
RC
tracking low-pass filter (LPF). The modulating signal itself is used to proportionally modulate the bias of a field effect transistor (FET), acting as the variable resistance in the LPF, so that a near perfect tracking is obtained. This ensures a very low-distortion frequency-modulated sinusoidal carrier at the output of the LPF. The conditions for ideal tracking have been presented, along with the description of an experimental circuit and test results.