Linear frequency modulation of voltage-controlled oscillator using delay-line feedback

This letter proposes a structure for a voltage-controlled oscillator (VCO) circuit which operates in a frequency range of 4.0-4.3 GHz and can achieve a highly linear frequency sweep without any additional compensation circuit. The VCO consists of an amplifier and a feedback circuit only. The feedback circuit compensates for the phase delay of a transmission line with a varactor and sets the closed-loop phase change close to 360 degrees. The measured maximum deviation from a linear frequency sweep is similar to 1.2 MHz when the varactor capacitance C-v of the VCO is related to the control voltage V-c by C-v proportional to V-c(-1.06). The spectral distribution of the beat frequency between the VCO output and delayed VCO output shows that the proposed VCO has excellent linearity in frequency modulation.