Characteristics of Voltage Controlled Oscillators Implemented With Tunable Ferroelectric High-k Capacitors

Ferroelectric capacitors made from Ba_0.5Sr_0.5TiO₃ (BST) are applied as varactors in tunable, high frequency circuit applications. In this context, a voltage-controlled oscillator (VCO) based on Colpitts architecture has been designed and implemented using discrete RF BJTs and tunable ferroelectric capacitor. The tunable ferroelectric capacitor has been fabricated on quartz substrate with BST deposited by RF magnetron sputtering and platinum electrodes. At 1 MHz, the ferroelectric capacitor shows a tunability of about 30% for applied DC bias voltage of plusmn5 V and quality factor ´Q´ of 160. The fabricated VCO on RF prototype board can be tuned from frequency 577 MHz to 602 MHz with a control voltage of plusmn3 V and a gain of 8.3 MHz/V and a tunability of 4%.