Low phase noise p-HEMT VCO design

HEMT's offer the lowest noise figure when compared to all other devices. They are however, not the first choice for VCO's. The main reason for this the high 1/f noise. HEMT's have very high 1/f noise corner frequencies when compared to other devices, especially HBT's. This 1/f noise gets upconverted close to the carrier and gives poor phase noise performance. 1/f noise upconversion can be reduced by good circuit design. The main aim of this work was to reduce the 1/f noise upconversion by circuit enhancements. No work was done to reduce the device 1/f noise.; The VCO's were fabricated using commercial p-HEMT foundries. These VCO's were designed to operate in the X-Ku band (10-16GHz). The LC tank in the VCO's was optimized for the best Q to get the phase noise performance. The voltage dependent capacitors are one of the main sources for 1/f noise upconversion by amplitude modulation leading to phase modulation(AM-to-PM). The reduction in the AM-to-PM conversion in the designs led to a significant improvement in the 1/f noise upconversion in the VCO's. The use of switched resonators also helped in the reduction AM-to-PM conversion due to a smaller varactor. The dependence of 1/f noise upconversion on the device size and the bias conditions is analyzed in this work. The best phase noise achieved was -128dBC at 3MHz offset for a VCO operating at 12.2GHz.; A completely monolithic PLL was fabricated as a part of this work. The PLL consisted of a VCO, an injection locked frequency divider, static frequency dividers and a phase detector. The loop filter was implemented off chip. Injection locked frequency dividers provide a good narrow band alternative to static frequency dividers when the static frequency dividers are unable to operate at the high frequencies due to bandwidth limitations of the process. This work showed promising results for an integrated communication system solution using p-HEMT's.