Development of ultra low phase noise X-band oscillators

This paper reports on the design and the measurement of low phase noise X-band oscillators combining a room temperature high-Q whispering gallery mode (WGM) sapphire resonator and an ultra-low phase noise sustaining amplifier. The resonator thermal configuration has been optimized leading to a thermal frequency sensitivity of -0,05 ppm/K. Compact microstrip DBR (Dual Behavior Resonators) filters have been realized to suppress cavity spurious modes. High performance commercially available amplifiers have been tested. Owing to the low phase noise, the measurement of oscillators requires crosscorrelation and some unusual solutions. X-band oscillators typical phase noise as low as -36 dB.rad(2).Hz at 1 Hz Fourier frequency, -145 dB.rad2/fHz at 10 kHz offset and -160 dB.rad(2)/Hz at 100 kHz from the carrier have been measured. Parallely, two excellent and original double stage amplifiers based on a Si-SiGe transistors cascade (Power gain=8.2 dB - Phase noise performances : 168 dB.rad(2)/HZ at 100kRz offset) have been designed, precisely modelled (non-linear and noise modelling) and optimized thanks to dedicated CAD techniques.