Varactorless, tuneable LC-VCO for microwave frequencies in a 0.25 μm SiGe BiCMOS technology

Many LC-VCOs use a cross-coupled differential pair as active negative resistance. The cross-coupled differential pair realizes un damping for frequencies up to fcross. An active negative resistance can also be realized based on a capacitively loaded emitter follower. Such a topology enables higher oscillation frequencies, and is therefore more suited for microwave frequencies. Moreover, by realizing the capacitive load from the input capacitance of a second, resistively loaded emitter follower, the negative resistance and output signal buffering functions can be combined. The varactor typically dominates the losses of the LC-tank at microwave frequencies. This paper demonstrates an LC-VCO based on a capacitively loaded emitter follower, with frequency tuning realized via the collector-base capacitance of the first emitter followers connected to the inductor. No additional varactor is required. The oscillator, implemented in a 0.25μm SiGe:C BiCMOS technology, achieves a measured frequency tuning range from 53.2 to 57.6 GHz and a phase noise of -100 dBc/Hzat 1 MHz from the carrier.