Investigation and reduction of frequency pulling in SiGe mm-wave VCOs at limited power consumption

The influence of load variation on the oscillation frequency of wideband mm-wave VCOs in a SiGe bipolar technology is investigated theoretically and experimentally. This load variation can change the real part of the voltage gain of the oscillating transistor stage, which varies the input impedance of this stage via C_CB, thus leading to the so called frequency pulling δf_osc. This effect can be reduced, at relatively low power dissipation (240 mW in total for the 80 GHz VCOs under investigation), by use of a common-base stage at the output of the oscillator and, furthermore, by two cross-coupled compensation diodes in the oscillator core. The latter measure slightly reduces the tuning range (from Δf_osc = 24.5 GHz to still remarkable 18.7 GHz), but does not degrade phase noise and output power (-97 dBc/Hz at 1 MHz offset frequency and 11 dBm, respectively, at center frequency).