The role of baseband noise and its upconversion in HBT oscillator phase noise

The phase noise spectral density (£(f_m)) of an 11.02-GHz heterojunction bipolar transistor (HBT) dielectric resonator oscillator (DRO) has been investigated in terms of the HBT´s low frequency noise and the oscillator´s upconversion coefficient. Experimental studies have been used for this purpose and the measured £(f_m) ranged from -89 dBc/Hz to -101 dBc/Hz at a 10-kHz offset frequency (best phase noise spectral density performance was -124 dBc/Hz at 100 kHz). It was shown that, in most test cases, £(f_m) can be described by the upconversion of the HBT´s baseband noise. As a result the frequency dependence, of £(f_m ), is dictated by the low frequency noise spectrum rather than the upconversion itself. Deviation from pure 1/f frequency dependence found for the HBT´s baseband noise at frequencies above 100 Hz resulted in d£(f_m)/d(f_m) deviating from about -30-dB/decade rate. Reduced oscillator phase noise at high collector current is attributed to reduced upconversion in the oscillator