As long as FET oscillators have been employed for radio communications, flicker or 1/f noise upconverted into close-in phase noise has annoyed and mystified the circuit designer [1]. Upconverted flicker noise causes the phase noise to rise at a rate of 10 dB/decade faster close in to the oscillation frequency. This may render the FET oscillator unable to meet the specifications on close-in phase noise.

In digital cellular receivers such as for GSM, the specifications on close-in phase noise are fairly relaxed because the base station limits the transmitted power of immediately adjacent channels to a modest level higher than the channel of interest. Usually the much stronger channels are 3 MHz or more away, where thermal noise is responsible for phase noise. However, in narrowband broadcast systems such as wireless paging, where all channels are transmitted at the same power level and the channels are closely spaced, it is most difficult to meet the specifications on close-in phase noise in the receive VCO. For example in the FLEX paging system, the phase noise at 25 kHz offset from the oscillation frequency must be lower than–110 dBc/Hz [2]. This challenging specification can only be met today with discrete bipolar transistor VCO modules, which are widely used in FLEX paging receivers.