Technical Topics

摘要

The April, 2002 "TT' item 'The Quest for Low-Noise Oscillators' stressed the growing recognition that, once good strong-signal performance has been reached, the current limiting factor of high-performance receivers is usually the phase-noise of the local analogue VFO or the digital frequency synthesiser. The April item discussed the excellent results achieved by Colin Horrabin, G3SBI, using his 'double-tank' FET oscillator as the VHF VCO of a hybrid DDS / PLL synthesiser, and that of the basically similar approach used by John Thorpe for the AOR AR7030 HF receiver. But, in addition, 'TT' also reported the views of Mike Hall, G3USC, who was concerned that, by renewing interest (November 2001) in the long-established G3PDM 5MHz VFO, more attention needed to be paid these days to its phase-noise performance, so important for low reciprocal mixing. He wrote: "I do not believe that in this respect the G3PDM VFO can be anything to write home about. That 4k7 gate resistor and the low resistance presented to the circuit by the buffer amplifier must be affecting its close-in noise fairly significantly, not to mention the effect of the high flicker noise cornerfrequencyofthe2N3819 FET."