[EMAIL PROTECTED] wrote: > Thanks Bruce, > > > > it would be interesting to see how the different topologies affect > phase noise and stability etc, and what kind of performance can be > achieved. > > > > bye, > > Said > Said
When the output signal is filtered by the crystal, the phase noise floor is independent of the particular oscillator design. The phase noise floor only depends on the phase noise of the buffer amplifier. Techniques for designing low phase noise buffer amplifiers are well established. Basically the idea is to keep the dc and low frequency noise at the buffer amplifier output low and use RF negative feedback to stabilise the buffer amplifier gain (reduces flicker phase noise). Thus low power supply noise is required (it modulates amplifier phase shift and thus contributes to phase noise). For offsets within the crystal bandwidth, intrinsic crystal flicker noise contributes to the phase noise as does the phase noise of the oscillator active components. The oscillator can be run in class A if AGC is used, RF negative feedback can then be used to reduce the sustaining circuit phase noise. A Colpitts crystal oscillator typically operates in Class C with a small duty cycle, the transistor should never saturate. Thus with carefully optimised buffer amplifiers and a fixed crystal current and the same crystal, the various circuits should only differ substantially in their flicker phase noise characteristics. In a well designed circuit the stability will be largely determined by component quality, amplitude stability and residual temperature fluctuations. Without some stabilising RF negative feedback the flicker phase noise will be relatively high, however the transistor is on for part of the cycle which tends to reduce the phase noise. Using a 100k resistor from the FET gate to ground isnt exactly conducive to a low phase noise floor unless the amplitude of the signal at the gate is large. It would certainly be useful to evaluate the phase noise of the various oscillator circuits particularly in the flicker phase noise region as well as ADEV as a function of time. Bruce _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
