[time-nuts] Designing and building an OCXO and GPSDO
bruce.griffiths at xtra.co.nz
Wed Aug 13 18:13:32 EDT 2008
SAIDJACK at aol.com 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
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
Techniques for designing low phase noise buffer amplifiers are well
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.
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