[time-nuts] Notes on the Driscoll VHF Overtone Crystal Oscillator

dk4xp at arcor.de dk4xp at arcor.de
Tue Dec 29 20:20:38 UTC 2009


 
> My understanding is that Chris' variant of this particular Driscoll osc. 
> has been designed specifically for low close-in phase noise, and that is 
> why the phase-shift network has a low-pass response (to try and reduce 
> flicker noise) rather than the more common high-pass network.
> 
> The NE688xx was chosen for the active devices due to it's claimed low 
> flicker noise; the flicker noise parameters are actually specified on 
> the datasheet for the NE68833 - which is quite unusual.  The high Ft may 
> not be desirable, but it seems that is the price to pay for low flicker 
> noise.
> 
> I've built a couple of oscillators similar to Chris Bartram's design at 
> around 116MHz, albeit with the more conventional 'high-pass' phase shift 
> network, and they seem to perform quite well - certainly no sign of 
> spurious high frequency oscillation, but that may be a function of PCB 
> layout.
> 
> I'm not aware of anyone yet measuring the close-in phase noise of the 
> Bartram variant of this oscillator, and that's really the only way to 
> verify or otherwise that the new topology gives any advantage in terms 
> of close-in phase noise, compared to a similar, low cost design using 
> similar crystals.

When I get a sample in known working condition, I'll stress the friendliness
of my customer to get it tested on their signal source analyzer. 

What we already have seen is that crystals from the same production run
may yield up to 15 or 20 dB worse phase noise at 100 Hz than the best.
(in the same oscillator)

That confirms:
Close-in to the carrier, the phase noise is dictated by the resonator. [1]


73, Gerhard DK4XP


[1] Grant Moulton: Analysis And Prediction Of Phase Noise In Resonators
and Oscillators, HP signal analysis division 1985
<http://www.hparchive.com/seminar_notes.htm>





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