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

Bruce Griffiths bruce.griffiths at xtra.co.nz
Tue Dec 29 18:13:27 UTC 2009


Grant Hodgson wrote:
> I've had several discussions with Chris Bartram about this and similar 
> VHF oscillators.
>
> 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.
>
Since an overtone crystal is usually used a bandpass feedback network 
response is usually required to ensure that oscillation occurs on the 
desired overtone.
The dc biasing network has more effect on the flicker noise than the RF 
feedback 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.
>
Actually close in flicker noise is usually better for lower ft 
transistors with larger junction areas.
According to the datasheet the device is obsolete or about to be phased out.
I see no mention of flicker noise specs in the datasheet except perhaps 
in the models which are not guaranteed.

> 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.
>
> BTW I've tried simulating the phase noise of this oscillator using 
> ADS, but wasn't able to get meaningful results from the simulator, and 
> on this occasion Agilent technical support were not able to resole the 
> issues either.  Maybe Microwave Office or Ansoft Designer would yield 
> better results, but I haven't tried them.  (LT Spice is unable to 
> simulate phase noise of oscillators).
>
> regards
>
> Grant
>
Bruce





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