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

[Grant Hodgson]

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.

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.

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

 > >
 >> >> An inductor in series with the 220 ohm emitter resistor will 
improve the
 >> >> phase noise floor.
 >> >>
 > > In theory, yes. But already with only 220 Ohms, Q3 will oscillate 
wildly
 > > at a few hundred MHz.
 > >
 > > The mechanism is this: Somewhat hot RF transistor NE688, collector at
 > > RF ground, emitter at high-ish impedance --->  When you measure into
 > > the base, you see a negative resistance in series with a few pF.
 > >
 > >
Using a transistor with a higher ft than necessary in an oscillator
circuit isnt usually a good idea.