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According to the datasheet the device is obsolete or about to be phased out.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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