[time-nuts] Low noise quartz crystal oscillator by Bruc
Charles Steinmetz
csteinmetz at yandex.com
Tue Nov 10 04:23:11 EST 2015
Rick wrote:
>What Driscoll was talking about was self limiting in a
>transistor. That is discontinuous operation, although
>Driscoll doesn't call it that.
A transistor amplifier can self-limit at either end of its output
swing -- by going into saturation at one end, or by running out of
current at the other end. "Discontinuous operation" refers to the
latter -- the transistor does not draw current through the full 360
degrees of each cycle. This is better than running the transistor
into saturation because when the transistor saturates, it presents a
low impedance to the resonator that spoils the resonator Q -- leading
to increased phase noise (as well as high distortion). With
discontinuous operation, when the transistor is off it presents a
high impedance to the resonator and avoids the phase noise
penalty. It also causes less distortion than saturation.
FET transconductance is a quadratic function of drain current, so a
FET amplifier can usually reach limiting gain (and, therefore, stable
oscillation) without actually cutting off the drain current. If a
FET is kept in its saturation region (drain-source voltage greater
than several volts throughout each cycle), it also has very high
drain resistance and presents a light load to the resonator at all
times. [NOTE that a FET's "saturation region" has nothing to do with
voltage saturation ("clipping") -- it designates the
constant-current, "pentode" region of the FET's characteristic curves.]
Very low PN oscillators can be made with JFETs that have low input
voltage noise. Presumably, the reasons we haven't seen more
commercial designs using low-noise JFETs are (i) the large spread of
FET parameters, which may require selecting FETs, and (ii) for
ovenized oscillators, the rapid rise of gate leakage current with
temperature -- both of which complicate the mass production of a
consistent product.
There is another transistor operating mode that minimizes oscillator
phase noise, namely "very Class C." In this mode, the transistor
draws current during only a very small portion of each
cycle. Although it loads the resonator while it conducts, that is
only for a very short portion of each cycle. For the rest of the
cycle, the transistor is not conducting and does not load the
resonator. The net effect is a lightly loaded resonator and good PN.
Best regards,
Charles
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