[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,


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