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 _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
