View Single Post
Old 14th Feb 2018, 4:39 pm   #15
G0HZU_JMR
Dekatron
 
Join Date: Sep 2010
Location: Cheltenham, Gloucestershire, UK.
Posts: 3,077
Default Re: Quartz crystal accuracy over time

One other way to operate on an overtone is to starve the circuit of negative resistance at the fundamental. If you plot the negative resistance vs frequency looking into the base of the transistor it typically has an abrupt response where the transition into negative resistance occurs at a sharp knee point. Below this the resistance is positive all the way down to LF. It should be possible to get adequate negative resistance at the overtone but have positive resistance at the fundamental. This can be achieved by controlling the feedback and the resistive loading at the base of the transistor. The circuit would look the same as a classic Colpitts crystal oscillator but with different values for the resistors and capacitors. Some care would be needed to avoid having spurious responses at higher/unwanted overtones but in theory this approach would work. I'm not sure I recommend it though!
__________________
Regards, Jeremy G0HZU
G0HZU_JMR is offline