[time-nuts] Xtal-like Colpitts Oscillator

[Bruce Griffiths]

Jenny Lee wrote:
> Hi Everyone,  I was hoping to get some words of wisdom on my oscillator that only oscillates in simulation!  I have a MEMS device that resonates around 34 MHz at a given bias with Q of 100. I fit the impedance into a six-element equivalent model (similar to 4-element equivalent model of Quartz) and placed in a Colpitts configuration for the simulation. In the open-loop gain/phase plot, the peak barely passes 0 dB no matter what C1 and C2 I chose.  For transient simulation, it does oscillates, but on a PCB, the circuit won't oscillate! I'm including the device impedance, circuit schematics and simulation results as an attachment. I've managed to oscillate 20 MHz device which has a higher maximum impedance than 34 MHz devices both in simulation and on a PCB - but for this I followed the guidelines of Colpitts for Quartz ranging at 10~20MHz. But above 20 MHz, for Quartz, overtones seem to be the way to go due to the manufacturing so I couldn't find any reference or circuits that operates Quartz for 30 MHz~50MHz ranges at a fundamental mode. I can only use fundamental mode for my device...  I would really appreciate if you could share some of your experiences with Colpitts or any suggestions of what I can try to make it work...  Thank you!-Jenny
>   
Jenny

The so called overtone oscillators can also be made to oscillate at the 
fundamental by suitably selecting the mode selection components.
The Driscoll oscillators with a relatively low Q tuned collector tank 
are perhaps easiest to design.

Bruce