[time-nuts] Xtal-like Colpitts Oscillator

[Bruce Griffiths]

Max Robinson wrote:
> Jenny asked.
>
> 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.
>
> You have already received some very good advice.  You might also try a 
> transistor with a higher ft.   My personal experience is that it's easier to 
> get an FET to oscillate in a colpits configuration.  You won't be able to 
> drive a 50 ohm load from an FET.  A buffer of some kind will definitely be 
> needed.
>
> Regards.
>
> Max.  K 4 O D S.
>
>   
Max

Lowest noise is achieved when the oscillator transistor only conducts 
for part of the cycle.
Altering the ratio of C1 to C2 adjusts the duty cycle.

With the given values of C1 and C2 a high(~ 30ms) transconductance FET 
is required given the relatively high resonator ESR (~200 ohms).
Even when using something like a J310 the values of C1 and C2 will need 
to be reduced.

One option achieving a clean low noise output signal is to extract the 
output signal via the resonator.
Instead of grounding one end of the resonator it is connected to the 
emitter of a common base stage.

One could even use the Pound technique to interrogate the MEMs resonance.
In which case, determining the MEMs fundamental resonance frequency to 
within a few ppm of fr/Q should be possible.

Bruce