[time-nuts] Xtal-like Colpitts Oscillator

[Max Robinson]

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.

Email: max at maxsmusicplace.com

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----- Original Message ----- 
From: "Jenny Lee" <hyunjoo_lee at hotmail.com>
To: <time-nuts at febo.com>
Sent: Monday, March 31, 2008 4:20 PM
Subject: [time-nuts] Xtal-like Colpitts Oscillator



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
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