[time-nuts] Xtal-like Colpitts Oscillator
Max Robinson
max at maxsmusicplace.com
Mon Mar 31 19:17:28 EDT 2008
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
Transistor site http://www.funwithtransistors.net
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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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