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How to Tune Fet Colpiits to

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JoseLorca

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Hi there. I know this might sound too basic for some gurus here, but please i need help in designing/tuning the Fet Colpitts circuit shown below, to oscillate at 216MHz. The designed shown oscillate at 175MHz and the problem is, i have a 216 MHz quartz.

Thanks in advance for you help.
 

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RCinFLA

Well-Known Member
Is this a SAW resonator or a 5th overtone crystal? 5th OT crystal oscillators are a bit trickly to design for reliable operation. You are probably running on L-C oscillation at 175 MHz with the crystal just acting as a capacitor. You will see the 175 MHz move in freq significantly as you bring your finger close to the components when running as LC oscillator.

The circuit has to dampen out fundament mode and 3rd overtone mode oscillator circuit loop gain or crystal will likely start up on one of them. The crystal Rs of the higher overtone is greater and gain of device is lower at higher frequency so unless there is circuit frequency roll-off's to kill the loop gain at lower frequencies the fundamental or 3rd overtone will ignite and take over oscillator frequency. If resonant tanks are used to kill the lower freq gain then they have the chance to startup oscillation. So there is a fine line to get all the criteria established to get oscillator to startup on the crystal's 5th overtone.
 
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