overtone crystal oscillator


Posted on Feb 4, 2014

Crystals with higher frequencies than about 30 MHz are almost exclusively overtone crystals. Such a crystal is intended to be used on an odd multiple of it`s fundamental frequency. Unfortunately, the crystal doesn`t want to resonate on it`s overtone frequency voluntarily, the oscillator circuit needs to force the correct overtone. For a little 2m


overtone crystal oscillator
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band homebrew FM receiver I intent to use channel crystals and a first IF of 10. 7 MHz. After a while of thinking I determined taht I want the crystal (xtal) frequency to be defined by the following equation: Fxtal = (Freceive - 10. 7 MHz) : 3 For an initial lab test I had a hand full of crystals for the frequency 145. 625 MHz available. This frequency is more commonly used across Europe than here in the US. The crystal frequency is 44. 975 MHz and the crystal needs to be operated in 3rd overtone mode. If such a crystal is used in a regular oscillator, like for instance my crystal tester, the crystal will resonate on it`s fundamental frequency and not the intended third overtone. A little LC tank circuit can be used to get the crystal oscillator on the designated frequency. The tank circuits resonance frequency needs to be equal to the target frequency of the crystal oscillator. The exact frequency is not very critical as long as the resonance frequency is closer to the intended overtone frequency than to the fundamental mode frequency and other overtones. One possible approach is a series LC-Tank circuit in series with the crystal. This option works with almost every oscillator where the crystal is being used in series resonance. The oscillator works very reliable and always starts on the third overtone without any difficulties. I tested the circuit between about -20 and +40 degrees Celsius. In case you where wondering...




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