VHF Overtone Oscillator

Posted on Feb 5, 2014

VHF overtone crystals can be difficult to use due to the many frequencies at which the crystal is perfectly happy to oscillate. A typical 100 MHz, 5th-overtone crystal will oscillate at approximately 20 MHz, 60 MHz, 100 MHz, and even 140 MHz, not to mention other, unintended modes that such crystals often have. Most VHF overtone oscillators use some sort

VHF Overtone Oscillator
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of tuned circuit or frequency selective filter to select the desired mode and reject the other, equally active modes. The oscillator circuit shown below uses a simple tuned circuit to select the desired mode and is suitable for VHF crystals, including high overtone types. A tank circuit in the collector of the exciter transistor is tuned to the desired crystal resonance, either by a trimmer capacitor as shown in the schematic or by a variable inductor in place of the 0. 1 uH. The frequency of the tank is determined by the 0. 1 uH choke, the parallel 9-35 pF trimmer capacitor, the series combination of the 15 pF and 39 pF capacitor and the reactance of the collector of the transistor and other stray reactance. The 15 pF and 39 pF provide a low voltage "tap" for driving the crystal and the output buffer. In practice, the two selected components in series with the crystal would be replaced with jumpers and the tank would be peaked for maximum signal on the 39 pF, observed with a very low capacitance probe. Alternately, the trimmer may be set to the middle point between the extremes where oscillation stops by observing the output of the gate. Once the optimum tuning is achieved, selected components are added in series with the crystal to center the frequency. Either jumper position may be used for fixed tuning or a mechanical trimmer could be added on one side and an additional selected component could center the tuning range....

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