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Quartz Crystals

Overtone Circuits

[ Up ] [ 3rd Overtone Oscillator Circuit ] [ 5th overtone ] [ 5th-7th-9th -- 200 MHz ]

With overtone operation of crystals, the overtone frequency is not an exact multiple of the fundamental frequency and the circuit design must ensure that the crystal does in fact operate at the overtone frequency.

In order to be certain of correct operation, it is preferable to specify series resonant crystals, and to provide external circuit elements to prevent oscillation at the fundamental frequency. Where parallel resonant overtone crystals are required a load capacitance must be specified

On pages accessed by the above hypertext links, circuits are detailed for 3rd overtone crystals 15 to 65MHz and 5th overtone crystals 60 to 105 MHz operating in their series resonant mode. In both of these circuits with the crystal short circuited, the oscillator should operate at or near the required frequency. With the crystal in circuit L1 should be adjusted for either (a) minimum voltage across the crystal or (b) for the exact frequency required. Ideally, these two points would coincide but they rarely will due to the need for a manufacturing tolerance on crystal frequency.

If L1 is of incorrect size it is possible for the oscillator to operate on a different order of overtone, for this reason it is important to accurately check the output frequency.

Under no circumstances should a tuned circuit at the crystal overtone frequency be included in the collector circuit of TR1 as this configuration will result in oscillation not controlled by the crystal. However it is possible to include a tuned circuit at that point which is twice or three times the crystal frequency. It is then possible to extract from the collector the harmonics of the crystal frequency.

Unless otherwise specified QuartSLab.com supplies 3rd overtone crystals between 21 and 60 MHz, 5th overtone between 60 and 126 MHz and 7th overtone between 126 and 175 MH.

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