Crystal units manufactured by Quartslab Marketing Ltd for use over the frequency range 1 to 21 MHz use an AT cut quartz plate and operate in their fundamental mode. Crystal units may be specified for operation in either a series or parallel resonant condition. This operating condition should be correctly specified when ordering or the crystal unit will not operate on the correct frequency. Typically, over the frequency range of 1 - 21 MHz there may be between 2 and 15 kHz difference between the parallel resonant and series resonant conditions, the series resonant condition being lower in frequency.

If parallel resonance is required, it is necessary to specify a load capacitance for the Crystal. This capacitance is not related to any capacitance present in the crystals or the circuit but refers to the load capacitance used when crystals are measured in standard test sets. The preferred value of capacitance is 30 pF but crystals can be manufactured for other values.

These circuits are designed for non-critical adjustment and operation with easily available components. In each case, an output of at least 500 millivolts RMS is available across a load of 1000 ohms in parallel with 15 pF. The circuits are suitable for use with supply voltages from 5 to 10 volts DC. Over this voltage range frequency stabilities of 0.001% (10 ppm) should be achieved.

Please note that where no load capacitance is specified for fundamental crystals we supply them calibrated for a 30 pF load and although the circuit is shown as suitable for crystals at 0.95 MHz (950 kHz) we do not offer crystals below 1.5 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 following pages circuits are shown 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 we supply 3rd overtone crystals between 21 and 60 MHz. 5th overtone between 60 and 126 MHz and 7th between 126 and 175 MHz.