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Quartz Crystals

Fundamental Oscillators

Crystal units supplied by QuartSLab.com for use over the frequency range 1.5 to 27 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 1.5 - 27 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. Load capacitance is defined as being the total capacitance present in an oscillator circuit as measured or calculated across the pins of the crystal socket. Load capacitance has the effect of increasing the frequency of a crystal unit. The preferred value of capacitance is 30 pF but crystals can be manufactured for other values. For example, if the load capacitance (CL) of a parallel resonant crystal is specified as 20pF, the value of the load capacitors used in a parallel resonant oscillator circuit can be calculated using the following formula to approximate the value of capacitors needed:

CL=((C1 x C2) / (C1 + C2)) + Cstray

Cstray is the stray capacitance in the circuit, typically 2-5pF. If the output frequency is high, the capacitor values should be increased to lower the frequency. If the frequency is low, the capacitor values should be decreased, thus raising the frequency of oscillation. When CL =20pF, C1 and C2 will be approximately 27-33pF each, depending on the amount of stray capacitance.

The oscillator circuits referenced in this section 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 capacity (CL) and although the circuit is shown as being suitable for crystals at 0.95 MHz (950 kHz) Quartslab does not offer crystals below 1.5 MHz.

Due to the physical size of the crystal there are some restrictions in the availability of types of crystal holders.

  • 1.5 to 225 MHz HC6/u, HC33/u
  • 2.0 to 225 MHz HC49/u, HC50/u (HC18/u, HC25/u)
  • 6.0 to 225 MHz HC49/T (HC18/T), HC25/T (HC25/T)
  • 12.0 to 225 MHz HC45/u, HC18M (HC49M), HC25M (HC50M)

QuartSLab.com can supply fundamental crystals up to 27.0 MHz and these circuits should work at that frequency but some minor component changes may be required above 21.0 MHz.


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