The details which must be specified when ordering crystals are:

1. Frequency. Normally specified in kHz up to 9999.999 kHz and in MHz from 10.0MHz upwards except for integer values which are all specified in MHz. The frequency must be described to seven significant figures, otherwise any figure that might follow those given will be taken as zero.

2. Mounting or holder style.

3. Frequency tolerance. This is the cutting or calibration tolerance acceptable at 25◦C. It should be borne in mind that cost rises with increased manufacturing accuracy and a slight adjustment (pullability) is possible in the circuit.

4. Frequency stability. Normally specified as a plus or minus value measured over a defined temperature range. A crystal designed for a restricted range has a better performance over that range than one designed for a wider range so it is important not to overspecify.

5. Temperature range. The range over which the crystal is required to operate and meet the performance specified in 4. Standard temperature ranges are:

0 to 5◦C
−10 to 60◦C
−20 to 70◦C
−30 to 80◦C
−40 to 90◦C
−55 to 105◦C
−55 to 125◦C

It is sufficient when ordering from some manufacturers to quote only the lower temperature limit. For ovened operation the quoted figure, say 80◦C, would denote the oven temperature.

6. Circuit condition. This specifies the shunt capacitance that the circuit will place across the crystal in parallel mode operation.

7. Drive level. The maximum power that the crystal can safely dissipate. 1mW is a typical value for crystals used in radio transmitters and receivers.

A typical specification therefore reads: 16.66667MHz HC49 20 30 10 30
1 2 3 4 5 6 referring to the items listed above

When the crystal is for operation in series mode, it is usually sufficient to replace the last figure with ‘S’. The drive level is not normally specified in the ordering details.

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