Specifying Crystals

 

Frequency

The frequency of a crystal is dependent upon circuit design parameters and should be expressed to as many significant figures as demanded by the frequency tolerance; 7 figures are usually adequate. For example, if the frequency of a ±10ppm crystal was given as 48.583MHz it can be interpreted as either 48.5830 or 48.58333 possibly resulting in crystals being made to the wrong frequency. Eska will add a zero to the last figure after the decimal point if fewer than seven figures are given.

 

Mode

Unless otherwise specified, AT-cut crystals below 24MHz will be made as fundamental mode units. At 24MHz and above economics usually dictate whether a crystal is to be made a fundamental, or one of the odd (3rd, 5th, 7th etc.) overtones. The high frequency limit of each normally being set by 21xn MHz, where n is the overtone order. If a particular mode is important to ensure satisfactory circuit operation, it should be stated. Since the fundamental frequency of an overtone crystal is not exactly related to its overtone frequency, do not quote a fundamental frequency where overtone operation is required. For crystal frequencies above 24MHz it is always preferable to specify 'fund' for fundamental or 'n' Overtone, where 'n' = overtone.

 

Holder Style

A crystals' holder style depends upon its application, design and frequency. ESKA adopts what has become common industry practice in using the American (HC-xx) nomenclature for metal cans. However, to avoid confusion, mechanical outlines and dimensions are given throughout these pages.

 

Calibration Tolerance

Expressed in parts per million, (ppm) calibration tolerance indicates the permissible tolerance at the reference temperature of 25°C. This temperature is usually assumed for ambient temperature operation as most crystals exhibit an approximately symmetrical frequency/temperature curve around this temperature. If a crystal is to be used in an ovenized, temperature-controlled environment, the oven operating temperature should be stated. For certain applications instead of a plus or minus tolerance crystals are specified with an offset tolerance.

 

Temperature Stability

The allowable frequency deviation in parts per million from the reference temperature over the operating temperature range is known as the Temperature Stability. Note that the operable temperature range is usually considerably wider than the operating range; the stated tolerance is only guaranteed for the latter. Frequency tolerance may also be expressed as an overall tolerance, that is the sum of calibration and temperature tolerances. Military specification and low-frequency crystals are often specified in this way.

 

Overall Tolerance

The allowable frequency deviation over the specified temperature range with respect to nominal frequency is known as an Overall Tolerance. It is the sum of calibration tolerance and temperature stability.

 

Drive Level

For high frequency AT-cut crystals, the maximum recommended drive level for reasonable stability is in the region of 1-2 milliwatts, but for better results is should be limited to 0.5mW or less. Up to 10 times this value may not cause damage but will result in significant frequency shifts; this is particularly apparent with VHF overtone crystals. Most low frequency crystals cuts should be limited to 100mW. Miniature 'watch' type crystals to only ½mW or so.

 

Circuit Condition

Crystals have both a series and a parallel (or load) resonance frequency, the latter being dependent upon the effective circuit capacitance presented to them. For many applications, (for example micro-processors not needing an overly accurate real-time clock) the small difference between the two resonance frequencies is irrelevant. (It is more likely that the oscillator circuit itself will be producing greater errors.) If the crystal must oscillate at a precise frequency then the circuit condition, either series resonance or parallel resonance with a stated value of load capacitance must be stated. Should this not be stated ESKA will supply made-to-order fundamental mode crystals calibrated at 30pF and overtone crystals at series resonance.

 

Motional Parameters

 

   The crystal motional parameters, motional capacitance (C1), motional inductance (L1) and motional resistance (R1 - alternatively referred to as
        ESR,) can be important for correct operation of a crystal in an oscillator circuit. As motional parameters need to be designed-in to the                   crystal specification prior to manufacture if there is any doubt contact ESKA  

 

 

Crystal Specification Code

 

For most crystal specifications, ESKA uses an 'industry-standard' method of describing a crystal specification:

 

FREQUENCY/CRYSTAL HOLDER/CALIBRATION TOLERANCE/TEMPERATURE STABILITY/TEMPERATURE RANGE/CIRCUIT CONDITION/MODE/SPEC

 

For example:

 

16.000MHz HC49/20/30/10/30/ATF/SPEC

 

This translates as:

 

FREQUENCY - Nominal crystal frequency.

 

CRYSTAL HOLDER - Style of package in which the crystal is manufactured.

 

CALIBRATION TOLERANCE - Permitted maximum calibration tolerance in ±ppm at 25°C.

 

TEMPERATURE STABILITY - Permitted maximum frequency deviation over the operating temperature range, in ±ppm.

 

TEMPERATURE RANGE - Operating temperature range. As most cuts of crystal exhibit a symmetric performance around the temperature of 25°C, it is usual practice to only state the lower figure of the temperature range and to omit the '-' (minus sign). In the example above, '10' indicates that the operating temperature range is from -10° to +60°C. If -20° to +70°C operating temperature was required then this figure would be written as '20', etc.

 

CIRCUIT CONDITION - This refers to the configuration of the oscillator circuit in which the crystal will be used. As it is usually appropriate to macth a crystal to the conditions it will 'see' in circuit. This will be either 'S' for series or a numeric value which directly relates to load capacitance, the above example indicating a circuit load of 30pF.

 

SPEC - This refers to any other crystal parameters that are necessary to ensure correct operation of the crystal in circuit. These parameters could include ESR (Effective Series Resistance) parameters, shunt capacitance (C0), motional capacitance (C1), capacitance ratio, or motional inductance (L1). However, if several parameters need to be specified it may be best to request ESKA to raise a separate and unique part number for your crystal; this will avaoid an over-long and unwieldy part number, possibly prone to typographical error being produced.

 

ESR - If you wish to ensure a minimum crystal activity you may specify ESR in the form 'R50' for instance; this will indicate that the crystal is to be produced with ESR <50 Ohms.