Quartz Crystal Cuts
- the way in which a crystal blank is cut from the main quartz crystal has a major impact on its properties.
Looking at the specifications of quartz resonators mention is often made about the quartz crystal cut. Terms including AT-cut, CT-cut and SC-cut appear and have a major relevance to the operation of the crystal.
When defining the specification for a particular crystal, it is often necessary to define what quartz crystal cut is required.
Crystal cut basics
A quartz crystal has a complicated structure, but one that is the same for all crystals. There is an infinite number of ways this can be cut relative to the x, y, and z, axes. However there are several cuts that have defined angles to the axes that give properties that are of particular use.
The structure of a quartz crystal showing the different faces
The blank for the final finished crystal is cut with respect to the crystallographic axis of a quartz bar shown above. The type of crystal cut, i.e. angle of cut with respect to the three axes determines many of the properties of the crystal and influences many others. Factors including mode of vibration, ageing, frequency stability and many other parameters.
Some of the basic cuts include those cut along the axes. The cuts are then labelled according to the plane to which they are perpendicular.
X, Y, and Z crystal cuts
One of the most widely used cut is the AT cut. This is at 35° 25' to the Z axis as shown. The crystal blanks are cut in this orientation to the crystal axes and then the blank is machined and finished to the required size.
Quartz crystal AT cut
Summary of popular crystal cuts
It is possible to define an infinite number of crystal cuts. However, some have defined properties that are particularly useful, and these cuts have been given specific names.
|Crystal Cut||Typical Operational Frequency Range (MHz)||Vibration Mode||Details|
|AT||0.5 - 300||Thickness shear||The AT crystal cut is the most widely used cut and it is particularly used for electronic instruments, etc where oscillators are required to run in the range 500 kHz to around 300 MHz, although the top limit is increasing as technology develops.|
|SC||0.5 - 200||Thickness shear||The SC cut or stress compensated cut was developed in the late 1970s especially for use in precision crystal ovens where some key requirements are low sensitivity to thermal and mechanical stress. The SC cut crystal provides good phase noise and ageing characteristics. This cut uses a double rotation to the basic axes: 35°15' and 21° 54'. Whilst this cut provides excellent ageing and stability as well as a low phase noise performance, it has a higher ESR and also it is more susceptible to spurious resonances. One of the difficulties with the SC cut is that it creates difficulties during manufacture because the requirement for compound angles as used in the SC cut adds cost in terms of measuring the angles and then maintaining them during the subsequent lapping and polishing processes. Tolerances for the SC cut are tight. They typically require a tolerance of ±10" as opposed to ±30" for an AT cut.|
|BT||0.5 - 200||Thickness shear||This is another cut similar to the AT cut but it uses a different angle: 49° from the z axis. It provides repeatable characteristics and has a frequency constant 2.536 MHz?mm. However the temperature stability characteristics are not as good as the AT cut, but it can be used for higher frequency operation more easily as a result of its higher frequency constant.|
|XY||5 - 100 kHz||Length-width flexure||This crystal cut is widely used for low frequencies where one common frequency is 32.768 kHz. It has advantages that it is very small for the frequency, is less expensive than other low frequency crystal types and in addition to this it has a low impedance and low Co/C1 ratio.|
|GT||0.1 - 2.5||Width extensional||Cut at an angle of 51° 7', it has a temperature coefficient of nearly zero between ?25 and +75°C as a result of the effect of the two modes cancelling each other.|
|IT||0.5 - 200||Thickness shear||This crystal cut is very similar to the SC. However, with crystal ovens needing to work in the range 80 - 90°C this option has enabled the difficulties using the SC at these temperatures to be overcome. The IT cut has a upper turning point of between 85 and 105°C but it does not share the lower level of mechanical stress sensitivity of the SC.|
There are very many crystal cuts that are available. Some that were available and used many years ago have fallen out of use because they have been superseded by superior cuts that are now available with the more advanced manufacturing techniques.
By Ian Poole
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