Quartz Crystal Resonator Specification
- summary, overview or tutorial about the basics of a crystal specification
and specifying a quartz crystal resonator for radio and electronics
applications.
Quartz crystals used in radio and electronics circuits are
precision electronic components and when buying them it is necessary to be able
to specify them precisely. There are normally several elements to a crystal
specification, many of which are specific to quartz crystals and not widely used
elsewhere in radio applications. Also there are a number of elements to a
crystal specification that may be set down by the manufacturer for a given range
of crystals and when ordering a component it is necessary to be aware of them.
Frequency specification
The frequency of the quartz crystal is obviously a
fundamental specification. It is normally expressed to as many significant
figures as demanded by the frequency tolerance, although seven figures is
normally the maximum. It is wise to express the frequency to the right number of
significant figures to avoid misunderstandings in this area of the quartz
crystal specification.
Crystal resonator mode
Quartz crystals may either operate in a fundamental mode or
in an overtone mode. Below frequencies of around 25 MHz crystals are normally
designed to operate in their fundamental mode, whereas above this they will
normally be designed for overtone operation, although with manufacturing
techniques improving higher frequency crystals are becoming available. The mode
is therefore an important element of the crystal specification.
When ordering an overtone crystal quote the exact frequency
of operation and not what is expected to be the fundamental frequency as
confusion may arise over the frequency required, and as the overtone frequency
of the crystal is not an exactly the same as the harmonic of the fundamental
frequency this may result in an incorrect frequency being supplied. The
frequency of overtone crystals is normally expressed in MHz, whereas one
operating at its fundamental frequency is normally expressed in kHz.
Resonance type
There are two types of resonance that are applicable to
quartz crystals. One is parallel resonance and the other is series resonance.
The actual type required will depend on the circuit in use. Although crystals
will operate in either mode, the frequency of resonance for each type of
resonance is slightly different. For some applications such as microprocessor
clock generators the small difference between the two frequencies may not be a
problem, but for many others it is. Accordingly the crystal specification should
clearly include the type of resonance required.
Parallel resonance is the more commonly used type. However
when specifying this type a load capacitance is required because the external
capacitance forms part of the resonant circuit. One common value of load
capacitance is 30pF, although 20pF is also becoming common.
Holder style
Crystals come in a variety of packages. There are a number of
standard varieties used with through-hole mounting and sockets. Styles such as
HC43, etc are still widely available, but there are also many new packages for
use with surface mount soldering. It is necessary to consult the manufacturers
datasheets to make the final choice.
Calibration tolerance
This is the final frequency of the crystal at manufacture at
a temperature of 25C which is normally assumed to the operating temperature of
electronic equipment. However if the crystal is to be used in an oven then the
temperature of the oven should be stated instead. The calibration tolerance
itself is expressed in ppm (parts per million).
Temperature stability
The temperature stability is another important area of the
crystal specification and it is the allowable frequency deviation as the
temperature varies. Again normally expressed in ppm, from the frequency at the
reference temperature per degree Celsius. Sometimes the crystal specification
may use a frequency tolerance consisting of the sum of the calibration and
temperature stability tolerances is quoted.
Ageing
Ageing of quartz crystals will depend upon a number of
factors and in particular the encapsulation. It is generally greatest in the
first few weeks of operation, and as a result crystals to be used in high
quality oven oscillators are run in before use. Figures for ageing are expressed
in a certain number of ppm over a given time, often a day and/or a year.
Activity
A measure of the activity of a crystal is the resistive
component that is seen in the motional arm of its equivalent circuit. As would
be expected the resistance and Q are inversely proportional to each other.
Spurious responses
In some applications the spurious responses may be of
importance, but there are some responses that may be within a few hundred
kilohertz of the main frequency. These are normally low, and rarely cause
problems in oscillator circuits except if the tuned circuit used in the
oscillator resonates on the same frequency as a nearby response. They may be
more important in filter applications, and it may be necessary to specify maxim
response levels relative to the main response. It is likely that in a filter
several crystals will be used, and they will not all use the same frequency.
This will result in the responses also appearing on different frequencies,
making the problem less severe.
Crystal specification summary
When ordering a quartz crystal resonator for any application
it is necessary to ensure that the crystal specification is correct and
expressed in the right format. As there are many elements to a crystal
specification, it is necessary that it is well checked before issuing it to the
manufacturer, and in this way the item that is delivered should perform as
needed.
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