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Quartz
Crystal General Information
Go to all [ Quartz
Crystal
] [ Surface
Mount Quartz Crystal
]
1.
Constant Electrical Parameters of Quartz Crystals
Generally,
Quartz Crystals are manufactured according to industry standard
guidelines pertaining to such variables as package type, (a.)
Mode of Operation, (b.) Equivalent Series Resistance, (c.)
Shunt Capacitance, and (d.) Drive Level. Although these parameters
may vary slightly between manufactures, and review of these
parameters should be done when crossing between manufacturers,
one can generally assume that these parameters will remain
constant.
a.
Mode of Operation (Resonance Frequency):
A Quartz Crystal has two basic modes of operation comprising
Fundamental and Overtone Modes. For frequencies below 30MHz,
a crystal manufacturer will use the Fundamental Mode of operation.
For frequencies exceeding 30 MHz, overtone modes are recommended
(overtones are not precise integer multiples of the fundamental
as in a similar concept found in harmonics) Higher Fundamental
mode Crystals are possible, however the crossover at 30MHz
is generally considered the point where the Fundamental mode
Crystal becomes more expensive to produce due to thinning
of the quartz blank (increasing breakage during production).
Using Overtone Modes in production therefore reduces manufacturing
costs for higher frequency applications. A good rule of thumb
is to assume the a Crystal manufacturer will produce a Fundamental
Crystal below 30MHz, and an Overtone Crystal above 30MHz.
If higher frequencies are required in the Fundamental Mode,
please contact Oscilent for support and details on Inverted
Mesa technology.
b.
Equivalent Series Resistance (ESR):
The Equivalent Series Resistance is the resistive element
of the quartz crystal equivalent circuit. This range is normally
found to be 25-100 ohms for most Quartz Crystals and is usually
specified as a maximum value. Typically, these values are
specified according to frequency ranges and are very similar
across multiple manufactures because similar holder types,
electrodes, modes of operation, and mounting structures are
used in achieving specific frequencies. More significant at
lower frequencies, it should be noted that ESR values at a
given frequency for an AT- strip cut crystal (commonly used
in Surface Mount devices) are higher than that of the standard
(round blank) design (commonly found in Through Hole devices)
c.
Shunt Capacitance:
The electrodes present in a Quartz Crystal, plus the stray
capacitance of the holder, form a capacitance in parallel
with the LC model. Generally, the Shunt Capacitance is sought
to be as low as possible in manufacturing the device, as some
oscillators cannot tolerate an excessive value. Because Shunt
Capacitance may vary according to the size of the Crystal,
manufacturers will normally designate a maximum value usually
at 7.0pF. If a lower value is required, we suggest contacting
Oscilent Engineering Support.
d.
Drive Level:
Specified by Quartz Crystal manufacturers to provide a guideline
for maximum values, the Drive Level should be held to a minimum
to avoid problems with Aging, Stability, nonlinear coupled
modes, and other nonlinear effects. Improvements in phase
noise performance can be realized with increased drive levels,
however, so careful consideration should be given to this
Electrical Parameter when approaching the maximum suggested
level.
2.
Specifying an Oscilent Quartz Crystal
(Variables that must be selected
by the design engineer)
Variable
Electrical parameters are those that the Design Engineer must
specify when selecting a Quartz Crystal device for any particular
application. Although Oscilent offers a wide range of these
variables in the Part Number Guide section of our Specification,
other, more custom parameters, are available:
a. Package Type: Will the package be Through Hole or Surface
Mount, and are there size constraints with either.
b. Frequency
c.
Load Capacitance: The specified Load Capacitance is dependent
on the Resonance Mode required in the application ( for more
detailed information on this subject Definitions
of Load Capacitance ) For Series resonance, no Load Capacitance
is required ("SR" in the Oscilent part number). For Parallel
Resonance, the Load Capacitance specified by the Design Engineer
will be used to calibrate the Quartz Crystal thereby effecting
the major operating characteristics of the device, including
initial Frequency Tolerance.
d.
Frequency Tolerance: At 25 degree Celsius, an amount of initial
frequency deviation acceptable for the application is required.
Tighter specifications of Frequency Tolerance lower yield
in Quartz Crystal Blank production thereby serving to increase
production costs.
e.
Stability: Over the Operating Temperature Range, an amount
of total deviation acceptable for the application. Tighter
specifications of Stability lower yield in Quartz Crystal
Blank production thereby serving to increase production costs.
f.
Operating Temperature: Standard Operating Temperature ranges
are generally considered as -20-+70 degrees Celsius (considered
"commercial" Operating Temperature), and -40-+85 degrees Celsius
(considered "Industrial" Operating Temperature) Other Operating
temperature ranges are available and should be specified and/or
discussed with an Oscilent Technical advisor.
For
more more technical information, including Term Definitions,
Suggested Reading Material, and Engineering Support, please
visit the Oscilent Engineering
Support page.
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