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Crystal
Oscillator General Information
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Oscillator
] [ Surface
Mount Crystal Oscillator
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Crystal
Oscillators are oscillators where a quartz crystal is
used to provide the primary frequency element. The characteristics
of the internal quartz crystal provide that the Crystal Oscillator
can achieve a high degree of frequency stability and accuracy.
A
Crystal Oscillator is made up of an amplifier and feedback
network that selects a portion of the amplifier output and
returns it to the amplifier input. The oscillator circuit
is dependent on two key conditions: First, the loop gain needs
to be greater than losses around the oscillator loop, or equal
to unity. Second, the loop phase shift must be equal to 0
or 360 degrees. Loop phase angle shifts determine the frequency
at which the oscillator will operate. A change in net loop
phase angle results in a change in output frequency of the
oscillator circuit. In order to minimize the net phase shift,
a quartz crystal is placed in the feedback loop. Because of
the changes in impedance of the quartz crystal due to changes
in applied frequency, all other components of the circuit
should be considered constant reactance resulting in an adjustment
of the quartz crystal frequency to a reactance that satisfies
the loop phase characteristics.
Selecting
an Oscilent Crystal Oscillator:
1.
Operating Environment In selecting a Crystal Oscillator,
first consideration should be made to the environment in which
the device will operate. Careful consideration should be given
to the size constraints applicable to the PCB, whether a through
hole or SMT device is needed, and to the temperature environment
in which the Crystal Oscillator will operate:
a.
Selecting a Package: The most common industry wide standard
package for a Crystal Oscillator through hole device is a
4-pin dip half size (Oscilent 320 Series) The most common
package for an surface mount device is the 5x7mm Ceramic 4
pad device (Oscilent 430 Series)
b.
Temperature Environment: Generally, the industry standard
Operating Temperature ranges are 0-+70 degrees celsius (considered
commercial temperature), and -40-+85 degree celsius (considered
industrial temperature) These common ranges make up approximately
80% of all products available in the market. For common uses
in military and automotive applications, some Crystal Oscillators
can be designed for more extreme conditions. The higher the
Operating Temperature specified, the higher the cost of the
device.
2.
Electrical Parameters Generally, the Crystal Oscillator selected
will have common characteristics that will be standard across
industry participants. These characteristics include standards
in available frequencies, drive currents, capacitance, pin
outs, etc. However, key Electrical Parameters of the Crystal
Oscillator must be selected by the consumer:
a.
Package: Discussed above
b.
Output Logic: By far, the most common Output Logic is CMOS/HTML.
However, other Output Logics are available including PECL
and LVDS.
c.
Frequency
d.
Voltage: The two most common Operating Voltage options are
3.3 Volt (considered low voltage) and 5.0 Volt (considered
standard) Other less common Operating Voltages are available
including voltages ranging from 1.8-3.0 Volts. Whenever possible
the Crystal Oscillator should be specified in either 3.3 Volt,
or 5.0 Volt to minimize costs and lead times of the device.
e.
Stability: The most standard Stability for a Crystal Oscillator
is 50ppm. The Stability is most dependant on the Operating
Temperature Range, and, hence, 50 ppm over 0-+70 degrees celsius
in most common. 15-20 ppm is normally considered the tightest
production spec available, but special accommodations are
possible for tighter specs.
f.
Operating Temperature: Discussed above
g.
Pin One Option: Output Enable (Tristate) or No Connection
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