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TCXO
General Information
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- Temperature Compensated Crystal Oscillators
]
A
TCXO is best viewed as a derivative of a standard Crystal
Oscillator for use in applications where an improvement in
the devices Temperature Stability is required. The premise
holds that the TCXO designer can flatten the Temperature versus
Frequency curve in order to spec a device meeting these more
precise characteristics. Aging characteristics can also be
improved upon versus a standard Crystal Oscillator.
A
Temperature Compensated Crystal Oscillator (TCXO) utilizes
the following design features differing substantially from
a basic Crystal Oscillator design:
1.
Specially processed tight tolerance crystal manufactured for
both accuracy and to offer minimized long term aging.
2.
Temperature/frequency compensation network made up of thermistors,
resistors, and capacitors in an oscillation loop. The thermistor
network is matched to the tight tolerance crystal causing
the voltage to vary with temperature thereby compensating
for the inherent frequency versus temperature characteristics.
3.
A frequency/tuning adjustment allowing for manual trimming
of the TCXO to compensate for aging and/or other Environmental
influences. The method of frequency adjustment can also be
electrical via an additional pin located on the package (utilizing
an external potentiometer, for instance), or a TCVCXO utilizing
both temperature compensation and external voltage control.
4.
Buffering to minimize the effects of external changes in the
circuit.
5.
An internal regulator to minimize the effects of variation
in voltage.
Specifying
an Oscilent TCXO:
Generally,
TCXO's are manufactured according to industry standard guidelines
pertaining to such variables as package type, output types,
input voltages and currents, rise/fall times, and duty cycles.
Although these parameters may vary slightly between manufacturers,
and review of these parameters should be done when crossing
between manufacturers, one can generally assume that these
parameters will remain constant. When selecting a TCXO/VCTCXO
product, careful consideration should be given to the following
variables for which designations are made when selecting an
Oscilent Part Number:
a.
Package Type: Will the package be Through Hole or Surface
Mount, and are there size constraints with either.
b.
Output: Usually one of TTL, HCMOS, Clipped Sine Wave, or Compatible
(simultaneous TTL and HCMOS compatible) while reviewing the
characteristics of Load, Output Levels, and Input Currents
associated with each Output type referenced in the Electrical
Parameters portion of the Oscilent TCXO spec.
c.
Frequency
d.
Supply Voltage: Most TCXO products are designed to operate
at either 5.0V, or 3.3V, although other options are available.
The Oscilent Specification assumes this standard. For other
Supply Voltage options, please contact Oscilent for assistance
Frequency
Stability:
Over
the Operating Temperature Range, an amount of total deviation
acceptable for the application. Oscilent provides a Frequency
Stability versus Temperature chart on most TCXO product Specifications.
This chart should be used as a general guide only as, variations
to this parameter are available. For a TCXO, industry standard
selections range from 1.0ppm to 5.0ppm over temperature. For
applications requiring <1ppm Frequency Stability, please contact
Oscilent Engineering Support for assistance.
Operating
Temperature:
Because
of the more precise characteristics of a TCXO compared to
standard Crystal Oscillators, an increased focus on Operating
temperature is recommended. For TCXO's the Operating Temperature
Ranges are smaller and offer many more options. Generally,
a TCXO has a standard range of -40-+80 degrees Celsius, however
the Operating Temperature is a significant influence on what
is possible in production from Stability perspective, in addition
to influences on production costs, so special consideration
should be given in an attempt to avoid "over-specifying" the
required Operating Temperature.
Frequency
Deviation:
In
TCXO product design, a package pin/pad is normally chosen
to provide for initiating the VCTCXO (voltage control) option.
For normal TCXO operation, this pin is designated as "No Connection."
However, when the Voltage Control option is exercised in part
number selection, a designation must be made that, not only
specifies that this function is needed, but also specifies
the amount of deviation required for this adjustment.
For
more technical information, including Term Definitions, Suggested
Reading Material, and Engineering Support, please visit the
Oscilent Engineering Support
page.
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