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- Voltage Control Oscillators
VCXO is Crystal Oscillator, only the output
frequency can be adjusted by varying the external control
voltage across a varactor diode within the oscillator circuit.
The change in frequency versus control voltage is otherwise
referred to as pullability. To achieve this adjustment, a
VCXO utilizes a variable capacitor (varactor diode) that,
when used in series with an internal Quartz Crystal, combined
with a change in control voltage, causes the diode capacitance
to change. This capacitance change results in a change in
the internal Crystal load capacitance thereby resulting in
a change in frequency. VCXO's are typically used in phase
locked loops (PLLs) for data acquisition or signal regeneration
applications, and their inherent capabilities are sometimes
referred to as Frequency Modulation.
an Oscilent VCXO:
VCXO's are manufactured according to industry standard guidelines
pertaining to such variables as package types, 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 reasonably constant. When selecting
a VCXO product, careful consideration should be given to the
following variables for which parameter selections are made
when specifying an Oscilent Part Number:
a. Package Type: Will the package be Through Hole or Surface
Mount, and are there size constraints with either.
Output: Usually one of TTL, HCMOS, Compatible (simultaneous
TTL and HCMOS compatible), or ECL Output.
Supply Voltage: Most VCXO 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
the Operating Temperature Range, an amount of total deviation
(in ppm) acceptable for the application. The VCXOFrequency
Stability includes the initial adjustment (with the control
pin set to center trim voltage of 2.5V) at 25 degrees Celsius,
the Tolerance over Operating Temperature Range, the Load,
and the effects of Supply Voltage variation, unless otherwise
defined on the Oscilent data sheet. Normally there are three
(3) industry standard options (100ppm, 50ppm, and 25ppm),
however tighter parameters are possible for most Oscilent
the industry standards for VCXO 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 VCXO products can be designed for more
extreme conditions. However, the higher the Operating Temperature
specified, the higher the cost of the device. Special consideration
should be given in an attempt to avoid "over-specifying" the
required Operating Temperature. Although, in general, a VCXO
will continue operation outside their normal temperature range
with a degradation in stability, damage to the VCXO will result
if the temperatures reached are excessive.
referred to as Symmetry, the Duty Cycle measures the time
that the output waveform is in a logic high state, expressed
as a percentage. A perfectly symmetrical output waveform would
be in the logic high state exactly 50% of the time, and in
a logic low state the other 50% of the time producing an oscillator
Duty Cycle specification of 50%/50%. However, when specifying
this parameter in an oscillator (VCXO), a tolerance for this
parameter must be specified. Typically, this tolerance is
specified at +/-10% creating a Duty Cycle parameter of 40%/60%
(+/- 10% from the 50% reference) Industry standards allow
for specifying a tighter tolerance Duty Cycle, usually allowing
for a 5% maximum deviation (expressed as 45%/55%) These standards
allow for a selection of either 40/60% or 45/55% Duty Cycle
in the Oscilent Part Number Guide.
Pulling Range is the amount of Frequency Deviation realized
by a corresponding change in voltage. The Pulling Range of
a VCXO is determined by the motional capacitance characteristics
of the Quartz Crystal, among other less critical factors.
The Voltage Control function typically utilizes Pin 1 of both
Through Hole and Surface Mount packages. The deviation is
normally expressed in Parts Per Million ranging from 15-200ppm
MIN. on most Oscilent devices.
plotting the Output frequency versus Control Voltage, Linearity
(expressed as a percentage) refers the maximum allowable deviation
away from the best straight line drawn through the tuning
curve. For example, a VCXO with a 1kHz tuning range, and a
10% Linearity parameter, would allow for an excursion of 100Hz
away from the best straight line.
is a manufactuer of VCXO and a supplier worldwide for various
applications and markets. Oscilent offers standard products
and have designers in-house for custom VCXO devices.