MK3771-1 Datasheet, PDF (1/6 Pages) Integrated Circuit Systems – USING MICROCLOCKS INTEGRATED VCXO PRODUCTS

USING MICROCLOCK’S
INTEGRATED VCXO PRODUCTS
MicroClock Application Note MAN05
MicroClock has developed a patented technique to
integrate a Voltage Controlled Crystal Oscillator
(VCXO) function into clock synthesizer products. The
VCXO oscillator circuit, in conjunction with an external
crystal, allows the output clocks to be pulled (varied up
or down in frequency) more than 100 parts per million
(+100ppm or 0.01%), under control of an analog input
voltage. The MK27XX and MK37XX series of products
integrate a VCXO oscillator and a PLL, creating a cost
effective VCXO clock solution. Our communication
synchronization products (MK2049, MK2058, MK2059,
MK2069) use the integrated VCXO as a circuit block,
and this information applies to these products also.
the temperature extremes, hot and cold. This variation
is characteristic of a quartz crystal, and the slope and
magnitude is controlled by the type of crystal cut and
the crystal lattice angle at which the crystal is cut. This
parameter is specified with a maximum and minimum
frequency deviation, expressed in percent (%) or parts
per million (ppm). It is typically +/-30ppm for IC VCXO
designs.
X1
C_T
C_T
VCXO IC
Crystal Selection
The crystal is the frequency reference of the VCXO and
the overall performance of the circuit depends on the
characteristics of it. It is important that the crystal
meets all required specifications if the VCXO is
expected to work reliably. ICS works with crystal
vendors to define, build, and certify crystals that meet
these requirements, and the crystal vendors maintain
an inventory of these devices in stock. Please see our
web site for recommended part numbers.
Using a packaged clock oscillator to drive these VCXO
products will not work correctly. The clock will be
generated, but the frequency cannot be pulled because
the VCXO circuit has no control over the oscillator
frequency.
Crystal Specifications
The crystals defined for use with VCXO products have
specifications common to all crystals, and additional
requirements to insure VCXO performance. All crystals
have specifications for:
1. Frequency tolerance (often called Calibration
Accuracy). This is the allowable frequency error from a
specified center frequency of the crystal at 25 c. This
parameter is specified with a maximum and minimum
frequency deviation, expressed in percent (%) or parts
per million (ppm). It is typically +/-20ppm for IC VCXO
crystal designs. The source of this error term is
principally variation in the manufacturing process.
2. Temperature stability. This is the change in
frequency allowed as temperature is varied from 25 c to
Oscillator Circuit
C_L
C_L
Figure 1 - VCXO Circuit
3. Load capacitance. This is the capacitance, specified
in picofarads (pF), which the oscillator circuit presents
to the crystal for the crystal to resonate on frequency.
Load Capacitance is comprised of a combination of the
circuits’ discrete load capacitance, stray board
capacitance, and capacitance internal to the ICS
device. Because this includes the stray capacitance of
the circuit board, we recommend that pads for small
capacitors (CT in figure 1) be provided in your layout to
make small adjustments to the total capacitance.
Details are given in the Layout Considerations section.
4. Equivalent Series Resistance. This is a term that
represents (in ohms) all the losses within the crystal. If
this value is too high, the oscillator may have startup
problems.
5. Aging. This specifies the amount that the frequency
is allowed to drift, long term, and is typically 5ppm in
the first year, and logarithmically declines each year
thereafter, to not exceed a total of +/-20ppm in 10
years.
MAN05 G
1
Revision 050603
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