MAN05 J 1 Revision 052312
Integrated Device Technology l Tech Support: www.idt.com/go/clockhelp l www.idt.com
USING IDT’S INTEGRATED VCXO PRODUCTS
IDT Application Note MAN05
IDT 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, ICS726A) use the integrated VCXO as a
circuit block, and this information applies to these
products also.
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. IDT 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
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.
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 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
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Oscillator Circuit
VCXO IC
Figure 1 - VCXO Circuit
USING IDT’S INTEGRATED VCXO PRODUCTS
Additional requirements for VCXO crystals
The ratio C0/C1 is inversely proportional to pullability;
lower ratios indicate a more pullable crystal. Crystals
intended for use with IDT VCXOs must have C0/C1
ratio no higher than 250 if they are to meet minimum
pull requirements.
Crystals can be made to resonate either at the
fundamental frequency, or on the third, fifth, or even
higher overtone. VCXO crystals are always
fundamental mode, because overtone modes are much
less pullable and require additional oscillator circuitry
for proper operation.
The third overtone mode is not necessarily at exactly
three times the fundamental frequency. The
mechanical properties of the quartz element dictate the
position of the overtones relative to the fundamental,
and in a VCXO circuit, the third overtone is not typically
exactly three times the fundamental, or the oscillator
circuit may excite both the fundamental and overtone
modes simultaneously. This will cause a nonlinearity in
the transfer curve such as the one in Figure 3. This
potential problem is why VCXO crystals are required to
be tested for absence of any activity inside a +/-100
ppm window at three times the fundamental frequency.
Crystals for VCXO applications are always parallel
resonant because series resonant oscillators cannot be
pulled. The designation for the lattice angle of these
Figure 2 - Equivalent Circuit of a Crystal
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Vin (V)
ppm
normal curve
curve with
perturbation
Figure 3 - Perturbation due to third overtone