MK3771-1 ICST [Integrated Circuit Systems], MK3771-1 Datasheet - Page 3

• Manufacturer Part Number: MK3771-1
• Description: USING MICROCLOCKS INTEGRATED VCXO PRODUCTS
• Manufacturer: ICST [Integrated Circuit Systems]

Absolute Pull Range
VCXOs are usually used as a narrowband local
frequency source that is locked to some external
frequency reference. The VCXO must have sufficient
accuracy and pullability to be able to lock to that
reference, and Absolute Pull Range is the measure of
that ability. All the frequency errors of the VCXO are
subtracted from the nominal pull range, and the
remaining range can be guaranteed over all conditions.
Here is an example using the standard specifications
for the recommended crystals:
The VCXO will be able to output a frequency +/- 35ppm
under all conditions, which is sufficient for applications
such as MPEG transport (32 ppm), SONET (20 ppm),
PDH communication links (32 ppm), and others. If your
application requires more APR, the best approach is to
respecify the crystal to reduce the error terms. Contact
ICS for advice.
Packaging and Assembly Considerations
The special requirements of a VCXO crystal are best
met with a full size AT-cut round quartz crystal blank.
Unfortunately, this blank will only fit into the traditional
full size HC/49U metal can or the smaller UM1. These
packages are available with wire leads for through hole
mounting, or a third lead may be added to the top of the
crystal can and the three leads may then be formed
into a surface mount gull wing device. Smaller surface
mount packages, including HC49/US (the “short” can),
require a smaller piece of quartz (often called AT strip
resonators). The resulting mechanical limitations of
strip resonators restrict the performance of the crystal.
The smaller electrodes mean that C0/C1 is higher and
the pullability spec is harder to meet. Greater
mechanical losses in the quartz mean that the ESR
MAN05 G
Integrated Circuit Systems
115 ppm guaranteed pull range for the MicroClock
-20 ppm subtract initial accuracy
-30 ppm subtract temperature stability
-20 ppm subtract aging
-10 ppm subtract for circuit variation
-----------
35 ppm
VCXO circuit with a minimum pull
(C0/C1=250) crystal
Absolute Pull Range
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specification is harder to meet. And the most subtle
and potentially most
Figure 4- The HC/49U or UM1 crystal can be ordered
with gull-wing leads for surface mounting.
devastating effect is that, in the smaller crystal,
undesired vibrational modes can be active over narrow
temperature ranges and cause serious frequency
perturbations (also known as activity dips) such as
those in figure 6. In this example, severe discontinuities
exist at 38 C and 59 C. This could cause unlocking in a
feedback control system at these temperatures, yet
there is no hint of this problem at room temperature.
ICS maintains a list of crystals that have been tested
and approved for use with our VCXO chips.
click here for a current list of approved
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Revision 050603
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RODUCTS