MK3771-1 ICST [Integrated Circuit Systems], MK3771-1 Datasheet - Page 5

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

Figure 6 -
Determining Value of Fixed Centering Capacitors
VCXO parts from ICS require that locations be
provided to tune the load capacitance of the pullable
crystal. This tuning serves to center the crystal's
operating frequency relative to the VCXO, thereby
increasing the range of frequencies that can be
locked-to by the VCXO over that of an untuned board.
The MicroClock applications department can perform
this procedure. Send us two PC boards (stuffed or
unstuffed) and we will calculate the value of the
capacitors needed.
Many boards will not need any tuning capacitors, but
for consistent long-term performance of a system, two
load capacitor pads should be put into every design.
What follows is the general procedure for tuning these
load capacitors to match the specific board layout.
Typically, the required capacitors will range from 0 to 4
pF.
Procedure
To determine the need for and value of these
capacitors, you will need a PC board of your final
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Integrated Circuit Systems
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layout, a frequency counter capable of less than 1 ppm
resolution and accuracy, two power supplies, and some
samples of the crystals which you plan to use in
production, along with measured initial accuracy for
each crystal at the specified crystal load capacitance,
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the crystal manufacturer by obtaining datalogged
crystals, or by using a crystal tester.
To determine the value of the crystal capacitors:
1. Connect VDD of the ICS part to either 5.0V or 3.3V
(depending on part and system requirements). Connect
the loop filter voltage to the second power supply.
Adjust the voltage for the loop filter to 0V. Measure and
record the frequency of the clock output.
2. Adjust the voltage on the loop filter to 3.0V (for a 5 V
part) or 3.3V (for a 3.3V part). Measure and record the
frequency of the same output (this is f
To calculate the centering error:
Centering error
Where:
f
=
target
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. In practice, this measurement can be performed at
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ICRO
6
= nominal crystal frequency
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Revision 050603
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