Appendix A
Overview of the Calibration Constants
©
National Instruments Corporation
A-3
NI 54XX Calibration Procedure
Voltage-Controlled Crystal Oscillator (VCXO) Constants
When external clocking is not being used, the NI 54
XX
devices use a
voltage- controlled crystal oscillator. If the device is not being phase locked
to an external signal, the driver must determine what value to write to the
VCXO DAC that controls the voltage to the oscillator. Calibrating the
VCXO involves iteratively converging on the value that sets the update
clock rate as near to 40 MHz as possible. For the NI 5431, two additional
constants are stored. These constants represent the proper VCXO settings
for generating phase alternation line (M-PAL) and National Television
System Committee (NTSC) video signals, which require slightly different
update rates.
