Chapter 3
Adjustment
©
National Instruments Corporation
3-25
•
sessionHandle
—The handle of the calibration session for the
device
•
measurementMode
—
NI54XX_SET_D1
•
actualData
—
NULL
•
measuredData
—
NULL
Determining the VCXO Calibration Constants
This stage of calibration determines the correct value to write to the
VCXO DAC,whichcontrols the device oscillator,toachieve adesiredupdate
rate. For all devices, you must find a value that results in an update clock of
about 40 MHz. For the NI 5431, you must find two additional values. These
values correspond to the update rates needed for the M-PAL and NTSC video
modes. Like the duty cycle calibration, VCXO calibration is an iterative
process. First, initialize the VCXO calibration for the current generation
mode (normal, M-PAL, or NTSC). Then, call the function
ni54xx_CalibrateVCXO
. This function, like
ni54xx_SetDutyCycle
,
sets the VCXO with the current “best-guess.” Next, measure the output
frequency. If the measured frequency differs substantially from the desired
frequency, call
ni54xx_CalAdjust
, which calculates the next “best-guess”
value. Call
ni54xx_CalibrateVCXO
again, setting the VCXO to the new
“best-guess” and measure the frequency again. Repeat this process
until the output frequency is acceptable or until you have called
ni54xx_CalAdjust
12 times. Iterating more than twelve times produces no
further refinements.
For all devices:
1.
Enable the analog filter by calling the function
ni54xx_SetAnalogFilter
. Set the following parameters:
•
sessionHandle
—The handle of the calibration session for the
device
•
filterState
—
NI54XX_ENABLE
2.
Generate a full scale (10 V) sine wave at 1 MHz by calling the function
ni54xx_GenerateWaveform
. Set the following parameters:
•
sessionHandle
—The handle of the calibration session for the
device
•
waveType
—
NI54XX_SINE
•
amplitudeInVolts
—
10
•
frequencyInHz
—
1000000
