National Instruments NI 6601, Manual, Page: 5 / 17

© National Instruments Corporation 5 NI 6601/6602 Calibration Procedure
The frequency measurement will have an uncertainty that is equal to the
sum of the stable clock error and the quantization error caused by the
counter. The clock error should be 1 ppm or less, as specified in the
Test
Equipment section of this document. The quantization error, which is the
inherent uncertainty in digitizing an analog value, will always be up to
50 ns. Using a measurement duration of 50 ms gives a quantization error
as follows:
The sum of the stable clock error, 1 ppm, and the quantization error,
1 ppm, gives a total measurement uncertainty of 2 ppm. You can reduce
measurement uncertainty by using a more stable clock, by increasing the
measurement duration, or both. The calculations presented at the end of this
document allow for a 2 ppm measurement uncertainty.
Both Traditional NI-DAQ and LabVIEW programming instructions are
presented in the following sections. Choose the appropriate instructions for
your programming environment.
Setting Up the Stable Clock
This section describes how to set up the clock. NI recommends using the
PXI-6608 to provide the stable clock. You can use an alternative stable
clock if it meets the specifications described in the
Test Equipment section
of this document. Instructions for both clock alternatives are included in the
following two sections,
Setting Up a PXI-6608 Clock Source and Setting
Up an Alternative Clock Source .
Setting Up a PXI-6608 Clock Source
To set up a PXI-6608 as the stable clock source, complete the following
steps:
1. Power on the 6608 device.
2. Wait 15 minutes to allow the measurement circuitry in the device
to warm up.
50 ns
50 ms
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1 ppm
=