Measuring Low Frequencies with LabVIEW and FieldPoint Using [c]FP-CTR-500/502
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You can measure frequency in two ways using a counter.
1) Measure the time it takes for a set number of pulses to pass or,
2) Count the number of pulses that pass in a set amount of time.
For low-frequency signals it is best to measure the time duration of a set number of pulses. If we tried to calculate the number of pulses that passed in a set amount of time, we would need a long gate period to get accurate results.
The drawback of measuring the amount of time it takes for a set number of pulses to pass is that the sample period varies with the frequency of the signal.
To accurately measure low frequencies with a CTR module, select two counter channels, one output channel, and one gate input channel. Count Input 0 counts the pulses of the signal that we are measuring. The output of the counter is set to high for x pulses and low for x pulses, which is used as the gate of Count Input 1. The source of Count Input 1 is the internal clock reference (the example can be modified to use an external clock reference). During the high phase of Count Input 0, Count Input 1 counts the pulses of the clock reference.
During the low phase, we read channel 1 and reset the counter.
For example, if the output of Count Input 0 is set to high for 4 pulses and the signal has a frequency of 20 Hz, the output of Count Input 0 is high for 200 ms. During the high phase, we will count 200 counts if we are using the 1 kHz internal clock reference. So to calculate the frequency, we have F = #pulses/duration = timebase * terminal count/counts = 1000 Hz * 4 terminal counts/200 counts = 20 Hz.
Filename: 25143.vi
Application Software: LabVIEW Full Development System 7.1
Language(s): LabVIEW
Hardware Group: FieldPoint
Driver: NI-FieldPoint 4.1
Filename: 25142.vi
Application Software: LabVIEW Full Development System 6.1
Language(s): LabVIEW
Hardware Group: FieldPoint
1) Measure the time it takes for a set number of pulses to pass or,
2) Count the number of pulses that pass in a set amount of time.
For low-frequency signals it is best to measure the time duration of a set number of pulses. If we tried to calculate the number of pulses that passed in a set amount of time, we would need a long gate period to get accurate results.
The drawback of measuring the amount of time it takes for a set number of pulses to pass is that the sample period varies with the frequency of the signal.
To accurately measure low frequencies with a CTR module, select two counter channels, one output channel, and one gate input channel. Count Input 0 counts the pulses of the signal that we are measuring. The output of the counter is set to high for x pulses and low for x pulses, which is used as the gate of Count Input 1. The source of Count Input 1 is the internal clock reference (the example can be modified to use an external clock reference). During the high phase of Count Input 0, Count Input 1 counts the pulses of the clock reference.
During the low phase, we read channel 1 and reset the counter.
For example, if the output of Count Input 0 is set to high for 4 pulses and the signal has a frequency of 20 Hz, the output of Count Input 0 is high for 200 ms. During the high phase, we will count 200 counts if we are using the 1 kHz internal clock reference. So to calculate the frequency, we have F = #pulses/duration = timebase * terminal count/counts = 1000 Hz * 4 terminal counts/200 counts = 20 Hz.
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| Measure Frequency with LabVIEW and FieldPoint Using FP-CTR-500/502 |
Requirements
Filename: 25143.vi
Software Requirements
Application Software: LabVIEW Full Development System 7.1
Language(s): LabVIEW
Hardware Requirements
Hardware Group: FieldPoint
Driver: NI-FieldPoint 4.1
Software Requirements
Application Software: LabVIEW Full Development System 6.1
Language(s): LabVIEW
Hardware Requirements
Hardware Group: FieldPoint
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