Table 7-1. frequency measurement method 1 – National Instruments DAQ M Series User Manual

Page 76

Advertising
background image

Chapter 7

Counters

NI 6236 User Manual

7-14

ni.com

80 MHz Timebase. Your measurement may return 1600 ± 1 cycles
depending on the phase of the signal with respect to the timebase. As
your frequency becomes larger, this error of ±1 cycle becomes more
significant; Table 7-1 illustrates this point.

Method 1b (measuring K periods of F1) improves the accuracy of the
measurement. A disadvantage of Method 1b is that you have to take
K + 1 measurements. These measurements take more time and
consume some of the available PCI or PXI bandwidth.

Method 2 is accurate for high frequency signals. However, the
accuracy decreases as the frequency of the signal to measure
decreases. At very low frequencies, Method 2 may be too inaccurate
for your application. Another disadvantage of Method 2 is that it
requires two counters (if you cannot provide an external signal of
known width). An advantage of Method 2 is that the measurement
completes in a known amount of time.

Method 3 measures high and low frequency signals accurately.
However, it requires two counters.

Table 7-1. Frequency Measurement Method 1

Task

Equation

Example 1

Example 2

Actual Frequency to Measure

F1

50 kHz

5 MHz

Timebase Frequency

Ft

80 MHz

80 MHz

Actual Number of Timebase
Periods

Ft/F1

1600

16

Worst Case Measured Number
of Timebase Periods

(Ft/F1) – 1

1599

15

Measured Frequency

Ft F1/(Ft – F1)

50.125 kHz

5.33 MHz

Error

[Ft F1/(Ft – F1)] – F1

125 kHz

333 kHz

Error %

[Ft/(Ft – F1)] – 1

0.06%

6.67%

Advertising