User's Manual

Choosing a Method for Measuring Frequency

The best method to measure frequency depends on several factors

including the expected frequency of the signal to measures, the desired

accuracy, how many counters are available and how long the measurement

can take.

• Method 1 uses only one counter. It is a good method for many

applications. However, the accuracy of the measurement decreases

as the frequency increases.

Consider a frequency measurement on a 50 kHz signal using an

80 MHz Timebase. This frequency corresponds to 1600 cycles of the

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, as Table 5 illustrates.

• Method 1b (measuring K periods of F1) improves the accuracy

of the measurement. A disadvantage of Method 1b is that K + 1

measurements are required. These measurements take more time

and consume some of the available USB 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 5. Frequency Measurement Method 1

Task Equation Example 1 Example 2

Actual Frequency to Measure F1 50 kHz 5MHz

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) 50.031 kHz 5.33 MHz

Error [Ft F1/(Ft – F1)] – F1 31 Hz 333 kHz

Error % [Ft/(Ft – F1)] – 1 0.06% 6.67%