# Choosing a Method for Measuring Frequency

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The best method to measure frequency depends on several factors including the expected frequency of the signal to measure, the desired accuracy, how many counters are available, and how long the measurement can take. For all frequency measurement methods, assume the following:
 fx is the frequency to be measured if no error fk is the known source or gate frequency Measurement Time (T) is the time it takes to measure a single sample Divide down (N) is the integer to divide down measured frequency, only used in large range two counters fs is the sample clock rate, only used in sample clocked frequency measurements
Here is how these variables apply to each method, summarized in the table below.
• One counter—With one counter measurements, a known timebase is used for the source frequency (fk). The measurement time is the period of the frequency to be measured, or 1/fx.

• Two counter high frequency—With the two counter high frequency method, the second counter provides a known measurement time. The gate frequency equals 1/measurement time.

• Two counter large range—The two counter larger range measurement is the same as a one counter measurement, but now the user has an integer divide down of the signal. An internal timebase is still used for the source frequency (fk), but the divide down means that the measurement time is the period of the divided down signal, or N/fx where N is the divide down.

• Sample clocked—For sample clocked frequency measurements, a known timebase is counted for the source frequency (fk). The measurement time is the period of the sample clock (fs).

Table 1. Frequency Measurement Methods
Variable Sample Clocked One Counter Two Counters
High Frequency Large Range
fk Known timebase Known timebase $\frac{1}{\mathrm{gating period}}$ Known timebase
Measurement time $\frac{1}{\mathit{fs}}$ $\frac{1}{fx}$ gating period $\frac{\mathit{N}}{\mathit{fx}}$
Max. frequency error $\mathit{fx}×\frac{\mathit{fx}}{\mathit{fk}×\left[\frac{\mathit{fx}}{\mathit{fs}}-1\right]}$ $\mathit{fx}×\frac{\mathit{fx}}{\mathit{fk}-\mathit{fx}}$ fk $\mathit{fx}×\frac{\mathit{fx}}{\mathit{N}×\mathit{fk}-\mathit{fx}}$
Max. error % $\frac{\mathit{fx}}{\mathit{fk}×\left[\frac{\mathit{fx}}{\mathit{fs}}-1\right]}$ $\frac{\mathit{fx}}{\mathit{fk}-\mathit{fx}}$ $\frac{\mathit{fk}}{\mathit{fx}}$ $\frac{\mathit{fx}}{\mathit{N}×\mathit{fk}-\mathit{fx}}$
Note

Accuracy equations do not take clock stability into account. Refer to the specifications document for your chassis for information about clock stability.