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 |
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).
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 | $\frac{1}{\mathit{fs}}$ | $\frac{1}{\mathrm{fx}}$ | gating period | $\frac{\mathit{N}}{\mathit{fx}}$ |
Max. frequency error | $\mathit{fx}\times \frac{\mathit{fx}}{\mathit{fk}\times [\frac{\mathit{fx}}{\mathit{fs}}-1]}$ | $\mathit{fx}\times \frac{\mathit{fx}}{\mathit{fk}-\mathit{fx}}$ | fk | $\mathit{fx}\times \frac{\mathit{fx}}{\mathit{N}\times \mathit{fk}-\mathit{fx}}$ |
Max. error % | $\frac{\mathit{fx}}{\mathit{fk}\times [\frac{\mathit{fx}}{\mathit{fs}}-1]}$ | $\frac{\mathit{fx}}{\mathit{fk}-\mathit{fx}}$ | $\frac{\mathit{fk}}{\mathit{fx}}$ | $\frac{\mathit{fx}}{\mathit{N}\times \mathit{fk}-\mathit{fx}}$ |