A frequency counter is an electronicinstrument, or component of one, that is used for measuring frequency. Since frequency is defined as the number of events of a particular sort occurring in a set period of time, it is generally a straightforward thing to measure it.
Most frequency counters work simply by using a counter which accumulates the number of events. After a preset period (1 second, say), the value in the counter is transferred to a display, and the counter is reset to zero, to begin accumulating the next sample period. The sampling period is called the timebase, and must be very accurately calibrated.
If the thing to be counted is already in electronic form, simple interfacing to the instrument is all that is required. More complex signals may need some conditioning to make them suitable for counting - most general purpose frequency counters will include some form of amplifier, filtering and shaping circuitry at the input. Other types of periodic event that is not inherently electronic in nature will need to be converted using some form of transducer. For example, a mechanical event could be arranged to interrupt a light beam, and the counter made to count the resulting pulses.
Frequency counters designed for radio frequencies (RF) are also common, and operate on the same principles as lower frequency counters, but often have more range before they overflow. For very high frquencies, many designs use a high-speed prescaler to bring the signal frequency down to a point where normal digital circuitry can operate. The displays on such instruments take this into account so they still read true.
The accuracy of a frequency counter is strongly dependent on the stability of its timebase. Highly accurate circuits are used to generate this for instrumentation purposes, usually using a quartzcrystal oscillator within a sealed temperature-controlled chamber, known as a crystal oven. Where the frequency does not need to be known to such a high degree of accuracy, simpler oscillators can be used. It's also possible to measure frequency using the same techniques in software in an embedded system - a CPU for example, can be arranged to measure its own frequency of operation provided it has some reference timebase to compare with.
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