A FREQUENCY COUNTER is an electronic instrument , or component of
one, that is used for measuring frequency .
* 1 Operating principle * 2 Accuracy and resolution * 3 I/O Interfaces * 4 See also * 5 References * 6 External links
Most frequency counters work by using a counter which accumulates the number of events occurring within a specific period of time. After a preset period known as the gate time (1 second, for example), the value in the counter is transferred to a display and the counter is reset to zero. If the event being measured repeats itself with sufficient stability and the frequency is considerably lower than that of the clock oscillator being used, the resolution of the measurement can be greatly improved by measuring the time required for an entire number of cycles, rather than counting the number of entire cycles observed for a pre-set duration (often referred to as the reciprocal technique). The internal oscillator which provides the time signals is called the timebase, and must be calibrated very accurately.
If the event 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. DSP technology, sensitivity control and hysteresis are other techniques to improve performance. Other types of periodic events that are 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.
ACCURACY AND RESOLUTION
Fluke PM6685R frequency counter
The accuracy of a frequency counter is strongly dependent on the stability of its timebase. A timebase is very delicate like the hands of a watch, and can be changed by movement, interference, or even drift due to age, meaning it might not "tick" correctly. This can make a frequency reading, when referenced to the timebase, seem higher or lower than the actual value. Highly accurate circuits are used to generate timebases for instrumentation purposes, usually using a quartz crystal oscillator within a sealed temperature-controlled chamber, known as an oven controlled crystal oscillator or crystal oven .
For higher accuracy measurements, an external frequency reference
tied to a very high stability oscillator such as a
Accuracy is often limited by the available resolution of the measurement. Resolution of a single count is generally proportional to the timebase oscillator frequency and the gate time. Improved resolution can be obtained by several techniques such as oversampling /averaging.
Additionally, accuracy can be significantly degraded by jitter on the signal being measured. It is possible to reduce this error by oversampling/averaging techniques.
I/O interfaces allow the user to send information to the frequency
counter and receive information from the frequency counter. Commonly
used interfaces include
* ^ Johansson, Staffan. "New frequency counting principle improves resolution". Spectracom. Retrie