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Afrequency counter is anelectronicinstrument, orcomponent of one, that is used for measuringfrequency. Frequency counters usually measure the number of cycles of oscillation or pulses per second in a periodic electronicsignal. Such an instrument is sometimes called acymometer, particularly one of Chinese manufacture.[citation needed]
All frequency counters rely on an internalelectronic oscillator, known as thetimebase, which serves as a time reference for measurements.
Most frequency counters work by using adigital counter to count the number of rising or falling signal edges occurring in the measured signal within a specific period of time, known as thegate time. At the end of the gate time, the accumulated count is transferred to a holding register and the counter is reset to zero to prepare for the next measurement. The value stored in the holding register directly indicates the measured frequency in Hertz (Hz) when the gate time is one second; for other gate times the holding register value must be scaled to convert it to Hz. In frequency counter instruments, the measured frequency is typically output to a display.
If the frequency of the measured signal is sufficiently stable and significantly lower than that of the timebase, the measurement resolution may be improved by measuring the period of the measured signal and computing the reciprocal to obtain frequency; this is sometimes referred to as thereciprocal measurement technique. Period measurement is done by measuring the time required for an integer number of cycles, rather than counting cycles for a constant gate time, and dividing that time by the number of cycles.
If the frequency to be measured is already in electronic form, simple interfacing with 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 ofamplifier,filtering, and shaping circuitry at the input.DSP technology, sensitivity control andhysteresis 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 oftransducer. 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 forradio frequencies (RF) are also common and operate on the same principles as lower frequency counters. Often, they have more range before they overflow. For very high (microwave) frequencies, many designs use a high-speedprescaler to bring the signal frequency down to a point where normal digital circuitry can operate. The displays on such instruments consider this so they still display the correct value. Microwave frequency counters can currently measure frequencies up to almost 56GHz. Above these frequencies, the signal to be measured is combined in amixer with the signal from alocal oscillator, producing a signal at the difference frequency, which is low enough to be measured directly.
The accuracy of a frequency counter is strongly dependent on the stability of its timebase. In particular, timebase stability may be degraded by vibration, electromagnetic interference, drift due to age, and temperature variations, thus making the measured frequency seem higher or lower than the actual value. Consequently, highly stable timebase circuits are usually employed in frequency counters. Simplecrystal oscillators are commonly used when high accuracy is not required; for higher accuracy measurements, aGPS disciplined oscillator oroven-controlled oscillator is typically used.
Accuracy is often limited by the availableresolution of the measurement. The 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 asoversampling/averaging.[1][2]
Additionally, accuracy can be significantly degraded by jitter on the signal being measured. It is possible to reduce this error by oversampling/averaging techniques.
It is also possible to measure frequency using the same techniques in software in anembedded system. Acentral processing unit (CPU), for example, can be arranged to measure its own frequency of operation, provided it has some reference timebase to compare with.
I/O interfaces allow the user to send information to the frequency counter and receive information from the frequency counter. Commonly used interfaces includeRS-232,USB,GPIB andEthernet. Besides sending measurement results, a counter can notify users when user-defined measurement limits are exceeded. Common to many counters are theSCPI commands used to control them. A new development is built-in LAN-based control via Ethernet complete withGUI's. This allows one computer to control one or several instruments and eliminates the need to write SCPI commands.
