This section provides overview, applications, and principles of frequency counters. Also, please take a look at the list of 11 frequency counter manufacturers and their company rankings.
A frequency counter is a digital device used to measure the frequency occurring in an electronic circuit. Electrical signals can be expressed in terms of three parameters: frequency, amplitude, and phase.
Frequency measurement is an essential parameter in the basic measurement of electrical signals because the reciprocal of the frequency can be used to determine the period. A frequency counter with several additional functions, such as duty cycle measurement, pulse rise time, and time interval, is called a universal counter.
Frequency counters are used as basic measuring instruments in electrical signal measurement, like ammeters and voltmeters. Although some frequency counters are available as stand-alone products, many other products have been developed that function as frequency counters as part of the functions of devices with many functions, such as digital multimeters, oscilloscopes, and optical spectrum analyzers.
Frequency counters are characterized by the fact that their principle is very simple. For this reason, it is possible to make your own frequency counter, and kits are available.
Kits for frequency counters are available from various companies for several tens of MHz. The reason is that direct method frequency counters are characterized by their uncomplicated operation.
In addition, some multi-testers are equipped with ranges that allow frequency measurement. These types are useful because they are easy to handle.
However, while they have the advantage of being easy to use, they are not suitable for applications that require high frequencies or a large number of significant digits. It is necessary to understand this disadvantage when using them.
Most kits are LSI-based. This does not allow you to learn all of the structure, but it does give you a feel for it.
A frequency counter is composed of a waveform shaping circuit, a gate, a crystal oscillator, and a counting circuit. First, the waveform shaping circuit converts the input signal into a pulse train.
The crystal oscillator generates pulses with a fixed time width, which is used as a time window for measuring the pulse train just described. The time window is necessary to perform frequency measurement.
The counting circuit is responsible for measuring the frequency. The time window generated by the waveform shaping circuit is used.
The counting circuit measures the frequency of the original signal by counting the number of pulses in the time window. The resolution of the frequency measurement is determined by the width of the time window generated by the crystal oscillator.
For example, if the time window is 1 second, the frequency can be displayed in 1 Hz units. If the time window is 0.1 second, the resolution is 10 Hz, and so on. The resolution is proportional to the inverse of the time width of the time window.
In a frequency counter, the most error occurs where the pulse train is generated. In particular, if the input signal contains noise, the rising edge of the pulses becomes unstable or extra pulses are generated that should not be there.
The device used to prevent the generation of errors is to average the noise components by repeated measurement. This reduces the errors that occur when generating a pulse train.
A frequency counter is a device that measures the frequency of an input signal and displays the result. There are two main methods of measurement. One is the "direct method," which has been used for a long time because it is easy to implement, and the other is the "reciprocal method," which is more expensive but can obtain a high number of significant digits.
The direct method frequency countermeasures the number of times the frequency of the input signal crosses at the zero point. If the input signal is a sine curve, it counts the number of times it crosses the zero point, either down or up per second.
The advantage of the direct method frequency counter is that it can be easily realized using only hardware. For this reason, this method has been used for a long time. The number of zero crossings per second is displayed as the measured frequency.
In the direct method frequency counter, an accurate reference clock is made inside the device, and the gate is opened for that time to measure the number of times.
The gate time and input frequency determine the number of significant digits in a direct method frequency counter. For example, if the input frequency is 1 GHz and the gate time is 1 second, the measured value is 1x10^9, and the number of significant digits is 10.
If the input frequency is 1 kHz, the number of significant digits is 4. In both cases, the resolution is 1Hz. Here, the longer the gate time, the higher the resolution. For example, if the gate time is set to 100 seconds, the number of significant digits is 6 at 1 kHz, and the resolution is 0.01 Hz.
However, taking at least 100 seconds for a single measurement is not realistic and significantly reduces workability. In addition, it is necessary to understand and use with the understanding that there is always an error of ±1 in the measured value.
If you are only measuring high-frequency signals, a direct method frequency counter can be used without problems. However, when measuring low frequencies, it is necessary to increase the gate time in order to improve accuracy with the direct method.
However, the disadvantage of lengthening the gate time in the direct method is that it also increases the measurement time, resulting in extremely low efficiency. Under these circumstances, the "reciprocal" frequency counter is an option.
Reciprocal frequency counters count the input waveform as it is or divides the waveform by an internal reference clock. The advantage of this method is that a high number of significant digits can be obtained, especially in low-frequency measurements.
The internal reference clock and gate time determine the number of significant digits in a reciprocal frequency counter. And it is not affected by the input frequency.
For example, if the internal reference clock is 10 MHz and the gate time is 1 second, the number of significant digits is 7. If the same reference clock is used and the gate time is 10 seconds, the number of significant digits is 8.
Although the reciprocal method can obtain a high number of significant digits in low-frequency range measurements, it has the disadvantage of being expensive because the operation of the counter itself is complex.
*Including some distributors, etc.
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