Frequency counter

A frequency counter ( true often referred to as counting frequency meter ) is an electronic measuring instrument that determines the fundamental frequency of the AC component of an electrical voltage.

Frequency measurement with gate time

A simple form of a frequency counter measures the frequency, characterized in that the number of oscillation periods or pulses during a defined period of time - the so-called gate time - to be counted.

The gate time is selected for this purpose so that the count immediately - can be displayed as a measured value - only added to point position and unit. This is always an arithmetic mean of the duration of the gate; Frequency changes during this time or the presence of a non-periodic event is not detected.

Often, these instruments have additional features such as a high-resolution time measurement. You will then be referred to as universal counter.

Because of the sheer size of possible measuring ranges and because of the small error limits, the frequency count is now preferred over analog frequency measurement. Most of the result in the unit Hz ( per second) is displayed.

A conventional frequency counter consists of the following parts:

• And trigger input stage: The signal to be counted is optionally amplified and converted into a sequence of right pulses. Counter for very high frequencies (above about 100 MHz) additionally have upstream fast frequency divider ( prescaler ).
• Gate: The square wave signal is combined in an AND circuit of the gate signal.
• Counter chain: a digital decadal event counter counts the number of pulses that come from the gate. Depending on the required resolution usually includes the count 4-8 decades.
• Display (Display): The count is displayed in numeric form. Most often, the display has a memory, so that the previous meter reading is displayed until the next measurement value is determined.
• Time base: The gate time (selectable in steps of a factor of ten, for example, 0.01 s, 0.1 s, 1.0 s, 10 s ) is generated by a precision quartz oscillator followed, switchable frequency divider.
• Flow control: 1 meter chain to zero again ( reset ): Through an electronic control system the following actions are performed sequentially Open second goal for the period specified by the time base and then close it again; during this period include pulses; Take 3 counter contents into the display memory.

The higher the count is, the more accurate the measurement, but the longer it lasts, too. The gate time should be chosen to achieve a low relative quantization distortion and high resolution so that the leading digit is greater than zero, but does not overflow.

Frequency measurement by timing

For slow processes whose period is measured in many cases instead of the frequency and calculates the frequency as the inverse. The gate time is made ​​up of a number of cycles of the unknown frequency. The number is preferably a smooth power of ten: 1, 10, 100, etc. are counted the periods of precision quartz oscillator, which fit into the gate time.

In an extension of this principle to every relevant input event is provided with a relative time stamp when the event occurred. The result is a stream of time-stamps, further parameters can be calculated from the frequency counter, the next period, for example, jitter or statistical data. The time stamps are derived from the reference frequency of the counter.

Analog input stage

The quality of a frequency counter is also determined by its analog input stage. Task of the input stage is to amplify a weak input signal to weaken a strong input signal to determine the trigger level for events to filter out disturbances and represent a constant input impedance for the signal source.

Simple counters have no analog input level, but offers a rectangular input signal to a logic level.

Accuracy

The margin of error of a frequency counter depends on the maximum permissible error of the time base error of the gate circuit ( switching time), the error of the trigger circuit and of the number of significant figures from. Typical values ​​are from 10 to 0.1 ppm. To increase accuracy, an external time base can sometimes ( highly accurate frequency of a quartz furnace, DCF77 or GPS - synchronized oscillator ) can be connected. In addition, incorrect measurements in the frequency measurement of frequency mixtures ( not spectrally pure signals, sounds, etc. ) are possible depending on the setting of the trigger circuit.

Application

Measuring the frequency of electrical signals (power frequency, low frequency, high frequency oscillators and radio channels). Together with sensors and the frequencies of mechanical vibrations, speeds, time periods or other physical processes can be measured.

Frequency counter can often operate as a pulse or event counters. The Zähltor is then permanently open and the current count is displayed.

A special form of the frequency counter is the electronic tuner for tuning musical instruments. These devices have a built-in microphone and calculate the frequency in pitch to another (eg micro tuner).

Digital Multimeters (desktop and handheld devices ) and digital oscilloscopes are often equipped with a Zählfrequenzmesser and indicate the frequency of an applied alternating voltage or periodic pulse train at.