Frequency counter
A frequency counter (also often referred to as a counting frequency meter ) is an electronic measuring device that determines the basic frequency of the alternating component of an electrical voltage.
Frequency measurement with gate time
A simple form of frequency counter measures a frequency by counting the number of oscillation periods or pulses during a defined period - the so-called gate time .
The gate time is selected in such a way that the counter reading can be displayed immediately as a measured value - only supplemented by the decimal point and unit . This is always an arithmetic mean over the duration of the gate time; Frequency changes during this time or the presence of a non-periodic event are not recognized.
These measuring devices often have additional functions such as high-resolution time measurement. They are then called universal counters.
Because of the very large scope of possible measuring ranges and because of the low error limits , frequency counting is preferred to analog frequency measurement today . Usually the result is displayed in the unit Hz (per second).
A conventional frequency counter consists of the following parts:
- Input stage and trigger : The signal to be counted is amplified if necessary and converted into a sequence of square-wave pulses . Counters for very high frequencies (over approx. 100 MHz) also have high-speed frequency dividers (prescaler) connected upstream .
- Gate circuit: The square-wave signal is linked with the gate signal in an AND circuit .
- Counter chain: A digital decade event counter counts the number of impulses that come from the gate circuit. Depending on the resolution requirements , the counter usually covers 4 to 8 decades.
- Display: The counter reading is shown in digit form. The display usually has a memory so that the previous counter reading is displayed until the next measured value has been determined.
- Time base : The gate time (switchable in steps of a power of ten, e.g. 0.01 s, 0.1 s, 1.0 s, 10 s) is generated by a precise crystal oscillator with a subsequent switchable frequency divider.
- Sequence control: The control electronics carry out the following actions one after the other:
- Set counter chain to zero (reset);
- Open the gate for the duration determined by the time base and then close it again; count pulses during this time;
- Transfer the counter content to the display memory.
The higher the count, the more accurate the measurement, but the longer it takes. In order to achieve a low relative quantization deviation or high resolution, the gate time should be selected so that the leading digit is greater than zero but does not overflow.
Frequency measurement by time measurement
In the case of slow processes, the period is often measured instead of the frequency and the frequency is calculated as a reciprocal value from this . The gate time is formed from a number of periods of the unknown frequency. The number is preferably an even power of ten: 1, 10, 100, etc. The periods of the precise crystal oscillator that fit into the gate time are counted.
- Example: The measurement of a frequency of 50 Hz with a five-digit display takes 1000 s. A clock with 1 MHz derived from the crystal oscillator measures a single 50 Hz period in five digits with a counter reading of 20,000 in 20 ms.
In an extension of this principle, each relevant input event is provided with a relative time stamp indicating when the event occurred. The result is a stream of time stamps, from which further measured variables can be calculated in the frequency counter, in addition to the period duration, for example jitter or statistical data. The time stamps are derived from the frequency reference of the counter.
Analog input stage
The quality of a frequency counter is also determined by its analog input stage. The 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 interference and to provide a constant input impedance for the signal source.
Simple counters do not have an analog input stage, but expect a square-wave input signal at a logic level .
accuracy
The error limit of a frequency counter depends on the error limit of the time base, the error in the gate circuit (switching time), the error in the trigger circuit and the number of significant digits . Values from 10 to 0.1 ppm are typical . To increase the accuracy, an external time base (high-precision frequency from a quartz furnace , DCF77 or GPS-synchronized oscillator ) can sometimes be connected. In addition, incorrect measurements are possible when measuring the frequency of frequency mixtures (non-spectrally pure signals, sounds, etc.) depending on the setting of the trigger circuit.
application
Measurement of the frequency of electrical signals (mains frequency, low frequency, high frequency of oscillators and radio transmitters). Together with sensors, the frequencies of mechanical vibrations, speeds, periods of time or other physical processes can also be measured.
Frequency counters can often also be operated as pulse or event counters. The counting gate is then always open and the current count is displayed.
A special form of the frequency counter is the electronic tuning device for tuning musical instruments . These devices have a built-in microphone and convert the frequency into pitches (e.g. microtuner ).
Digital multimeters (tabletop and handheld devices) and digital oscilloscopes are often equipped with a counting frequency meter and indicate the frequency of an applied alternating voltage or periodic pulse sequence.
See also
Web links
- Agilent AN200: "Fundamentals of electronic frequency counters" [1] (PDF; 226 kB)