sources:
https://en.wikipedia.org/wiki/Frequency_counter
https://www.ehow.com/list_7500738_frequency-counter-specifications.html
https://www.ehow.com/how-does_5164850_frequency-counter-works.html
https://www.ehow.com/how_5022745_use-frequency-counter.html
https://wiki.answers.com/Q/What_is_a_timebase
Frequency is the number of repetitions of a periodic process per 1 second and is measured in Hertz (Hz). Light or sound are examples of such periodic processes and are called electromagnetic waves. Frequency is a way to distinguish among different types of waves. For instance, the frequency of sound is between 20 to 20,000 Hz, while the frequency of light is about one trillion times higher. It can be calculated either from the energy or the wavelength using the fundamental physical constants.
A frequency counter is an electronic instrument, or component of one, that is used for measuring frequency. Frequency is defined as the number of events of a particular sort occurring in a set period of time. Frequency counters usually measure the number of oscillations or pulses per second in a repetitive electronic signal.
A frequency counter may be the simplest piece of electronic test equipment to operate, but perhaps the hardest to understand. You apply a signal to the input, and the frequency appears on a digital display. Although the manual may have several pages of specifications, you can understand them better if you break them down into those that will suit your application.
Frequency Range
The range tells you what frequencies it will measure. Audio frequencies run from 20 HZ to 20,000 HZ. Microwave frequencies run up to 30 or more GHz. Some counters have models or plug-in units for various frequency bands. Expect the price to increase with higher frequencies and wider ranges.
Power Range
The power range tells how strong a signal the counter will read. Also, some signals ride on DC voltage that the counter must withstand. If you want to read the frequency at the output to a transmitter amplifier, you need a counter that can withstand high power. But that's not necessary at the input to the amplifier.
Accuracy
Accuracy tells how closely the counter measures the actual signal. Specifications can be illustrated in percentages or parts per million. Although price usually increases with accuracy, technological advances tend to keep lowering the price of units offering adequate accuracy.
Resolution
The resolution specification tells how pure the signal must be for the counter to read it. Complex formulas describe resolution specifications and circuits, but your application really determines what resolution you need. Usually it isn't too great of a concern for most frequency-measurement applications.
Environmental
Environmental specifications tell what climates in which you can operate and store the counter, and include temperature, humidity and altitude. Some counters also include specifications for dust, pollutants, physical shock or vibrations.
Operating principle
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 (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.
The accuracy of a frequency counter is strongly dependent on the stability of its timebase. A time base is usually a fixed frequency or a fixed period in time used as a comparison against which other frequencies or timed events are calculated. A time base can be established to meet your requirements for purposes of measurement.
Input
You connect the signal you're measuring to the frequency counter's input. Since you may be sending it any kind of waveform, the frequency counter conditions the signal and converts it to rectangular pulses, which are easily counted. The input will have a range of sensitivities depending on the model, typically from a few millivolts to a few volts peak-to-peak.
Gate
Before the signal is counted, it passes through a gate circuit. It's a precisely timed digital switch that is either open or closed. It has two main modes: frequency count and event count. Frequency count lets you measure the frequency of the input signal. In this case, the gate will open for a time period, passing the signal to the counter and letting the counts accumulate. For example, the gate may turn on for 1 second, so the counter counts pulses from your signal for that long. After 1 second, the gate closes and lets the counter display its result. Then it resets the count to 0 and opens again.
Event count keeps the gate open, letting you count all the signal events you send to the counter. You can reset the count with a manual push-button switch or external pulse. Event counting lets you determine the total number of events occurring in an arbitrary time period, from seconds to weeks or longer.
Counter
The counter circuit has a set of divide-by-10 stages. The total number of stages is the number of display digits minus 1. The stages are chained together, so the first stage divides by 10, the next by 100 and so on. The counter outputs are used to drive the display.
Latch
The latch is a simple memory circuit that holds the last count. Typically, the counter will be counting for the current gate period, but the latch will have the results from the last count. When the current gate period is over, it resets the latch and gives it the latest count. This improves readability. Without the latch, you'd see a fluttering of numbers as the counter counts from 0 to your frequency every second. With the latch, you see only the actual frequency, with updates every second.
Display
The frequency counter's display is a set of decimal digits driven by the counter and latch. A typical frequency counter has six or seven digits. They may be seven-segment LED, LCD or other technology. In addition to showing the frequency count, the display may have indicators for mode, gate time or battery condition.
Quartz Standard
The frequency standard determines the accuracy of the gate. A typical standard is a quartz crystal in a heated chamber. Keeping it at a known temperature improves the counter's accuracy. If the crystal runs at a frequency of 100,000 hertz, a separate set of dividers reduces it to 1 pulse per second. A separate counter may divide it further to 1 pulse every 10 seconds. The longer the gate is open, the more precise the frequency measurement can be, with the trade-off that it takes longer.
Usually any frequency counter has some sort of input signal conditioning that turns your input signal into a square-wave. So it just counts the cycles.
If you fed white noise into something like that, you would just see garbage random numbers. There is really no point in trying to measure the frequency of a complex rapidly fluctuating signal, because it probably does not have one.
Of the things you'll find on a technician's workbench, a frequency counter is one of the easiest to use. Their main purpose, measuring frequency, is done by setting a few front panel switches. Spending a few minutes with a frequency counter and a test oscillator will show you what you need to know.
Things You'll Need
Frequency Counter
Oscillator or Function Generator
Cable with BNC female connectors at both ends
Instructions
1: Connect oscillator to frequency counter using the BNC cable.
2: Turn oscillator and frequency counter power on.
3: Select a pure, unmodulated waveform from the oscillator: sine, triangle, or pulse.
4: Set the oscillator's amplitude (output level) about halfway. Set its frequency to about 1000 Hz.
5: Set the frequency counter to the lowest frequency range. Set its gate to one per second. If it has a frequency/period mode, set it to frequency.
6: If the frequency counter has a "Hold" button, press it. The display should hold the same count. Press "Hold" again to resume normal operation.
7: Change the counter's mode to "Period," if it has that mode. It should now display a time interval of about .001 seconds.
8: Change the oscillator frequency. The counter should show the new frequency momentarily.
9: Change the "Gate" setting. The display should update less often but with higher resolution.