A frequency counter is an electronic instrument that is used for measuring frequency. They usually measure the number of oscillations or pulses per second in a periodic electronic signal. The frequency counter is simple to use, you just turn the counter on and apply the signal to the input. They can be used for measuring many signals from digital logic signals to RF and microwaves.

General guidelines on using a Frequency Counter

Do not overload input: This may occur when using some RF frequency counters that may have a 50Ω input.
Keep frequency counter time-base / clock warm: The time-base of clock within a frequency counter governs the overall accuracy of the instrument.
Requires accurate time-base: As the accuracy of the time-base directly reflects onto the accuracy of the measurement, it is necessary to use an instrument with an accurate time-base, or use an external one that is very accurate.
Calibrate: With various aspects such as crystal ageing that affects even the most accurate crystal- based clock oscillators or time-bases, it is necessary to ensure that the time-base frequency is maintained as accurately as possible and all other functions are operating within their specified ranges.
For noisy signals, pay attention to trigger error: When using a frequency counter or timer with a noisy signal, the noise can cause spurious counts which affects the reading.
Lock all time-bases to a single clock: When a system uses several time-bases, there are possibilities of uncertainty between the signals that can cause some inaccuracies. To overcome this, it is often wise to lock all the elements of a system to a single accurate time-base.

Applications include:

Measuring the frequency of a transmitter carrier
Measuring the frequency of an oscillator in a circuit
Measure the frequency of a signal on a line
Any application where the frequency of a steady repetitive signal needs to be measured

To be able to measure the frequency of an RF signal using a frequency counter there are a few prerequisites.

Frequency must be steady, i.e. not varying
The signal should not have modulation applied as this prevents proper counting
Signal must have sufficient amplitude – typically signals over about half a volt are suitable
Signals should not be so large that they overload the input

Frequency counters are an easy to use test instrument, have few problems and give accurate results.