RF frequency counters and timers are items of test equipment that operate by counting events within a set period or discovering what a period is by counting a number of precisely timed events.
The time periods within which events are counted, or the precisely timed events can be generated using a highly stable quartz crystal oscillator. This may even be oven controlled, and in this way a very accurate reference is obtained.
To look at the way in which a frequency counter or timer works, it is necessary to described the two approaches separately.
The two approaches may be termed direct counting and reciprocal counting.