Frequency counter is a measuring instrument which measures frequency range, sensitivity and resolution. The present-day frequency counters are usually of the multi-function type and they can perform the following measurements: frequency, period, ratio of frequencies, totalizing, scaling and time interval. In addition, frequency counters can also be designed to be compatible with the IEEE or IEC interface bus, to enable their operation in automated test and measurement systems. Frequency measurement is performed by totalizing the number of input cycles for a precisely known time period, so that the resulting total count is proportional to the unknown input frequency. The logic circuits internal to the counter position the decimal point automatically or under manual control, so that the display is directly the input frequency. The time reference is usually derived from a precision, high-stability quartz crystal oscillator internal to the counter. Using this basic technique and the ICs available now, it is possible to carry out frequency measurement up to 1000 MHz.