Frequency counter
A frequency counter is a test instrument which measures the frequency of an electrical signal. The frequency is typically determined by counting the number of times the oscillating signal crosses some voltage, usually a zero volt reference, during some precise time interval. Frequency counters typically measure frequency by counting the number of cycle pulses crossing a predetermined voltage threshold, usually zero volts, over a precise time interval. Frequency counters typically determine the frequency of a pulsed signal in two ways. The counter counts the number of reference clock events of known frequency that occur between successive input signal events. The signal frequency is then computed by dividing the known frequency of the reference signal by the number of counted reference events. In another method, the counter counts input signal events occurring during a known reference period. The frequency of the input signal is then computed by dividing the number of counted input events by the known reference period. A conventional frequency counter consists of a signal gate followed by a counter. The signal gate is caused to open for a known period of time, which is usually a predetermined number of cycles of a stable reference frequency. The counter counts the number of input cycles that occur during this gate time, this count being representative o the ratio of the input signal frequency to the reference frequency. The input signal is normally conditioned by known input circuitry, such as a Schmitt trigger circuit, which receives an input signal and outputs one pulse per cycle of input. Frequency counters generally employ a crystal oscillator for generating a reference frequency against which the frequency of the input signal is compared. Consequently, the accuracy of the frequency counter is dependent on the accuracy of the reference frequency and thus on the accuracy of the reference or master crystal oscillator. The harmonic heterodyne converter utilizes a programmable local oscillator which provides a local oscillator frequency that is mixed with the input signal in a sampler. The local oscillator frequency is varied until an output of the sampler falls within the intermediate frequency range. The frequency of the input signal is computed from the local oscillator frequency, the measured intermediate frequency and a harmonic number.
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