3-34
Figure 3-26.—Frequency counter.
Frequency Counters
In the early 1950s, the frequency counter was developed. The device could measure and accurately
indicate frequencies up to 10 MHz. Present-day frequency counters can accurately read frequencies as
high as 40 GHz. In addition to direct frequency measurement indication, some types of frequency
counters can measure the WAVE PERIOD, which is the inverse of frequency; RATIO, which compares
one frequency against another; and TIME INTERVAL, the time between two events or the time between
two functions of an event. In addition, frequency counters can totalize event indications. This is similar to
measuring the frequency except that a manual or an electronic start-stop gate controls the time over which
the measurement is taken. Frequency counters can also provide scaling in the form of a digital output
signal from the frequency counter that represents a frequency-related division of the input frequency.
All of the above functions have useful applications. For pulse timing, the period function is used;
totalizing is used in digital applications; and ratio is used in comparing harmonic-related signals. Scaling
is used for triggering other test equipment used in conjunction with the frequency counter; and time-
interval capability is used in measuring the interval between two pulses or between two sets of pulses.
Because of the wide variety of frequency counters in use, the technical manual for a specific frequency
counter should be consulted to determine the instrument's full capabilities.
Frequency Counter Accuracy
All frequency counter measurements are measured with 1 part in 108 of accuracy. However,
frequency counters have provisions for input from external frequency standards. This extends the
accuracy of the frequency to that of the standard. A frequency self-check capability is provided to
determine if the counting and lighting circuits are operating properly.
