One of the many frequency meters that directly indicates frequency is reed –type meter of the following figure.
Fig.1: (a) Reed-Type Meter
Fig.1: (b) Indicator Dial
In this type of meter, many reeds are mounted on a common support, with their free ends visible on the meter face. Each reed has its own natural frequency of vibration. When an internal electromagnet is excited by the current of unknown frequency, an alternating magnetic field is produced. If the frequency of the field corresponds to the vibration frequency of the reed, that particular reeds vibrate with considerable amplitude. If two adjacent reeds vibrate with the same amplitude, the unknown frequency is halfway between those indicated by the two vibrating reeds. The reed –type meter is useful only at low frequencies and only over a limited range of frequencies.
Another type of frequency meter is the digital frequency meter, which measures frequency up to about 100 MHZ and displays a digital readout of the measured frequency as shown in figure 2.
The digital frequency meter is commonly called a counter since it determines the frequency by electronically counting the number of cycles of an unknown signal in a standard time interval, usually 1 second. The following figure describes the basic working principle of digital frequency meter.
Fig.2: Working Principle of Digital Frequency Meter
In above figure, pulse conversion block converts the sinusoidal signal into a discrete set of pulses. The train of discrete pulses then passes through the gate which remains open for a certain time. The electronic counter counts these pulses from gate output. The resultant value is displayed on main meter screen. In addition to its basic function of measuring frequency, the counter can count uniform or random pulses or events and displays the total.