A frequency meter is an instrument that displays the frequency of a periodic electrical signal. Digital frequency meter is a general purpose instrument that displays the frequency of a periodic electrical signal to an accuracy of three decimal places. It counts the number events occurring within the oscillations during a given interval of time. As the preset period gets completed, the value in the counter display on the screen and the counter reset to zero. Various types of instruments are available which operates at a fixed or variable frequency. But if any frequency meter operates at a different frequency than specified range, it could carry out abnormally. For measuring low frequencies, the deflection type meters are widely used. The deflection of the pointer on the scale shows the change in frequency. The deflection type instruments are of two types: one is electrically resonant circuits and other is ratio meter.
A frequency meter has a small device which converts the sinusoidal voltage of the frequency into a train of unidirectional pulses. The frequency of input signal is the displayed count, averaged over a suitable counting interval out of 0.1, 1.0, or 10 seconds. These three intervals repeat themselves sequentially. As the ring counting units resets, these pulses pass through the time-base-gate and then entered into the main gate, which opens for a certain period of time interval.
The time base gate prevents a divider pulse from opening the main gate during the display time interval. The main gate acts as a switch when the gate is open, pulses are allowed to pass. When the gate is closed, pulses are not allowed to pass that means the flow of pulses get obstructed. The functioning of the gate is operated by the main gate flip-flop. An electronic counter at the gate output that counts the number of pulses passed through the gate while it was open. As the next divider pulse is received at main gate flip-flop, the counting interval ends and divider pulses are locked out. The resultant value displayed on a display screen which has the ring counting units of scale-of-ten circuits and each unit is coupled to a numeric indicator, which provides the digital display. As the reset pulse generator is triggered, ring counters get reset automatically and the same procedure starts again.The range of modern digital frequency meter is between the range from104 to 109 hertz. The possibility of relative measurement error ranges between from 10-9 to 10-11 hertz and a sensitivity of 10-2 volt.
They are ordinarily used for measuring low frequencies but capable of being used for frequencies as high as 900Hz. These operate by balancing two opposing forces. Changes in the frequency to be measured cause a change in this balance that can be measured by the deflection of a pointer on a scale. Deflection-type meters are of two types, electrically resonant circuits ratiometers.
2.Resonant Reed type:
They are ordinarily used in ranges from 10 to 1,000Hz, although special designs can operate at lower or higher frequencies.These work by means of tuned steel reeds that vibrate like a tuning fork , under the effect of electric current through a coil; only those reeds that are in resonance vibrate visibly.