Types of frequency meter and one working

Types of frequency meter

1. Mechanical resonance type (vibrating reed frequency meter).
2. Electrical resonance type (LC resonance circuit).
3. Weston (ferrodynamic / electrodynamic) type frequency meter.
4. Bridge type (Wien bridge) frequency meter.
5. Heterodyne (beat) frequency meter.
6. CRO method (Lissajous pattern).
7. Digital frequency counter.

Digital frequency counter (explained)

It counts the number of cycles of the input signal during an accurately known time interval (gate time) and displays the number as frequency.

 Input -> [Signal    ] -> [Schmitt ] -> [ AND  ] -> [Counter] -> [Latch/
          conditioner]    trigger        gate        (decade)     Display]
                                          ^
 [Crystal    ] -> [Divider ] -> [Gate control  ] (gate opens for T)
  oscillator       chain /time   flip-flop
                   base

Construction:

• Signal conditioner (attenuator/amplifier) to bring the input to a suitable level.
• Schmitt trigger to change the sine wave into a pulse train of the same frequency (one pulse per cycle).
• Crystal oscillator (e.g. 1 MHz or 10 MHz) and decade dividers which give the time base (gate time 1 s, 0.1 s, 10 ms).
• Gate (AND), decade counting units (DCU) and display with latch and reset circuit.

Working:

1. The time-base divider output opens the gate for a precise time T (say 1 s).
2. During this time, input pulses pass through the gate to the counter.
3. When the gate closes, the counter holds N pulses; the display shows

f = N / T

4. Then the reset clears the counter and the cycle repeats.

Example: N = 5632 pulses with T = 1 s gives f = 5632 Hz; with T = 0.1 s the same count means 56.32 kHz.

Features: high accuracy (set by the crystal stability), wide range (Hz to GHz with prescalers), direct digital readout. Error: ±1 count, which is small at high frequency; for low frequencies, the period is measured instead.