Page 313 - Electrician - TT (Volume 1)

ELECTRICIAN - CITS

Digital Frequency Meter

Objectives: At the end of this lesson you shall be able to:
• state the function of digital frequency meter
• describe the block diagram of digital frequency meters.

A frequency counter is a digital instrument that can measure and display the frequency of any periodic waveform.
It operates on the principle of gating the unknown input signal into the counter for a predetermined time.
If the unknown input signal were gated into the counter for exactly 1 second, the number of counts allowed into
the counter would be the frequency of the input signal. The term gated comes from the fact that an AND or an OR
gate is employed for allowing the unknown input signal into the counter to be accumulated. Fig 1
Discription of block diagram:

The simplified form of block diagram of frequency counter is in Fig 1. It consists of a counter with its associated
display/decoder circuitry, clock oscillator, a divider and an AND gate. The counter is usually made up of cascaded
Binary Coded Decimal(BCD) counters and the display/decoder unit converts the BCD outputs into a decimal
display for easy monitoring.
A GATE ENABLE signal of known time period is generated with a clock oscillator and a divider circuit and is
applied to one leg of an AND gate.
The unknown signal is applied to the other leg of the AND gate and acts as the clock for the counter. The counter
advances one count for each transition of the unknown signal, and at the end of the known time interval, the
contents of the counter will be equal to the number of periods of the unknown input signal that have occurred
during time interval, t.In other words, the counter contents will be proportional to the frequency of the unknown
input signal.
For instance if the gate signal is of a time of exactly 1 second and the unknown input signal is a 600-Hz square
wave, at the end of 1 second the counter will counts up to 600, which is exactly the frequency of the unknown
input signal
The wave form in Fig 2 shows that a clear pulse is applied to the counter at t0 to set the counter at zero. Prior to
t1, the GATE ENABLE signal is LOW, and so the output of the AND gate will be LOW and the counter will not be
counting. The GATE ENABLE goes HIGH from t1 t0 t2 and during this time interval t=(t2 - t1 ) the unknown input
signal pulses will pass through the AND gate and will be counted by the counter
After t , the AND gate output will be again LOW and the counter will stop counting. Thus, the counter will have
2
counted the number of pulses that occurred during the time interval, t of the GATE ENABLE SIGNAL, and the
resulting contents of the counter are a direct measure of the frequency of the input signal.

Fig 1 & 2

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CITS : Power - Electrician & Wireman - Lesson 50-53