---
title: "Types of frequency meter and one working"
subject: "Instrumentation I"
chapter: 6
key: "semester_4/instrumentation_i:frequency-meter-types-working"
source: https://www.bctengineer.com/q/semester_4/instrumentation_i/frequency-meter-types-working
site: BCT Engineering
---

# Types of frequency meter and one working

Instrumentation I · Chapter 6: Digital Instrumentation

- Asked in: year not given, 8 marks
- Video: [Ferrodynamic Type Frequency Meter | Electrical Resonance Type Frequency Meter | Frequency Meter](https://www.youtube.com/watch?v=h6qUAgjZfz0) — IB TECHNICAL CLASSES
- Page: https://www.bctengineer.com/q/semester_4/instrumentation_i/frequency-meter-types-working
- Whole chapter: https://www.bctengineer.com/md/semester_4/instrumentation_i/chapter-6

## Question

List out the different types of frequency meter. Explain the constructional detail and working principle of any one of them to measure frequency.

## Short answer

Types include vibrating reed, electrical resonance, Weston, bridge, heterodyne, CRO Lissajous, and digital frequency counter. The digital counter shapes the input into pulses, gates them for a precise time T from a crystal-controlled time base, counts N pulses, and displays frequency f = N/T, offering high accuracy.

## Answer

#### 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.

```text
 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.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. A digital frequency counter measures frequency by:
   A. Comparing with a bridge balance
   B. Counting input cycles during an accurately known gate time
   C. Measuring a Lissajous pattern
   D. Measuring mechanical resonance
   Answer: B. The gate opens for time T, N pulses are counted, and f = N/T. (Key point: Counter principle)

2. What does the Schmitt trigger do in the counter?
   A. Generates the time base
   B. Divides the crystal frequency
   C. Latches the display
   D. Converts the input sine wave to a pulse train, one pulse per cycle
   Answer: D. It squares the signal so the counter can count one pulse per cycle. (Key point: Schmitt trigger)

3. What produces the gate time in the counter?
   A. A crystal oscillator followed by decade dividers
   B. The input signal itself
   C. The Schmitt trigger
   D. The display latch
   Answer: A. A crystal oscillator and divider chain give accurate gate times such as 1 s, 0.1 s or 10 ms. (Key point: Time base)

4. With gate time T = 0.1 s and count N = 5632, the frequency is:
   A. 5632 Hz
   B. 563.2 Hz
   C. 56.32 kHz
   D. 563.2 kHz
   Answer: C. f = N/T = 5632/0.1 = 56 320 Hz = 56.32 kHz. (Key point: Calculation)

## Related in Instrumentation I

- [Single-phase electrodynamometer wattmeter](https://www.bctengineer.com/question.md?key=semester_4%2Finstrumentation_i%3Aelectrodynamometer-wattmeter) — Instrumentation I, chapter 7

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