--- title: "Frequency counter operation with block diagram" subject: "Digital Logic" chapter: 11 key: "semester_3/digital_logic:frequency-counter" source: https://www.bctengineer.com/q/semester_3/digital_logic/frequency-counter site: BCT Engineering --- # Frequency counter operation with block diagram Digital Logic · Chapter 11: Applications - Asked in: 2082 Baisakh, 4 marks; 2081 Baisakh, 4 marks; 2081 Baisakh, 5 marks; 2080 Bhadra, 5 marks; 2080 Baisakh, 6 marks; 2079 Bhadra, 4 marks; 2078 Bhadra, 5 marks; 2076 Chaitra, 4 marks; 2076 Asoj, 5 marks; 2075 Chaitra, 4 marks; 2074 Chaitra, 4 marks; 2073 Shrawan, 8 marks; 2070 Chaitra, 5 marks; 2069 Chaitra, 4 marks; 2068 Chaitra, 4 marks (asked 14 times) - Video: [Lecture 5: DIGITAL FREQUENCY METER](https://www.youtube.com/watch?v=tMHBv9PtaoI) — EG ELECTRONICS - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_3/digital_logic/frequency-counter - Whole chapter: https://www.bctengineer.com/md/semester_3/digital_logic/chapter-11 ## Question With the help of block diagram explain the operation of frequency counter. ## Short answer A frequency counter counts input cycles during a precise gate time from a crystal oscillator divider chain. The signal is shaped, gated by an AND gate, counted by BCD counters, latched, and displayed. Frequency f = N / T_gate. Example: 5000 counts in 1s gives 5 kHz. ## Answer A **frequency counter** measures the frequency of a signal by counting the number of its cycles during an accurately known time interval (the gate time) and displaying the count: $$ f = \frac{N}{T_{gate}} $$ #### Block diagram ```text Input +-----------+ pulses +-----+ +-----------+ signal->| Amplifier |--------->| |-->| Decade | | & Schmitt | | AND | | counters | | trigger | +---->|gate | | (BCD) | +-----------+ | +-----+ +-----+-----+ | | +---------+ +---------+ | gate +----------v----+ | Crystal |->| Divider |-+ pulse | Latch | | osc. | | chain |---+ +-------+-------+ +---------+ +---------+ | | +----v-----+ +-------v-------+ | Control | | BCD to 7-seg | | (reset, | | decoders and | | latch) | | displays | +----------+ +---------------+ ``` #### Working of each block 1. **Amplifier and Schmitt trigger (wave shaper):** amplifies the unknown input (sine, triangle, etc.) and converts it into clean rectangular pulses of the same frequency, one pulse per cycle. 2. **Time base:** a crystal oscillator (e.g. 1 MHz) followed by a chain of divide-by-10 counters gives a very accurate **gate pulse** of 1 s, 0.1 s, 10 ms, … selected by the range switch. 3. **AND (main) gate:** one input receives the shaped pulses, the other the gate pulse. Pulses pass to the counter only while the gate pulse is HIGH, i.e. for exactly $T_{gate}$. 4. **Decade counters:** cascaded BCD counters (e.g. 7490) count the pulses that pass the gate; each counter is one decimal digit. 5. **Control logic:** before each gate period it **resets** the counters; when the gate closes it sends a **latch (transfer) pulse**, then resets again for the next measurement. 6. **Latch, decoder and display:** the latch holds the final count so the display does not flicker while counting; BCD-to-7-segment decoders (e.g. 7447) drive the LED displays. #### Example With $T_{gate} = 1$ s, if 5000 pulses pass the gate, $f = 5000 / 1 = 5000$ Hz = 5 kHz. With a 0.1 s gate the same signal gives 500 counts, displayed as 5.00 kHz with the decimal point shifted. A longer gate gives better resolution (1 Hz for a 1 s gate); the accuracy depends on the crystal time base, with an error of ±1 count. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What is the basic principle of a frequency counter? A. Counting input cycles during an accurately known gate time, f = N / T_gate B. Measuring the amplitude of the input C. Counting clock pulses between two events D. Converting frequency to voltage Answer: A. The counter counts N input cycles in a known gate time and divides N by that time. (Key point: Principle) 2. What is the role of the amplifier and Schmitt trigger? A. To generate the gate pulse B. To convert the input signal into clean rectangular pulses, one per cycle C. To count the pulses D. To latch the result Answer: B. The shaper turns sine or triangle waves into clean pulses so the counter can count cycles reliably. (Key point: Schmitt trigger role) 3. How is the exact gate time obtained and used? A. Crystal oscillator and divider chain give an accurate gate pulse, which enables the AND gate for exactly T_gate B. The input signal itself sets the gate C. A monostable sets it from the display D. The decoder sets it Answer: A. Pulses pass the AND gate only while the accurate gate pulse is HIGH. (Key point: Gate and AND gate) 4. Why is a latch used, and when is the counter reset? A. The latch holds the final count so the display does not flicker; the counters are reset before the next gate period B. The latch generates the pulses; reset follows display C. The latch divides the clock; reset happens during counting D. The latch shapes the input; no reset is needed Answer: A. After the gate closes, a latch pulse stores the count, then the counters are reset for the next measurement. (Key point: Latch and reset) ## Related in Digital Logic - [BCD-to-7-segment decoder; frequency counter](https://www.bctengineer.com/question.md?key=semester_3%2Fdigital_logic%3Abcd-7segment-frequency-counter) — Digital Logic, chapter 11 - [Digital device applications; frequency counter](https://www.bctengineer.com/question.md?key=semester_3%2Fdigital_logic%3Adigital-devices-frequency-counter) — Digital Logic, chapter 11 - [n-bit full adder block diagram](https://www.bctengineer.com/question.md?key=semester_3%2Fdigital_logic%3An-bit-full-adder) — Digital Logic, chapter 5 - [Synchronous MOD-5 counter; counter applications](https://www.bctengineer.com/question.md?key=semester_3%2Fdigital_logic%3Amod5-sync-applications) — Digital Logic, chapter 8 - [Sync vs async; mod-12 synchronous counter](https://www.bctengineer.com/question.md?key=semester_3%2Fdigital_logic%3Amod12-synchronous-counter) — Digital Logic, chapter 8, asked 3× - [3-bit asynchronous up/down counter](https://www.bctengineer.com/question.md?key=semester_3%2Fdigital_logic%3Aasync-up-down-counter-3bit) — Digital Logic, chapter 8, asked 2× ## Related in other subjects - [Data logger block diagram and operation](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Adl-block) — Instrumentation II, chapter 5, asked 6× - [PID control operation with block diagram](https://www.bctengineer.com/question.md?key=semester_6%2Fembedded_system%3Apid-operation-block-diagram) — Embedded System, chapter 7, asked 4× - [Instrumentation system with block diagram](https://www.bctengineer.com/question.md?key=semester_2%2Fbasic_electronics_engineering%3Ainstrumentation-system) — Basic Electronics Engineering, chapter 7, asked 3× - [Digital control system block diagram and advantages](https://www.bctengineer.com/question.md?key=semester_6%2Fdigital_control_system%3Adcs-block-diagram-advantages) — Digital Control System, chapter 1, asked 3×