--- title: "Filter importance in communication; ideal vs practical" subject: "Filter Design" chapter: 1 key: "semester_5/filter_design:filter-communication-ideal-practical" source: https://www.bctengineer.com/q/semester_5/filter_design/filter-communication-ideal-practical site: BCT Engineering --- # Filter importance in communication; ideal vs practical Filter Design · Chapter 1: Introduction - Asked in: 2074 Chaitra, 1+2+3 marks; 2082 Chaitra (new course), 1+1+3 marks (asked 2 times) - Video: [Filters - Ideal Filters](https://www.youtube.com/watch?v=45jWSeyl2X4) — Daniel Cutshall - Milligan University - Answer checked by Claude Sonnet 5.5 on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_5/filter_design/filter-communication-ideal-practical - Whole chapter: https://www.bctengineer.com/md/semester_5/filter_design/chapter-1 ## Question What is a filter? What is its importance (application) in the field of communication? Explain (differentiate) ideal response and response of practical filters. ## Short answer A filter is a frequency-selective two-port network that passes desired frequency bands and attenuates others. In communication, it selects channels, removes noise, prevents aliasing, and separates transmit/receive bands. Ideal filters have flat passband, infinite stopband attenuation, and vertical edges but are unrealizable; practical filters have ripple, finite attenuation, a transition band, and are realizable with finite order. ## Answer #### Filter A **filter** is a frequency-selective two-port network that passes signals in a chosen band of frequencies (passband) with little attenuation and attenuates signals at other frequencies (stopband). Filters may be passive (R, L, C), active (R, C, op-amp), switched-capacitor or digital. #### Applications in communication - **Channel selection:** IF and RF band-pass filters in radio, TV and mobile receivers select one channel and reject adjacent ones. - **Frequency division multiplexing:** band-pass filters separate channels in FDM telephony and cable systems. - **Anti-aliasing and reconstruction:** low-pass filters before an ADC and after a DAC. - **Noise and interference removal:** removing hum (50 Hz notch), out-of-band noise and harmonics from transmitters. - **Modulation/demodulation:** removing the carrier after detection, SSB generation (sideband filters), pulse shaping (raised-cosine) in digital links. - **Duplexers and diplexers:** separating transmit and receive bands sharing one antenna. #### Ideal vs practical response An **ideal filter** passes all frequencies in the passband with constant gain and zero phase distortion and completely blocks all frequencies in the stopband, with a vertical edge between them. It cannot be built, because its impulse response is non-causal and infinitely long. A **practical filter** only approximates this. | Ideal filter | Practical filter | |---|---| | Flat gain in passband | Gain varies within $\alpha_{max}$ (ripple or droop) | | Zero gain (infinite attenuation) in stopband | Finite attenuation, at least $\alpha_{min}$ | | No transition band (brick wall) | Transition band between $\omega_p$ and $\omega_s$ | | Linear phase / constant delay | Non-linear phase, delay varies | | Non-causal, not realizable | Causal, realizable with R, L, C, op-amps | | Infinite order | Finite order $n$ | ```text |T| ideal |T| practical 1 +------+ 1 +~~~~~. <- ripple (amax) | | | \ | | | \ transition | | | `--..__ (amin) 0 +------+------> w 0 +------+---+------> w wc wp ws ``` The same idea applies to HP, BP and BS filters: the ideal response has rectangular edges, while the practical one has finite slopes, passband ripple up to $\alpha_{max}$ and stopband attenuation of at least $\alpha_{min}$. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What is a filter? A. A frequency-selective two-port that passes a chosen band and attenuates other frequencies B. A circuit that amplifies all frequencies equally C. A device that converts AC to DC D. A network whose gain is independent of frequency Answer: A. A filter passes the passband with little attenuation and attenuates the stopband. (Key point: Filter definition) 2. Which is a communication application of filters? A. Raising the carrier amplitude in transmitters B. Converting digital data to analog directly C. Band-pass filters selecting one channel and rejecting adjacent channels D. Increasing the sampling rate of an ADC Answer: C. IF/RF band-pass filters select the wanted channel in receivers. (Key point: Communication use) 3. Why can an ideal brick-wall filter not be built? A. Its passband gain is greater than 1 B. Its impulse response is non-causal and infinitely long (infinite order) C. It needs a negative resistance D. It requires an infinite power supply Answer: B. A vertical edge needs a non-causal, infinite-duration impulse response, so only approximations are realizable. (Key point: Ideal filter) 4. Which feature belongs to a practical filter but not to an ideal one? A. Flat gain in the passband B. Zero gain in the stopband C. Linear phase in the passband D. A transition band between wp and ws Answer: D. A practical filter has a finite transition band between the passband edge wp and the stopband edge ws; an ideal one has none. (Key point: Practical vs ideal) ## Related in Filter Design - [Ideal vs practical filters and scaling](https://www.bctengineer.com/question.md?key=semester_5%2Ffilter_design%3Aideal-practical-filters-scaling) — Filter Design, chapter 1, asked 2× - [Ideal filter and magnitude/frequency scaling](https://www.bctengineer.com/question.md?key=semester_5%2Ffilter_design%3Aideal-filter-magnitude-frequency-scaling) — Filter Design, chapter 1 ## Related in other subjects - [Impulse response of ideal low pass filter](https://www.bctengineer.com/question.md?key=semester_4%2Fsignals_and_systems%3Aideal-lpf-impulse-response) — Signals and Systems, chapter 5, asked 5× - [Impulse response of DT ideal low pass filter](https://www.bctengineer.com/question.md?key=semester_4%2Fsignals_and_systems%3Aideal-lpf-discrete-time) — Signals and Systems, chapter 6 - [Practical vs ideal transformer, phasor diagram](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines%3Apractical-vs-ideal-transformer-phasor) — Electrical Machines, chapter 2 - [Ideal band pass and low pass filters (DT)](https://www.bctengineer.com/question.md?key=semester_4%2Fsignals_and_systems%3Aideal-bandpass-lowpass-dt) — Signals and Systems, chapter 6