--- title: "ISB modulation; AM vs FM" subject: "Communication Systems" chapter: 2 key: "semester_6/communication_systems:isb-am-vs-fm" source: https://www.bctengineer.com/q/semester_6/communication_systems/isb-am-vs-fm site: BCT Engineering --- # ISB modulation; AM vs FM Communication Systems · Chapter 2: Amplitude Modulation - Asked in: 2077 Chaitra (CS I), 3+5 marks - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_6/communication_systems/isb-am-vs-fm - Whole chapter: https://www.bctengineer.com/md/semester_6/communication_systems/chapter-2 ## Question Explain ISB modulation with necessary derivation. Differentiate between Amplitude modulated signal and Frequency modulated signal. ## Short answer ISB transmits two independent messages on USB and LSB with a pilot carrier; bandwidth 2W for two channels. Derivation: s_ISB(t)=(Ac/2)[(m1+m2)cosωct-(m̂1-m2)sinωct]. AM varies carrier amplitude, FM varies frequency. AM bandwidth=2fm, FM bandwidth=2(Δf+fm) (Carson). AM has poor noise immunity, FM has good immunity via limiter. AM uses linear amplifiers, FM uses class C. ## Answer #### ISB modulation **Independent Sideband (ISB)** modulation transmits **two different messages** on the two sidebands of one carrier: message $m_1(t)$ on the USB and message $m_2(t)$ on the LSB. A reduced (pilot) carrier is usually sent for receiver synchronisation. ```text m1(t) --> SSB modulator (USB) --+ ^ | fc osc --------+---------------(+)--> ISB out v | (+ pilot carrier) m2(t) --> SSB modulator (LSB) --+ ``` **Derivation.** Using the phase-shift form of SSB: $$ \begin{aligned} s_{USB}(t) &= \frac{A_c}{2}[m_1(t)\cos\omega_ct - \hat m_1(t)\sin\omega_ct] \\ s_{LSB}(t) &= \frac{A_c}{2}[m_2(t)\cos\omega_ct + \hat m_2(t)\sin\omega_ct] \end{aligned} $$ $$ s_{ISB}(t) = \frac{A_c}{2}\{[m_1(t) + m_2(t)]\cos\omega_ct - [\hat m_1(t) - \hat m_2(t)]\sin\omega_ct\} $$ For single tones $m_1 = A_1\cos\omega_1t$, $m_2 = A_2\cos\omega_2t$: $$ s_{ISB}(t) = \frac{A_cA_1}{2}\cos(\omega_c+\omega_1)t + \frac{A_cA_2}{2}\cos(\omega_c-\omega_2)t $$ ```text m2 band | m1 band (LSB) | (USB) ----[////]----+----[\\\\]----> f fc-W fc fc+W ``` Bandwidth $= 2W$ for two channels, same as one DSB channel. Used in HF point-to-point radio telephony to carry two voice channels. #### AM vs FM | Point | AM | FM | |---|---|---| | Varied parameter | Carrier amplitude | Carrier frequency | | Amplitude | Varies with message | Constant | | Bandwidth | $2f_m$ | $2(\Delta f + f_m)$ (Carson), much wider | | Sidebands | Two side frequencies per tone | Infinite (Bessel functions) | | Noise immunity | Poor; noise adds to amplitude | Good; limiter removes amplitude noise | | Power | Varies with $\mu$; carrier wastes power | Constant, equal to unmodulated carrier | | Transmitter efficiency | Low (linear amplifiers needed) | High (class C amplifiers) | | Circuit | Simple | More complex | | Typical band | MW/SW broadcast (535–1605 kHz) | VHF broadcast (88–108 MHz) | ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What does ISB transmit? A. One message on both sidebands B. Two independent messages, one on each sideband C. One message on USB with LSB suppressed D. The carrier alone Answer: B. m1 is on the USB and m2 on the LSB, often with a pilot carrier. (Key point: ISB concept) 2. What is the bandwidth of an ISB signal carrying two channels of width W? A. W B. 4W C. 3W D. 2W Answer: D. The two sidebands occupy 2W, the same as a single DSB channel. (Key point: ISB bandwidth) 3. In AM vs FM, which parameter does the message vary? A. AM varies the carrier amplitude; FM varies the carrier frequency B. AM varies frequency; FM varies amplitude C. Both vary the amplitude D. Both vary the phase only Answer: A. This is the basic distinction. (Key point: Varied parameter) 4. Why is FM more noise-immune than AM? A. It has a narrower bandwidth B. It uses a larger carrier C. Its amplitude is constant, so a limiter can remove amplitude noise D. It has fewer sidebands Answer: C. Noise mainly adds to amplitude, which FM receivers clip. 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