--- title: "Phase shift method of SSB generation: pros and cons" subject: "Communication Systems" chapter: 2 key: "semester_6/communication_systems:ssb-phase-shift-pros-cons" source: https://www.bctengineer.com/q/semester_6/communication_systems/ssb-phase-shift-pros-cons site: BCT Engineering --- # Phase shift method of SSB generation: pros and cons Communication Systems · Chapter 2: Amplitude Modulation - Asked in: 2075 Baisakh (CS I), 6+2 marks; 2072 Magh (CS I), 5+3 marks (asked 2 times) - Video: [SSB-SC Generation by Phase Shift Method Explained: Basics, Block Diagram, Working, Pros, and Cons](https://www.youtube.com/watch?v=ocSHGQmaIKc) — Engineering Funda - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_6/communication_systems/ssb-phase-shift-pros-cons - Whole chapter: https://www.bctengineer.com/md/semester_6/communication_systems/chapter-2 ## Question Explain the phase shift method of generation of SSB AM modulated wave. What are the pros and cons of this method? ## Short answer The phase-shift method generates SSB by summing or differencing the outputs of two balanced modulators fed with m(t) and its Hilbert transform, using quadrature carriers. Pros: no sharp filter needed, works for low-frequency messages. Cons: a precise wideband 90° phase shifter is hard to realize, and imbalance causes residual sideband. ## Answer The **phase-shift (phase discrimination) method** generates SSB directly from its mathematical form, without a sideband filter: $$ s_{SSB}(t)=\frac{A_c}{2}\left[m(t)\cos\omega_ct\mp\hat m(t)\sin\omega_ct\right] $$ where $\hat m(t)$ is the Hilbert transform of $m(t)$ (each component shifted by $-90°$). Minus gives USB, plus gives LSB. #### Block diagram ```text +-->[Bal. mod 1]-- m cos wct --+ | ^ | m(t) -------+ | cos wct v | [Carrier osc] ( + / - )--> SSB | | ^ | [-90 deg] | | | sin wct | +->[-90]->[Bal. mod 2]-- m^ sin wct m^(t) ``` #### Working 1. **Balanced modulator 1** multiplies $m(t)$ by $\cos\omega_ct$, giving DSB-SC with both sidebands. 2. A **wideband $-90°$ phase shifter** (Hilbert transformer) produces $\hat m(t)$, and a carrier phase shifter gives $\sin\omega_ct$. **Balanced modulator 2** gives $\hat m(t)\sin\omega_ct$. 3. The **summer/subtractor** combines them. One sideband appears in phase in both paths and adds; the other is in antiphase and cancels. **Proof with a tone** $m=A_m\cos\omega_mt$, $\hat m=A_m\sin\omega_mt$: $$ \begin{aligned} m\cos\omega_ct&=\tfrac{A_m}{2}[\cos(\omega_c-\omega_m)t+\cos(\omega_c+\omega_m)t]\\ \hat m\sin\omega_ct&=\tfrac{A_m}{2}[\cos(\omega_c-\omega_m)t-\cos(\omega_c+\omega_m)t] \end{aligned} $$ Subtracting gives $A_m\cos(\omega_c+\omega_m)t$ (USB only); adding gives $A_m\cos(\omega_c-\omega_m)t$ (LSB only). #### Pros 1. **No sharp sideband filter** is needed, so SSB can be generated directly at any carrier frequency (no multiple up-conversion). 2. Works for messages with **energy near zero frequency** (no need for a gap between sidebands, unlike the filter method). 3. Switching between USB and LSB is easy: just change adder to subtractor. 4. Lighter, smaller circuits for low carrier frequencies. #### Cons 1. A **wideband $90°$ phase shifter** that gives exactly $90°$ at all audio frequencies (e.g. 300–3400 Hz) with equal gain is very difficult to build. 2. Any phase or amplitude imbalance leaves a residual **unwanted sideband** (poor sideband suppression, typically 30–40 dB vs 50–60 dB for filters). 3. Two balanced modulators must be closely matched, and the circuit needs careful adjustment and is sensitive to drift. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. The phase-shift method produces SSB by: A. Filtering a DSB-SC signal with a sharp filter B. Combining m cos wct and m^ sin wct so one sideband cancels C. Squaring the message D. Frequency-modulating the carrier Answer: B. It implements (Ac/2)[m cos wct -/+ m^ sin wct] directly, with one sideband adding and the other cancelling. (Key point: Principle) 2. Which is an advantage of the phase-shift method over the filter method? A. No sharp sideband filter needed and it works for messages with low-frequency content B. Better sideband suppression C. Needs no matched modulators D. No 90 degree shifter required Answer: A. It avoids the sharp filter, so it works at any carrier and with energy near zero frequency. (Key point: Advantage) 3. What is the main difficulty of the method? A. Wideband 90 degree shifter with exact phase and gain is hard to build B. It needs a very high carrier C. It cannot switch sidebands D. It produces both sidebands Answer: A. Any imbalance in the 90 degree shifter or modulators leaves a residual unwanted sideband. (Key point: Main drawback) 4. In the tone proof, subtracting m cos wct and m^ sin wct gives: A. Am cos(wc - wm)t B. Am cos(wc + wm)t (USB) C. Am cos wm t D. Zero Answer: B. The lower terms cancel in the subtraction, leaving Am cos(wc+wm)t, the USB. (Key point: Sideband selection) ## Related in Communication Systems - [Phase shift method of SSB generation](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Assb-phase-shift) — Communication Systems, chapter 2 - [DSB-FC vs DSB-SC; phase shift method for SSB](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Adsbfc-dsbsc-ssb-phase-shift) — Communication Systems, chapter 2 - [Compare DSB-AM, DSB-SC, SSB, VSB; SSB by phase discrimination](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Acompare-am-variants-ssb-phase) — Communication Systems, chapter 2 - [SSB vs AM and DSB-SC; balanced modulator](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Assb-vs-am-balanced-modulator) — Communication Systems, chapter 2 - [VSB generation; VSB for TV; SSB applications](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Avsb-generation-tv-ssb-applications) — Communication Systems, chapter 2 - [DSB-AM vs SSB; ring modulator for DSB-SC](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Adsbam-ssb-compare-ring-modulator) — Communication Systems, chapter 2 ## Related in other subjects - [Surface wave propagation pros and cons](https://www.bctengineer.com/question.md?key=semester_6%2Fpropagation_and_antenna%3Asurface-wave-propagation) — Propagation and Antenna, chapter 4 - [Modulation; AM and FM modulated waves](https://www.bctengineer.com/question.md?key=semester_2%2Fbasic_electronics_engineering%3Amodulation-am-fm-wave) — Basic Electronics Engineering, chapter 5 - [Distribution layouts, pros/cons, improvements](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Adistribution-layouts-improvements) — Water Supply Engineering, chapter 7, asked 4× - [Collective intelligence pros and cons](https://www.bctengineer.com/question.md?key=semester_8%2Finformation_system%3Acollective-intelligence-pros-cons) — Information System, chapter 4