--- title: "Phase shift method of SSB generation" subject: "Communication Systems" chapter: 2 key: "semester_6/communication_systems:ssb-phase-shift" source: https://www.bctengineer.com/q/semester_6/communication_systems/ssb-phase-shift site: BCT Engineering --- # Phase shift method of SSB generation Communication Systems · Chapter 2: Amplitude Modulation - Asked in: 2071 Bhadra (CS I), 6 marks - Video: [SSB Generation Methods : Phase Shift Method and Filter Method Explained](https://www.youtube.com/watch?v=UCa7rOFp8js) — ALL ABOUT ELECTRONICS - 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 - Whole chapter: https://www.bctengineer.com/md/semester_6/communication_systems/chapter-2 ## Question With the help of block diagram and expression explain the phase shift method for generation of SSB-AM wave. ## Short answer The phase shift method uses two balanced modulators with 90° phase-shifted message (Hilbert transform) and carrier. Their outputs are added or subtracted: subtraction yields USB, addition yields LSB, implementing s_SSB(t)=A_c[m(t)cosωct ∓ m̂(t)sinωct]. No sharp filter needed but requires accurate wideband phase shifter. ## Answer The **phase shift (phase discrimination) method** generates SSB by using two balanced modulators and $90^\circ$ phase shifters, so that one sideband cancels and the other adds. No sharp sideband filter is needed. #### Block diagram ```text +-------> Balanced ---- v1 ----+ | modulator 1 | | ^ v m(t) -------+ | Ac cos wc t (+/-) --> SSB | Carrier osc ^ | | | [-90 deg] [-90 deg] | | | Ac sin wc t | | m^(t) v | +-------> Balanced ---- v2 ----+ modulator 2 ``` - Upper path: $m(t)$ and carrier $A_c\cos\omega_ct$. - Lower path: message shifted by $-90^\circ$ (Hilbert transform $\hat m(t)$) and carrier shifted by $-90^\circ$ ($A_c\sin\omega_ct$). #### Expressions Single tone $m(t) = A_m\cos\omega_mt$, so $\hat m(t) = A_m\sin\omega_mt$. $$ \begin{aligned} v_1(t) &= A_cA_m\cos\omega_mt\cos\omega_ct = \frac{A_cA_m}{2}[\cos(\omega_c - \omega_m)t + \cos(\omega_c + \omega_m)t] \\ v_2(t) &= A_cA_m\sin\omega_mt\sin\omega_ct = \frac{A_cA_m}{2}[\cos(\omega_c - \omega_m)t - \cos(\omega_c + \omega_m)t] \end{aligned} $$ $$ \begin{aligned} v_1 - v_2 &= A_cA_m\cos(\omega_c + \omega_m)t \quad \text{(USB)} \\ v_1 + v_2 &= A_cA_m\cos(\omega_c - \omega_m)t \quad \text{(LSB)} \end{aligned} $$ For a general message: $$ s_{SSB}(t) = A_c[m(t)\cos\omega_ct \mp \hat m(t)\sin\omega_ct] $$ (minus → USB, plus → LSB). **Merits:** no sharp filter, any carrier frequency, low audio frequencies preserved, easy sideband switching. **Demerit:** the wideband $90^\circ$ phase shifter must be accurate over the whole audio band (e.g. 300–3400 Hz); phase or amplitude errors leave part of the unwanted sideband. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. How does the phase shift method remove the unwanted sideband? A. The carrier is doubled B. Two balanced modulator outputs are added or subtracted so one sideband cancels C. A sharp band-pass filter rejects it D. A diode clips it Answer: B. v1 and v2 contain the same LSB term and opposite USB terms, so combining them cancels one sideband. (Key point: Principle) 2. What feeds the lower balanced modulator? A. The message shifted by -90 degrees and the carrier shifted by -90 degrees B. m(t) and the unshifted carrier C. Only the carrier shifted by 90 degrees D. The unshifted message and sin 2wc t Answer: A. The lower path uses the Hilbert-transformed message and Ac sin wc t. (Key point: Lower path) 3. With v1 = AcAm cos wm t cos wc t and v2 = AcAm sin wm t sin wc t, which gives the USB? A. v1 x v2 B. v2 - 2 v1 C. v1 + v2 D. v1 - v2 Answer: D. v1 - v2 = AcAm cos(wc + wm)t, the upper sideband. (Key point: USB or LSB) 4. What is the main demerit of this method? A. It cannot be used at high carrier frequencies B. The wideband 90 degree phase shifter must be accurate, else some unwanted sideband remains C. It loses low audio frequencies D. It needs a very sharp filter at fc Answer: B. Phase or amplitude errors in the shifter leave part of the unwanted sideband. (Key point: Demerit) ## Related in Communication Systems - [Phase shift method of SSB generation: pros and cons](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Assb-phase-shift-pros-cons) — Communication Systems, chapter 2, asked 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 - [AM wave diagram, modulation index; why near 100%](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Aam-diagram-index-100-percent) — 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 ## Related in other subjects - [Communication system and AM block diagram](https://www.bctengineer.com/question.md?key=semester_2%2Fbasic_electronics_engineering%3Adefine-comm-am) — Basic Electronics Engineering, chapter 5 - [Hydropower generation with block diagram](https://www.bctengineer.com/question.md?key=semester_4%2Fpower_system_analysis_i%3Ahydropower-generation-block-diagram) — Power System Analysis I, chapter 1 - [Barkhausen criteria and RC phase shift oscillator](https://www.bctengineer.com/question.md?key=semester_3%2Felectronics_devices_and_circuit%3Abarkhausen-rc-phase-shift) — Electronics Devices and Circuit, chapter 5, asked 3× - [RC phase shift oscillator](https://www.bctengineer.com/question.md?key=semester_3%2Felectronics_devices_and_circuit%3Arc-phase-shift-oscillator) — Electronics Devices and Circuit, chapter 5, asked 6×