Assertion (A) : Amplitude modulation is wastage of power. Reason (R) : Amplitude modulation is wastage of bandwidth.
Both (A) and (R) are true, but (R) is not correct explanation of (A).
Both statements are true, but they identify two separate inefficiencies, so neither explains the other — option 2.
A — the waste of power lies in the carrier. Total transmitted power is
\(P_{t}=P_{c}\left(1+\dfrac{m^{2}}{2}\right)\)
and the carrier carries no information whatever — it is a constant tone unaffected by the message. Only the sidebands convey anything, and their share is
\(\eta=\dfrac{m^{2}/2}{1+m^{2}/2}\)
Even at full modulation, \(m=1\), that is only 33 %; at a more typical \(m=0.5\) it falls to 11 %. Two-thirds or more of an AM transmitter's output is spent radiating a component that could be removed entirely, as DSB-SC does.
R — the waste of bandwidth lies in the second sideband. An AM signal occupies
\(BW=2f_{m}\)
but the upper and lower sidebands are mirror images carrying identical information. One of them is pure redundancy, and removing it — as SSB does — halves the occupied spectrum with no loss at all.
| Scheme | Carrier | Sidebands | Bandwidth | Fixes |
|---|---|---|---|---|
| AM | Present | Two | 2fm | — |
| DSB-SC | Suppressed | Two | 2fm | The power waste |
| SSB-SC | Suppressed | One | fm | Both |
Why R cannot explain A — and the table shows it. The two faults have different causes and different cures. DSB-SC removes the carrier and fixes the power waste while leaving the bandwidth exactly as it was; a vestigial-sideband system trims the spectrum while retaining a carrier. The independence of the two remedies proves the independence of the two faults. Transmitting a redundant sideband is not the reason the carrier is wasteful; the carrier would be equally wasteful in a single-sideband-plus-carrier system.
Why AM survives despite both. Retaining the carrier lets a diode, a capacitor and a resistor recover the envelope. Transmitter power is bought once; receivers are bought by millions — which is the whole economic case for broadcast AM.
Hence, both are true, but (R) is not the correct explanation of (A).
Similar Questions
Assertion (A) : In amplitude modulation technique the modulation index should be close to 1.
Reason (R) : The power carried by message signal in the side bands increases with increase in modulation index.
Read the statements :
i. DSB has two side bands and SSB has one
ii. DSB has carrier and two side bands and SSB has a carrier and a side band
iii. DSB has carrier and two side bands and SSB without carrier and two different side bands.Which statements are correct ?
The peak carrier voltage is 150 V. If the resistor is 200 Ω and the modulation index is 0.5, the total power in the AM signal is :
The modulation index of an AM wave is given by :
Consider an AM (amplitude modulated) signal
\(s(t) = 20\left[1 + 0.9\cos 2\pi \times 10^4 t\right]\cos 2\pi \times 10^6 t\)
The power efficiency (η) in the AM signal is
In amplitude modulation:
A. Amplitude of carrier is varied by modulating signal
B. Modulation index is between 0 and 1
C. Bandwidth is infinite
D. Bandwidth is twice of minimum modulating frequency.
Choose the correct answer from the options given below:
Match List I with List II
LIST I LIST II A. Power of AM Wave I. \(P_c\left(\frac{m^{2}}{4}\right)\) B. Power of VSB II. \(\frac{m^{2}}{4}P_c+F\left(\frac{m^{2}}{4}P_c\right)\) C. Power of SSB III. \(\frac{m^{2}}{4}\left(\frac{V_c^{2}}{2R}\right)+\frac{m^{2}}{4}\left(\frac{V_c^{2}}{2R}\right)\) D. Power of DSBSC IV. \(\frac{V_{carr}^{2}}{R}+\frac{V_{LSB}^{2}}{R}+\frac{V_{USB}^{2}}{R}\) Choose the correct answer from the options given below:
If 1 MHz carrier is amplitude modulated with a 5 kHz audio signal, the Upper Side Band (USB) and Lower Side Band (LSB) frequencies are ________ respectively.
Which of the following is associated with single balanced modulator circuit :
Important Questions from Amplitude Modulation
The highest modulation frequency typically used in AM broadcast is
Which of the following is NOT the advantage of amplitude modulation?
Bandwidth requirement for Amplitude modulated wave is:
The frequency range of AM radio is:
In an amplitude modulated system, a sinusoidal carrier signal of 1 MHz is modulated by a 10 kHz sinusoidal signal. If the lower sideband and the carrier are suppressed and if the amplitude modulated signal is sampled for further processing, what should be the minimum sampling frequency for baseband sampling?
