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.
1005 kHz & 995 kHz
What amplitude modulation produces. Multiplying a carrier by a message shifts the message spectrum up to sit either side of the carrier. Expanding the AM wave shows exactly three components:
\(s(t)=A_c\cos\omega_c t+\dfrac{\mu A_c}{2}\cos(\omega_c+\omega_m)t+\dfrac{\mu A_c}{2}\cos(\omega_c-\omega_m)t\)
the carrier, the upper sideband and the lower sideband.
Step 1 — put both frequencies in the same units.
\(f_c=1\ \text{MHz}=1000\ \text{kHz}, \qquad f_m=5\ \text{kHz}\)
Step 2 — add and subtract.
\(f_{USB}=f_c+f_m=1000+5=1005\ \text{kHz}\)
\(f_{LSB}=f_c-f_m=1000-5=995\ \text{kHz}\)
Check the bandwidth.
\(BW=f_{USB}-f_{LSB}=2f_m=10\ \text{kHz}\)
— the familiar result that an AM channel occupies twice the highest audio frequency, which is why medium-wave broadcast channels are spaced 9 or 10 kHz apart and audio is limited to about 5 kHz.
Why the other options are wrong. Option 4 (1010 and 990 kHz) uses 10 kHz instead of 5 kHz — it mistakes the whole bandwidth for the offset on each side. Option 3 moves the sidebands by 500 kHz and 50 kHz, and option 2 places the "lower" sideband at 9.05 MHz, far above the carrier, which is self-contradictory. A quick sanity rule: for a 5 kHz message the sidebands must sit only 5 kHz away from a 1000 kHz carrier, so both answers must be very close to 1000 kHz.
Note on the information content. The two sidebands are mirror images and carry identical information, and the carrier carries none — the reason SSB transmits just one sideband and halves the bandwidth.
Hence, USB = 1005 kHz and LSB = 995 kHz.
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 ?
Assertion (A) : Amplitude modulation is wastage of power.
Reason (R) : Amplitude modulation is wastage of bandwidth.
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:
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?
