When Emax=12 V and Emin=4 V, what is the AM index?
A 0.25
B 0.6
C 0.8
D 0.5
Answer: D. 0.5. m = (12 − 4)/(12 + 4) = 0.5.
Suppose Am=3 V and Ac=15 V; find m.
A 0.1
B 0.2
C 0.5
D 0.8
Answer: B. 0.2. The single-tone modulation index is Am/Ac = 3/15 = 0.2.
With a 4 kHz tone on a 100 kHz carrier, which pair is correct?
A 96 and 104 kHz
B 94 and 106 kHz
C 96 and 100 kHz
D 100 and 104 kHz
Answer: A. 96 and 104 kHz. The sidebands occur at 100 − 4 and 100 + 4 kHz.
With baseband bandwidth B=5 kHz, what is DSB AM bandwidth?
A 5 kHz
B 7.5 kHz
C 10 kHz
D 15 kHz
Answer: C. 10 kHz. Double-sideband AM occupies 2B, or 10 kHz.
At m=0.5, total sideband power is what fraction of Pc?
A 6.25%
B 25%
C 50%
D 12.5%
Answer: D. 12.5%. Total sideband power is m²Pc/2, which is 0.125Pc.
At m=1, conventional AM has what maximum efficiency?
A One-third
B One-quarter
C One-half
D Two-thirds
Answer: A. One-third. At full modulation, useful sideband power is one-third of total AM power.
Carrier power is 40 W and m=1. Find each sideband power.
A 5 W
B 20 W
C 10 W
D 40 W
Answer: C. 10 W. Each sideband has m²Pc/4 = 40/4 = 10 W.
Which scheme removes the carrier from a double-sideband signal?
A DSB-LC
B DSB-SC
C VSB-LC
D Narrowband AM
Answer: B. DSB-SC. Double-sideband suppressed-carrier modulation removes the carrier component.
How is a DSB-SC message usually recovered?
A Envelope detection
B Slope detection
C Thermal detection
D Coherent detection
Answer: D. Coherent detection. A synchronized local carrier is needed to demodulate DSB-SC.
Multiplying DSB-SC by a synchronized carrier uses which detector?
A Product detector
B Envelope detector
C Ratio detector
D Crystal detector
Answer: A. Product detector. A product detector performs coherent multiplication with the recovered carrier.
Relative to DSB, how much bandwidth does ideal SSB use?
A 2B
B B
C 3B
D 4B
Answer: B. B. Single-sideband modulation transmits one sideband, so its ideal bandwidth is B.
VSB sends one full sideband plus what?
A Both full sidebands
B Carrier only
C Vestige of the other
D A pilot tone
Answer: C. Vestige of the other. Vestigial-sideband modulation retains one sideband and a small vestige of the other.
Which scheme is commonly used for analog TV video?
A VSB
B DSB-SC
C SSB-SC
D Pulse FM
Answer: A. VSB. Vestigial-sideband modulation balances bandwidth saving with practical low-frequency response.
With m=0.8, total AM power equals what multiple of Pc?
A 1.08 Pc
B 1.16 Pc
C 1.64 Pc
D 1.32 Pc
Answer: D. 1.32 Pc. Ptotal = Pc(1 + m²/2) = 1.32Pc.
For Pc=100 W and m=0.4, total sideband power is?
A 4 W
B 16 W
C 8 W
D 20 W
Answer: C. 8 W. The two sidebands together carry m²Pc/2 = 8 W.
Given fc=1 MHz and fm=6 kHz, locate the lower sideband.
A 988 kHz
B 994 kHz
C 996 kHz
D 1006 kHz
Answer: B. 994 kHz. The lower sideband is 1000 − 6 = 994 kHz.
How much ideal SSB bandwidth is needed for a 0–3 kHz message?
A 3 kHz
B 1.5 kHz
C 6 kHz
D 9 kHz
Answer: A. 3 kHz. SSB transmits one sideband spanning the 3 kHz message band.
Which spectral component is suppressed in DSB-SC?
A Upper sideband
B Lower sideband
C Carrier
D Message band
Answer: C. Carrier. DSB-SC retains both sidebands but suppresses the carrier.
When does an envelope detector produce distortion?
A Zero modulation
B Carrier presence
C Small RC ripple
D Overmodulation
Answer: D. Overmodulation. Overmodulation makes the envelope non-proportional to the message.
In single-tone AM, what sets the sideband spacing?
A Carrier amplitude
B Message frequency
C Modulation phase
D Detector resistance
Answer: B. Message frequency. Each sideband is separated from fc by the modulating tone frequency.
Given a 50 W carrier and m=0.6, find total AM power.
A 53 W
B 56 W
C 59 W
D 68 W
Answer: C. 59 W. Ptotal = 50(1 + 0.36/2) = 59 W.
With a 2 kHz tone around 500 kHz, what are the sideband offsets?
A ±2 kHz
B ±1 kHz
C ±4 kHz
D ±500 kHz
Answer: A. ±2 kHz. Each sideband lies one message frequency away from the carrier.
How much RF bandwidth does a 0–3 kHz DSB-SC message need?
A 3 kHz
B 4.5 kHz
C 9 kHz
D 6 kHz
Answer: D. 6 kHz. Double-sideband transmission uses twice the 3 kHz message bandwidth.
Which detection method needs carrier phase alignment?
A Envelope detection
B Coherent detection
C Diode clipping
D Slope detection
Answer: B. Coherent detection. Coherent demodulation requires a phase-aligned local carrier.
What does SSB reduce compared with DSB?
A Message amplitude
B Carrier frequency
C Occupied bandwidth
D Audio information
Answer: C. Occupied bandwidth. SSB transmits one sideband instead of two, halving ideal bandwidth.