In standard AM, which carrier property varies with message amplitude?
A Carrier amplitude
B Carrier frequency
C Carrier phase
D Message period
Answer: A. Carrier amplitude. Amplitude modulation changes carrier amplitude according to the message.
What does a conventional AM signal contain?
A Only lower sideband
B Only upper sideband
C Carrier and sidebands
D No carrier component
Answer: C. Carrier and sidebands. Conventional double-sideband AM transmits the carrier and both sidebands.
Before modulation, which frequency marks the carrier?
A fm
B fc + fm
C fc − fm
D fc
Answer: D. fc. The carrier appears at its own frequency fc.
During single-tone AM, where do sidebands appear?
A At zero frequency
B fc ± fm
C At fc only
D At 2fc only
Answer: B. fc ± fm. The message creates components at fc plus and minus fm.
Sidebands or carrier, which components carry information?
A Sidebands
B Carrier alone
C DC component
D Noise bands
Answer: A. Sidebands. The sidebands contain the shifted message spectrum; the carrier mainly aids simple detection.
For a 2 kHz tone on a 100 kHz carrier, locate the upper sideband.
A 98 kHz
B 100 kHz
C 102 kHz
D 104 kHz
Answer: C. 102 kHz. The upper sideband is fc + fm, or 102 kHz.
How is the AM modulation index defined?
A Carrier-to-message frequency
B Message-to-carrier amplitude
C Power-to-bandwidth ratio
D Sideband-to-carrier spacing
Answer: B. Message-to-carrier amplitude. For a single-tone signal, m is the message amplitude divided by carrier amplitude.
At zero modulation, what remains transmitted?
A Two sidebands
B Only message tone
C No RF signal
D Unmodulated carrier
Answer: D. Unmodulated carrier. With m equal to zero, the carrier remains but no sidebands are produced.
At what envelope minimum is modulation 100%?
A Maximum reaches zero
B Envelope stays constant
C Minimum reaches zero
D Carrier disappears
Answer: C. Minimum reaches zero. At m equal to one, the AM envelope just touches zero at its minimum.
What occurs when AM modulation index exceeds one?
A Envelope overmodulation
B Carrier suppression
C Frequency inversion
D Perfect detection
Answer: A. Envelope overmodulation. Overmodulation makes the envelope cross zero and distorts simple envelope recovery.
For a 5 kHz message, how wide is conventional AM?
A 5 kHz
B 10 kHz
C 15 kHz
D 20 kHz
Answer: B. 10 kHz. Single-tone double-sideband AM occupies twice the message frequency.
For standard AM, what circuit commonly demodulates it?
A Balanced mixer
B Frequency discriminator
C Phase detector
D Diode RC detector
Answer: D. Diode RC detector. A diode and RC network can follow the envelope of conventional AM.
During detection, what slow feature does an envelope detector follow?
A Slow signal envelope
B Carrier phase
C RF zero crossings
D Noise spectrum
Answer: A. Slow signal envelope. The detector tracks the slowly varying amplitude outline that represents the message.
Simple envelope recovery requires which condition?
A Suppressed carrier only
B m greater than one
C Carrier with m ≤ 1
D No sidebands
Answer: C. Carrier with m ≤ 1. Envelope detection needs a transmitted carrier and a non-crossing envelope.
Between carrier and message, which normally has higher frequency?
A Message frequency
B Carrier frequency
C Both are equal
D Message bandwidth
Answer: B. Carrier frequency. The carrier is chosen far above the message band for transmission.
Why is a carrier used in AM transmission?
A It removes all sidebands
B It stores the audio
C It lowers message frequency
D It shifts the message spectrum
Answer: D. It shifts the message spectrum. Multiplication by the carrier translates the baseband spectrum around the RF carrier.
How do AM sidebands arise mathematically?
A Envelope clipping
B Phase-only rotation
C Carrier-message multiplication
D Noise averaging
Answer: C. Carrier-message multiplication. Amplitude modulation multiplies the message by a carrier, creating sum and difference frequencies.
The low-frequency information signal is called what?
A Message signal
B Carrier wave
C Local oscillator
D Image frequency
Answer: A. Message signal. The baseband waveform carrying information is the message signal.
Message amplitude tracks what quantity in standard AM?
A Carrier spacing
B RF phase only
C Sideband count
D Carrier envelope
Answer: D. Carrier envelope. The carrier’s envelope changes in proportion to the message waveform.
In standard AM, how does carrier frequency behave?
A It follows message
B It stays constant
C It becomes zero
D It doubles fm
Answer: B. It stays constant. AM varies carrier amplitude; the carrier frequency stays fixed.
What describes an AM signal with m below one?
A Under-modulated
B Over-modulated
C Suppressed-carrier
D Frequency-modulated
Answer: A. Under-modulated. Modulation depth below unity is called under-modulation.
Envelope maxima and minima help estimate which quantity?
A Mean and variance
B Phase and frequency
C Modulation index
D Power and bandwidth
Answer: C. Modulation index. m = (Emax − Emin)/(Emax + Emin), using the envelope extrema.
If Emax=8 V and Emin=4 V, what is m?
A 1/2
B 2/3
C 3/4
D 1/3
Answer: D. 1/3. The envelope formula gives (8 − 4)/(8 + 4) = 1/3.
For a 4 kHz tone and 200 kHz carrier, where is the lower sideband?
A 192 kHz
B 196 kHz
C 200 kHz
D 204 kHz
Answer: B. 196 kHz. The lower sideband is fc − fm, which equals 196 kHz.
At fc, what spectral line appears?
A Upper sideband
B Lower sideband
C Carrier component
D Message harmonic
Answer: C. Carrier component. The unmodulated carrier appears at fc; sidebands lie on either side.