AM and FM Modulation Interview Questions

Q1. What is amplitude modulation and how is the AM signal expressed mathematically?

In amplitude modulation, the amplitude of a high-frequency carrier is varied proportionally to the instantaneous value of the message signal, giving s(t) = Ac[1 + ka·m(t)]cos(2πfct), where Ac is the carrier amplitude, ka is the amplitude sensitivity, and m(t) is the message. For a single-tone message m(t) = Am·cos(2πfmt), the AM signal becomes Ac[1 + μ·cos(2πfmt)]cos(2πfct), where μ = ka·Am is the modulation index. The spectrum of this signal contains three components: the carrier at fc and two sidebands at fc ± fm, showing that AM transmission is inherently bandwidth-wasteful since the carrier carries no information.

Follow-up: What fraction of total AM signal power is in the sidebands for a modulation index of 1?

Q2. What is the modulation index in AM and what happens when it exceeds 1?

The AM modulation index μ = Am/Ac is the ratio of message amplitude to carrier amplitude, and it must be kept below 1 (100%) to prevent overmodulation. When μ = 0.5, 50% modulation means the envelope varies between 50% and 150% of Ac — an AM radio receiver's envelope detector can recover the message cleanly. When μ > 1, the carrier envelope inverts at some points, creating waveform crossings that an envelope detector interprets as a different (distorted) signal — this is overmodulation, causing severe harmonic distortion in broadcast AM systems.

Follow-up: How is the power efficiency of an AM transmitter calculated and why is it low?

Q3. What is the bandwidth of an AM signal and how is it determined?

The bandwidth of a double-sideband full-carrier AM signal is 2W, where W is the highest frequency in the message signal bandwidth, because the upper and lower sidebands each occupy a width of W on either side of the carrier. A standard AM broadcast station transmitting audio up to 5 kHz occupies a bandwidth of 10 kHz, which is why AM stations are spaced 10 kHz apart (9 kHz in ITU Region 1) on the medium-wave broadcast band. Bandwidth is exactly twice the message bandwidth because each message frequency fm creates sidebands at fc + fm and fc - fm symmetrically around the carrier.

Follow-up: How does DSB-SC differ from conventional AM in terms of bandwidth and power efficiency?

Q4. What is DSB-SC modulation and how does it differ from full AM?

Double-sideband suppressed-carrier (DSB-SC) modulation removes the carrier component, transmitting only the two sidebands as s(t) = Ac·m(t)·cos(2πfct), which doubles the power efficiency since no power is wasted in the carrier. A standard AM broadcast station at 100% modulation (μ = 1) wastes 66.7% of its total power in the carrier — the same total transmitter power in DSB-SC delivers the entire signal power useful to the receiver. The disadvantage of DSB-SC is that an envelope detector cannot recover the message — coherent detection with a synchronized carrier at the receiver is required, increasing receiver complexity.

Follow-up: What is a product detector and why is it needed for DSB-SC demodulation?

Q5. What is single sideband (SSB) modulation and what is its advantage?

SSB modulation transmits only one sideband (upper or lower) of the DSB-SC signal, halving the bandwidth to W Hz while maintaining full information content, because both sidebands carry identical information due to the symmetry of real message signals. A 3 kHz voice signal in SSB occupies only 3 kHz of spectrum, compared to 6 kHz for DSB and 6 kHz for conventional AM — this is why SSB is universally used in HF single sideband shortwave radio communication for long-distance links. SSB is the most spectrum-efficient analog modulation, and it also has a 6 dB power advantage over DSB-SC in the presence of white noise at equal bandwidth.

Follow-up: How is SSB generated using the phase-shift method, and what is its practical limitation?

Q6. What is frequency modulation and what is the FM modulation index?

In frequency modulation, the instantaneous frequency of the carrier is varied proportionally to the message signal: fi(t) = fc + kf·m(t), where kf is the frequency sensitivity in Hz/V. For a single-tone message m(t) = Am·cos(2πfmt), the FM modulation index β = kf·Am/fm = Δf/fm is the ratio of peak frequency deviation to message frequency. An FM broadcast station with a peak deviation Δf = 75 kHz transmitting audio at fm = 1 kHz has β = 75, meaning the carrier frequency swings ±75 kHz in sync with the 1 kHz tone — a much wider signal than AM for the same message.

Follow-up: How does the FM modulation index change if the message frequency is doubled at the same amplitude?

Q7. What is Carson's rule for FM bandwidth?

Carson's rule approximates the FM signal bandwidth as BW ≈ 2(Δf + W) = 2(β + 1)W, where Δf is the peak deviation and W is the message bandwidth, and captures approximately 98% of total FM signal power. For commercial FM broadcasting with Δf = 75 kHz and W = 15 kHz: BW = 2(75 + 15) = 180 kHz, which is why FM broadcast channels are spaced 200 kHz apart in most countries. Carson's rule underestimates the actual bandwidth for large β (wideband FM) but is accurate to within a few percent for practical modulation indices.

Follow-up: Why is FM broadcasting allocated much wider bandwidth than AM despite carrying similar audio content?

Q8. Why does FM have better noise performance than AM?

FM noise performance is better than AM because the FM demodulator (discriminator or PLL) is sensitive to frequency variations, not amplitude, so amplitude-riding noise on the received carrier can be removed by a limiter before detection — this amplitude noise rejection is completely absent in AM. At high SNR (above the threshold), an FM system with modulation index β achieves an output SNR improvement of 3β²(β+1) compared to baseband, so wideband FM with β = 5 gives a 225× improvement, while AM with μ = 1 gives only a 1/3 improvement. The tradeoff is bandwidth — the FM noise advantage is directly bought at the cost of increased spectral occupancy according to Carson's rule.

Follow-up: What is the FM threshold effect and below what SNR does FM's noise advantage disappear?

Q9. What is pre-emphasis and de-emphasis in FM broadcasting?

Pre-emphasis boosts high-frequency audio components (above about 2.1 kHz) by a fixed RC network (75 µs time constant in most countries) before FM modulation, and de-emphasis applies the inverse filter after demodulation, restoring the flat audio frequency response. Since FM noise power at the output of a discriminator increases as f², high audio frequencies are noisier than low frequencies; pre-emphasis ensures they receive more FM deviation (and therefore more SNR protection), while de-emphasis simultaneously reduces the high-frequency noise boosted by the demodulation process. This system improves high-frequency audio SNR by up to 13 dB at 15 kHz for the standard 75 µs system, which is why FM audio quality sounds much cleaner than AM.

Follow-up: What time constant is used for FM pre-emphasis in India and what does it correspond to in frequency?

Q10. What is the difference between narrowband FM (NBFM) and wideband FM (WBFM)?

NBFM has a modulation index β ≤ 0.5 and bandwidth approximately equal to 2W (similar to AM), while WBFM has β >> 1 and bandwidth much larger than the message bandwidth, gaining noise performance at the cost of spectrum. Public safety and aviation voice radios use NBFM with 5 kHz deviation and 25 kHz channel spacing, while commercial FM broadcast uses WBFM with 75 kHz deviation and 200 kHz channel spacing. The distinction matters practically because NBFM and WBFM require different receiver architectures and demodulators — NBFM can use a narrow-band discriminator, but WBFM requires a wider IF filter and more linear discriminator.

Follow-up: Why does narrowband FM have approximately the same bandwidth as AM?

Q11. How is an AM signal demodulated using an envelope detector?

An envelope detector uses a diode to half-wave rectify the AM signal, followed by an RC low-pass filter that tracks the slowly varying envelope while filtering out the carrier frequency, effectively extracting the message from the amplitude variations. For an AM broadcast signal at 1 MHz carrying 5 kHz audio, the RC filter is chosen with a time constant 1/fc << RC << 1/W, typically around 10–50 µs — small enough to follow the 5 kHz audio envelope but large enough to reject the 1 MHz carrier ripple. The envelope detector fails when μ > 1 because the envelope no longer faithfully represents the message — the detector interprets envelope inversions as positive excursions, causing distortion.

Follow-up: What is the squaring loss of an envelope detector compared to coherent detection?

Q12. What is FM capture effect?

The capture effect in FM means that when two FM signals on the same frequency are received simultaneously, the stronger one completely suppresses the weaker one's output — a receiver 'captures' the dominant signal. If two FM stations are on the same channel with a signal strength difference of more than about 6 dB, an FM receiver in between will receive only the stronger station with no audible interference from the weaker one. This is the reason FM cochannel interference is much less noticeable than AM cochannel interference, where both signals blend linearly and both are heard simultaneously.

Follow-up: What is the capture ratio of an FM receiver and how is it specified?

Q13. What is vestigial sideband (VSB) modulation and where is it used?

VSB modulation transmits the full upper sideband and a vestige (partial remnant) of the lower sideband, occupying 1.25× the message bandwidth — a compromise between SSB efficiency and the simpler filtering requirements of DSB. Analog terrestrial TV broadcasting (NTSC, PAL) used VSB because the video signal has significant energy near DC (which SSB cannot transmit due to phase singularity at the carrier) while the full DSB bandwidth would double the occupied channel. The ATSC digital TV standard in the USA also uses 8-VSB for the same reason — the asymmetric vestigial sideband filter is easier to implement than ideal SSB for wideband video signals.

Follow-up: Why can't SSB modulation transmit signals with significant DC content?

Q14. What is the power efficiency of a conventional AM signal at 100% modulation?

At 100% modulation (μ = 1), the total AM signal power is Pc(1 + μ²/2) = 1.5Pc, of which Pc is in the carrier and 0.5Pc is split equally between the two sidebands — the carrier contains 2/3 (66.7%) of total power while the two information-bearing sidebands together contain only 1/3 (33.3%). A 1 kW AM broadcast transmitter at full modulation radiates 667 W in the carrier (wasted) and only 333 W total in the sidebands that carry the audio. This is why DSB-SC and SSB are used in power-limited applications like HF radio, while conventional AM is retained for broadcasting where simple envelope detection in millions of inexpensive receivers is more important than transmitter efficiency.

Follow-up: How does the sideband power change if the modulation index is reduced from 1.0 to 0.5?

Q15. What is the principle of superheterodyne receiver and how does it relate to AM/FM demodulation?

A superheterodyne receiver mixes the received RF signal with a local oscillator to produce a fixed intermediate frequency (IF) — 455 kHz for AM broadcast, 10.7 MHz for FM — where the amplification, filtering, and demodulation are performed, enabling excellent selectivity and sensitivity across a wide tuning range. For an AM receiver tuned to 900 kHz, the LO runs at 900 + 455 = 1355 kHz, mixing to produce the 455 kHz IF regardless of which station is tuned; at the IF, a narrow ceramic filter provides the channel selectivity and an AGC-controlled IF amplifier provides consistent demodulation input. The advantage is that the sharp IF filter is fixed and can be optimized once, rather than requiring a variable-frequency sharp filter that would be impractical.

Follow-up: What is the image frequency in a superheterodyne receiver and how is it rejected?