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NSWEngineering StudiesQuick questions
HSC Module: Telecommunications Engineering
Quick questions on Modulation techniques (AM, FM, PM, digital): HSC Engineering Studies Telecommunications Engineering
15short Q&A pairs drawn directly from our worked dot-point answer. For full context and worked exam questions, read the parent dot-point page.
What is the carrier wave?Show answer
A sinusoidal carrier is described by:
What is amplitude modulation (AM)?Show answer
The message signal modulates the carrier amplitude:
What is frequency modulation (FM)?Show answer
The message signal modulates the carrier frequency:
What is phase modulation (PM)?Show answer
The message signal modulates the carrier phase. Mathematically related to FM (PM and FM differ by an integration of the message). In analog form, PM is less common than FM standalone but is important as a building block for digital modulation schemes.
What is digital modulation?Show answer
Digital modulation uses discrete symbol states to carry binary data. The four main schemes:
What are engineering selection trade-offs?Show answer
The choice between AM and FM for the early-to-mid 20th century broadcast bands illustrates the modulation trade-offs.
What is bandwidth?Show answer
For a message of bandwidth B, AM uses bandwidth 2B (the message generates both upper and lower sidebands around the carrier). For voice (3 kHz) the AM channel needs about 6 kHz; broadcast AM is typically allocated 10 kHz.
What is noise immunity?Show answer
Poor. Atmospheric and electrical noise add to amplitude and are not separated from the message at the receiver. AM broadcast at night famously suffers from interference.
What is power efficiency?Show answer
Poor. Most of the transmitted power is in the carrier itself rather than the sidebands (the sidebands carry the information). Variants like SSB (single sideband) suppress the carrier and one sideband to save power and bandwidth.
What is complexity?Show answer
Low. AM receivers are simple (envelope detector); AM transmitters are simple.
What is used for?Show answer
Long-distance broadcast at HF/MF (because lower-frequency AM signals propagate via the ionosphere over long distances), aviation voice radio (typically AM at VHF for safety reasons).
What is aSK?Show answer
Two amplitude levels (typically 0 and full) represent 0 and 1. Simple but vulnerable to amplitude noise. Used in low-cost remote controls and short-range RF.
What is fSK?Show answer
Two frequencies represent 0 and 1. Better noise immunity than ASK. Used in early modems (Bell 103, V.21), some legacy paging systems.
What is pSK?Show answer
Discrete phase states represent symbols. BPSK uses 2 phases (0 and 180 degrees); QPSK uses 4 phases (0, 90, 180, 270 degrees). Used in Wi-Fi, cellular, satellite communications.
What is qAM?Show answer
Combines amplitude and phase. The constellation diagram has multiple symbol points (16-QAM uses 16 points, 64-QAM uses 64, 256-QAM uses 256). Each symbol carries log2(N) bits, so 16-QAM carries 4 bits per symbol.