Amateur Extra E8C: Digital signals and bandwidth
These are the 15 official questions in group E8C of the 2024–2028 Amateur Extra question pool, with the correct answer marked and a plain-English explanation of why it's right. The real exam asks one question from this group.
- E8C01Question 1
What is Quadrature Amplitude Modulation or QAM?
- AA technique for digital data compression used in digital television which removes redundancy in the data by comparing bit amplitudes
- BTransmission of data by modulating the amplitude of two carriers of the same frequency but 90 degrees out of phase Correct
- CA method of performing single sideband modulation by shifting the phase of the carrier and modulation components of the signal
- DA technique for analog modulation of television video signals using phase modulation and compression
Why: QAM modulates the amplitude of two same-frequency carriers 90° apart.
- E8C02Question 2
What is the definition of symbol rate in a digital transmission?
- AThe number of control characters in a message packet
- BThe maximum rate at which the forward error correction code can make corrections
- CThe rate at which the waveform changes to convey information Correct
- DThe number of characters carried per second by the station-to-station link
Why: Symbol rate is how fast the waveform changes to carry information.
- E8C03Question 3
Why should the phase of a PSK signal be changed at the zero crossing of the RF signal?
- ATo minimize bandwidth Correct
- BTo simplify modulation
- CTo improve carrier suppression
- DAll these choices are correct
Why: Changing phase at the zero crossing keeps the bandwidth small.
- E8C04Question 4
What technique minimizes the bandwidth of a PSK31 signal?
- AZero-sum character encoding
- BReed-Solomon character encoding
- CUse of sinusoidal data pulses Correct
- DUse of linear data pulses
Why: Sinusoidal data pulses keep PSK31 narrow.
- E8C05Question 5
What is the approximate bandwidth of a 13-WPM International Morse Code transmission?
- A13 Hz
- B26 Hz
- C52 Hz Correct
- D104 Hz
Why: CW bandwidth ≈ 4 × WPM, so 13 WPM ≈ 52 Hz.
- E8C06Question 6
What is the bandwidth of an FT8 signal?
- A10 Hz
- B50 Hz Correct
- C600 Hz
- D2.4 kHz
Why: An FT8 signal is 50 Hz wide.
- E8C07Question 7
What is the bandwidth of a 4,800-Hz frequency shift, 9,600-baud ASCII FM transmission?
- A15.36 kHz Correct
- B9.6 kHz
- C4.8 kHz
- D5.76 kHz
Why: 1.2 × 4,800 + 9,600 = 15,360 Hz = 15.36 kHz.
- E8C08Question 8
How does ARQ accomplish error correction?
- ASpecial binary codes provide automatic correction
- BSpecial polynomial codes provide automatic correction
- CIf errors are detected, redundant data is substituted
- DIf errors are detected, a retransmission is requested Correct
Why: ARQ requests a retransmission when it detects errors.
- E8C09Question 9
Which digital code allows only one bit to change between sequential code values?
- ABinary Coded Decimal Code
- BExtended Binary Coded Decimal Interchange Code
- CExtended ASCII
- DGray code Correct
Why: Gray code changes only one bit between neighboring values.
- E8C10Question 10
How can data rate be increased without increasing bandwidth?
- AIt is impossible
- BIncreasing analog-to-digital conversion resolution
- CUsing a more efficient digital code Correct
- DUsing forward error correction
Why: A more efficient code raises the data rate without more bandwidth.
- E8C11Question 11
What is the relationship between symbol rate and baud?
- AThey are the same Correct
- BBaud is twice the symbol rate
- CBaud rate is half the symbol rate
- DThe relationship depends on the specific code used
Why: Symbol rate and baud are the same thing.
- E8C12Question 12
What factors affect the bandwidth of a transmitted CW signal?
- AIF bandwidth and Q
- BModulation index and output power
- CKeying speed and shape factor (rise and fall time) Correct
- DAll these choices are correct
Why: Keying speed and rise and fall time set CW bandwidth.
- E8C13Question 13
What is described by the constellation diagram of a QAM or QPSK signal?
- AHow many carriers may be present at the same time
- BThe possible phase and amplitude states for each symbol Correct
- CFrequency response of the signal stream
- DThe number of bits used for error correction in the protocol
Why: A constellation diagram shows every phase and amplitude state a symbol can take.
- E8C14Question 14
What type of addresses do nodes have in a mesh network?
- AEmail
- BTrust server
- CInternet Protocol (IP) Correct
- DTalk group
Why: Mesh nodes use IP addresses.
- E8C15Question 15
What technique do individual nodes use to form a mesh network?
- AForward error correction and Viterbi codes
- BActing as store-and-forward digipeaters
- CDiscovery and link establishment protocols Correct
- DCustom code plugs for the local trunking systems
Why: Nodes form a mesh using discovery and link-establishment protocols.