Amateur Extra E8A: Waveforms and analog-to-digital conversion
These are the 11 official questions in group E8A 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.
- E8A01Question 1
What technique shows that a square wave is made up of a sine wave and its odd harmonics?
- AFourier analysis Correct
- BVector analysis
- CNumerical analysis
- DDifferential analysis
Why: Fourier analysis shows a square wave is a sine wave plus its odd harmonics.
- E8A02Question 2
Which of the following is a type of analog-to-digital conversion?
- ASuccessive approximation Correct
- BHarmonic regeneration
- CLevel shifting
- DPhase reversal
Why: Successive approximation is a type of ADC.
- E8A03Question 3
Which of the following describes a signal in the time domain?
- APower at intervals of phase
- BAmplitude at different times Correct
- CFrequency at different times
- DDiscrete impulses in time order
Why: The time domain shows amplitude at different times.
- E8A04Question 4
What is “dither” with respect to analog-to-digital converters?
- AAn abnormal condition where the converter cannot settle on a value to represent the signal
- BA small amount of noise added to the input signal to reduce quantization noise Correct
- CAn error caused by irregular quantization step size
- DA method of decimation by randomly skipping samples
Why: Dither is a little noise added to reduce quantization noise.
- E8A05Question 5
What is the benefit of making voltage measurements with a true-RMS calculating meter?
- AAn inverse Fourier transform can be used
- BThe signal’s RMS noise factor is also calculated
- CThe calculated RMS value can be converted directly into phasor form
- DRMS is measured for both sinusoidal and non-sinusoidal signals Correct
Why: A true-RMS meter measures RMS correctly for sine and non-sine signals.
- E8A06Question 6
What is the approximate ratio of PEP-to-average power in an unprocessed single-sideband phone signal?
- A2.5 to 1 Correct
- B25 to 1
- C1 to 1
- D13 to 1
Why: Unprocessed SSB voice has a PEP-to-average ratio of about 2.5:1.
- E8A07Question 7
What determines the PEP-to-average power ratio of an unprocessed single-sideband phone signal?
- AThe frequency of the modulating signal
- BSpeech characteristics Correct
- CThe degree of carrier suppression
- DAmplifier gain
Why: The characteristics of speech set SSB's PEP-to-average ratio.
- E8A08Question 8
Why are direct or flash conversion analog-to-digital converters used for a software defined radio?
- AVery low power consumption decreases frequency drift
- BImmunity to out-of-sequence coding reduces spurious responses
- CVery high speed allows digitizing high frequencies Correct
- DAll these choices are correct
Why: Flash converters are very fast, so they can digitize high frequencies.
- E8A09Question 9
How many different input levels can be encoded by an analog-to-digital converter with 8-bit resolution?
- A8
- B8 multiplied by the gain of the input amplifier
- C256 divided by the gain of the input amplifier
- D256 Correct
Why: 2⁸ = 256 levels.
- E8A10Question 10
What is the purpose of a low-pass filter used at the output of a digital-to-analog converter?
- ALower the input bandwidth to increase the effective resolution
- BImprove accuracy by removing out-of-sequence codes from the input
- CRemove spurious sampling artifacts from the output signal Correct
- DAll these choices are correct
Why: The low-pass filter removes sampling artifacts from a DAC's output.
- E8A11Question 11
Which of the following is a measure of the quality of an analog-to-digital converter?
- ATotal harmonic distortion Correct
- BPeak envelope power
- CReciprocal mixing
- DPower factor
Why: Total harmonic distortion is a measure of ADC quality.