Amateur Extra · correct answers only, in question-pool order.
Fourier analysis decomposes a periodic waveform into sinusoidal components. A square wave is represented by its fundamental sine wave plus odd harmonics.
Successive approximation is one standard technique for converting an analog voltage into a digital value.
A time-domain representation describes how a signal's amplitude changes as time progresses.
Dither is a small amount of deliberately added noise that can reduce quantization artifacts in analog-to-digital conversion.
A true-RMS meter calculates RMS value for both sinusoidal and non-sinusoidal signals rather than assuming a sine-wave relationship.
The Extra pool gives an approximate peak-envelope-power to average-power ratio of 2.5 to 1 for unprocessed SSB phone.
The instantaneous peaks and average power of unprocessed SSB speech depend primarily on the characteristics of the speech waveform.
Direct or flash analog-to-digital converters are used in SDR applications because their very high conversion speed allows high-frequency signals to be digitized.
An n-bit converter can represent 2^n levels. With 8 bits, 2^8 equals 256 possible input codes.
A low-pass reconstruction filter at a digital-to-analog converter output removes unwanted high-frequency sampling artifacts from the analog waveform.
Total harmonic distortion is one measure of how faithfully an analog-to-digital converter reproduces an input signal without adding nonlinear distortion.
For FM, modulation index is the ratio of peak frequency deviation to the modulating signal frequency.
For a phase-modulated emission, modulation index is determined by the phase variation, not by the RF carrier frequency itself.
FM modulation index is deviation divided by modulating frequency: 3000 Hz / 1000 Hz = 3.
With 6 kHz peak deviation and 2 kHz highest modulating frequency, the modulation index is 6/2 = 3.
Deviation ratio is maximum carrier deviation divided by the highest audio modulating frequency. 5/3 is about 1.67.
Using maximum deviation divided by highest modulating frequency gives 7.5/3.5, approximately 2.14.
Orthogonal frequency-division multiplexing is a digital communications technique that distributes information across multiple coordinated subcarriers.
OFDM uses many subcarriers whose frequency spacing is selected so their spectra remain mathematically orthogonal, allowing efficient overlap.
Deviation ratio compares the maximum FM carrier frequency deviation with the highest modulating audio frequency.
Frequency-division multiplexing divides available spectrum into separate frequency channels, each carrying a different information stream.
Digital time-division multiplexing lets multiple signals share one channel by assigning each one discrete intervals in time.
Quadrature Amplitude Modulation carries data by modulating two carriers of the same frequency that are 90 degrees out of phase.
Symbol rate is the rate at which the transmitted waveform changes state to convey information.
Changing PSK phase at an RF zero crossing reduces abrupt waveform discontinuities and therefore helps minimize occupied bandwidth.
PSK31 uses shaped, sinusoidal data pulses to reduce abrupt transitions and keep the transmitted spectrum narrow.
The Extra pool gives approximately 52 Hz as the bandwidth of a 13-WPM International Morse Code transmission.
The Extra pool identifies an FT8 signal as occupying about 50 Hz of bandwidth.
For the specified 9,600-baud ASCII FM signal with a 4,800-Hz shift, the Extra pool gives 15.36 kHz occupied bandwidth.
Automatic Repeat reQuest detects an error and asks the sending station to retransmit the affected data.
Adjacent Gray-code values differ by only one bit, reducing ambiguity during transitions.
A more efficient digital code can carry more useful information within the same symbol-rate and bandwidth constraints.
Baud is the number of symbols transmitted per second, so baud rate and symbol rate refer to the same quantity.
Faster keying and sharper keying transitions create more high-frequency spectral components, increasing CW bandwidth.
A QAM or QPSK constellation diagram shows the allowed symbol states in terms of phase and, where applicable, amplitude.
The Extra pool identifies Internet Protocol addresses as the addressing method used by nodes in an amateur mesh network.
Individual mesh nodes find neighboring nodes and establish links using discovery and link-establishment protocols.
A spread-spectrum receiver correlates for the expected spreading sequence, so unrelated signals are suppressed relative to the desired signal.
Direct-sequence spread spectrum uses a high-rate pseudorandom binary sequence to rapidly alter the phase of the RF carrier.
Frequency-hopping spread spectrum moves the transmitted carrier rapidly among frequencies according to a pseudorandom sequence.
Extremely short CW rise and fall times contain strong high-frequency components that create audible key clicks and broaden the transmitted spectrum.
Slowing the CW keying waveform edges reduces abrupt transitions and suppresses the spectral energy that causes key clicks.
A parity bit adds a simple consistency check to a character, allowing certain transmission errors to be detected.
Driving the transmitter with too much AFSK audio causes excessive modulation and signal distortion.
Intermodulation distortion is used to evaluate distortion in an AFSK signal caused by excessive audio drive.
The Extra pool considers -30 dB an acceptable maximum intermodulation-distortion level for an idling PSK signal.
Baudot uses five data bits and separate letters/figures shift characters, while ASCII uses seven or eight data bits and does not use those shifts.
One advantage of ASCII over older codes such as Baudot is that it directly supports both uppercase and lowercase letters.