CRAC Class A (2025) — Questions 421–430

AFSK (Audio Frequency Shift Keying, e.g. for RTTY/PSK) is a constant-data tone pair whose envelope must stay clean. The speech-derived ALC loop was designed for voice and will pump/limit the AFSK envelope, distorting the shift and raising the bit error rate (误码率, BER). So ALC is disabled for data. Option A is correct. B (power runaway), C (polarity reversal) and D (timbre) are not the issue.

Speech compression (语音压缩 / speech processing) reduces the peaks and lifts the troughs of the voice waveform so the average level — and thus the perceived loudness/SNR at the far end — is higher without exceeding the ALC limit. Option A is correct. B is the opposite (it reduces dynamic range, not increases it usefully); C is equalization, not compression; D is untrue — compression does not shrink occupied bandwidth and can even widen it if overdone.

PROC = speech processor/compression (语音压缩) on transmit. It boosts low-level speech so the average modulation is higher. Option A is correct. B is PRE, C is AT/TUNE, D is ALC.

Over-driving the speech processor generates heavy harmonics and intermodulation products (互调成分) that spread outside the channel, degrading the other station's ability to copy you and causing adjacent-channel interference. Option A is correct. B is wrong (distortion does hurt copy); C (hum) is a ground/PSR issue, not compression; D — a processor will not damage the mic.

AFSK data tones need a clean, stable envelope; speech compression reshapes/limits that envelope, distorting the mark/space transitions (baseband) and raising the bit error rate (误码率). Hence compression must be off for data. Option A is correct. B and C are false (compression doesn't remove phase or all amplitude); D is backwards — compression lowers peak- to-average, not raises it dangerously.

Measuring by actually transmitting radiates a signal, so to avoid causing harmful interference (有害干扰) you must do it on a clear, unused frequency away from busy calling/simplex frequencies. Option A is correct. B (point north) is irrelevant; C (turn on compression) is wrong for a clean carrier measurement; D's specifics are not the required rule.

To measure SWR you need an unmodulated (or steady) carrier, not voice. The courteous method is CW key-down, or AM/FM with PTT held but silent (no speech), giving a clean continuous carrier, then return to SSB. Option A is correct. Whistling or voicing (B/C/D) sends modulated, intelligible — and thus potentially interfering — audio onto the air, which is neither clean nor courteous.

AT / TUNE = automatic antenna tuner (自动天线调谐): it matches the transmitter to the antenna by sensing the SWR and adjusting a matching network for the lowest SWR. Option A is correct. B is transmit AVC; C is ALC; D is AFC.

With separate RF/IF gain and audio gain, the best practice is: for a very weak signal maximize RF/IF gain (to keep the best SNR before the noise floor) and use audio gain only for comfort; for a very strong signal you can max audio gain but must reduce RF/IF gain (or use ATT) to avoid front-end overload, then set the other control for volume. Option A captures this. B reverses the weak-signal rule (max audio on weak signal just raises hiss); C/D's fixed-middle rules ignore the weak/strong distinction.

The defining feature of HF (short wave, 3–30 MHz) is skywave propagation — signals refract off the ionosphere (电离层) and return to Earth, enabling intercontinental communication. VHF/UHF are essentially line-of-sight. Option A is correct. B is false (HF fading is often severe); C is false (HF antennas are larger, e.g. 40 m dipole ≈ 20 m); D is false (HF channels are narrow, not broadband).