CRAC Class C (2025) — Questions 511–520

In an SSB transmitter, ALC limits the drive to the final amplifier so speech peaks do not over-drive it (which would cause splatter/distortion and possible damage). Option A is correct. Frequency stability (B) is set by the oscillator/synthesizer; matching (C) is the tuner; background noise (D) is a mic-gain issue.

Background noise heard in the speech gaps means the mic is too sensitive and is picking up room noise during pauses (and/or the compressor is boosting it). Lowering mic gain, and reducing compressor (PROC) depth if present, raises the noise floor relative to speech so the gaps quieten. Option A is correct. Power (B), antenna matching (C), and ALC depth (D) do not address room noise in the gaps.

ALC limits the average RF drive but cannot perfectly clean up a severely over-driven speech waveform — excessive mic gain still flattens/clips the audio (distortion) before ALC can fully compensate, producing splatter. Option A is correct. Over-gain does not raise power (B, ALC caps it), change frequency stability (C), or alter SWR (D, which is set by the antenna).

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.