CRAC Class B (2025) — Questions 551–560

Receiver overload (过载/阻塞) means an excessively strong input drives the front-end stages into saturation/blocking, which can even damage components; engaging the input attenuator (ATT) reduces the drive and cures or relieves it. So A and B are correct. C describes a power-supply overload, a different thing; D — turning down the speaker volume does nothing to the RF overload, since the overload happens before the audio stage.

When a weak desired signal coexists with a strong interferer, AGC tracks the strong signal and turns the receiver gain down, pulling the weak signal even further into the noise — so you disable AGC (or use a fast/separate scheme) to keep the weak signal audible. Option A is correct. For a plain weak signal (B) or strong signal (C) AGC normally helps; D is not the classic case for disabling it.

For constant-envelope signals (FM, FSK, PSK) the carrier amplitude is steady, so a short AGC time constant keeps the level tightly regulated without "pumping" the audio. Option A is correct. SSB/AM (B) have a varying voice envelope, so a long time constant is used to avoid breathing/distortion; C and D are unrelated to the constant-envelope rule.

SSB and AM voice have a continuously varying envelope; a long AGC time constant averages it out so the audio volume stays steady instead of "pumping" with every syllable. Option A is correct. Constant-envelope FM/FSK/PSK (B) want short AGC; C (DF) and D are not the envelope-variation case.

PRE = receiver preamplifier (前置放大器 / LNA), a low-noise amplifier at the front end that boosts weak signals. Option A is correct. B is AT/TUNE, C is ALC, D is PROC/compression.

ALC = Automatic Level Control (自动电平控制) on the transmit side: it samples the RF output and feeds back to the driver stage to keep PEP/RF output from over-driving the final amplifier, preventing distortion and damage. Option A is correct. B is audio AVC; C is AT/TUNE; D is AFC.

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.