CRAC Class C (2025) — Questions 501–510

MODE selects the emission/operating mode (工作方式) — FM, AM, SSB (LSB/USB), CW, RTTY, etc. Option A is correct. Squelch modes (B) are set elsewhere (CTCSS/DCS menus); receiver architecture (C) and audio monitoring (D) are not what the MODE knob does.

ATT = input attenuator (衰减器), a switchable pad at the receiver front end that reduces a very strong incoming signal so the first stages do not overload (过载/阻塞). Option A is correct. B is AT/TUNE, C is ALC, D is transmit audio AVC.

AGC = Automatic Gain Control (自动增益控制). In a receiver it samples the IF/Detection level and adjusts the gain so strong and weak stations come out at a steady audio level and the stages don't overload. Option A is correct. B describes audio AVC (not the standard AGC definition), C is AT/TUNE, D is transmit ALC.

AGC only acts after the front-end amplifier; a very strong signal that arrives before the AGC-controlled stages can drive the first mixer/RF amp into nonlinear operation, causing blocking, cross-modulation and intermodulation (互调). The ATT pad sits ahead of that stage and tames the strong signal before it overloads the front end. Option A is correct. B (gain range), C (speaker protection), D (TX self-damage) are not the real reason.

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.