CRAC Class A (2025) — Questions 631–640

A DC voltmeter reads the average (mean) value. A bipolar symmetric square wave that spends equal time at +V and −V has an average of zero, so the reading is zero (A).

A moving-coil AC meter rectifies the waveform and is calibrated to read the RMS value of a sine wave (using the sine form factor ≈1.11). A square wave has a rectified average equal to its peak (Vpk); multiplying by 1.11 yields about 1.11·Vpk, which exceeds the peak (A). It is neither the peak (B), peak-to- peak (C), nor the average (D).

A 0-to-Vpk square wave at 50% duty has an average = Vpk/2. A DC meter reads this average. For this waveform the RMS value also equals Vpk/2 (since RMS² = (Vpk²·0.5 + 0²·0.5) = Vpk²/2). So the reading is half the peak and equals the RMS value (A).

An ohmmeter supplies a small current to measure resistance. If a large capacitor is across the circuit, it initially acts like a short (low reading) while charging, then its impedance rises as it fills, so the reading climbs toward infinity (A). A large inductor would instead show a rising-then-settling or initially high reading, not this pattern; an AC source or fault (C/D/B) does not explain the gradual rise.

A sound electrolytic capacitor can be checked with an analog ohmmeter (it should show a charging swing to high resistance, A) and, more directly, with a digital multimeter's capacitance function if available (B). A current range (C) is unsuitable, and a charger plus short-circuit test (D) is dangerous and not a proper test.

The behavior — an initial charging deflection on first connection, then settling, and little or no movement on a repeat connection (the component having retained charge) — is consistent with an electrolytic capacitor measured on an ohmmeter (A). The other options make absolute ("肯 定/definitely") claims that are not supported and are incorrect as the intended answer.

Ohms-per-volt = the meter's input resistance per volt of range; a higher figure means higher input resistance, so the meter draws less current and disturbs the circuit less while measuring voltage (A). Lower ohms/volt means more loading (B wrong); it does not describe overload strength (C/D wrong).

A pure AC/audio signal has zero average, so a DC voltmeter would read zero and swapping leads would not change a zero reading. A non-zero reading that flips polarity when the leads are swapped proves a DC offset (a DC component) is present on top of the audio (A). Harmonics, bandwidth, or lead capacitance would not produce this polarity-reversing DC reading. 业余无线电台相关技术指标与知识 5 C H A P T E R Amateur-Station Technical Indicators and Related Knowledge 本章聚焦发射设备的国家技术指标要求与无线电设备相关标准。This chapter focuses on the national technical-indicator requirements for transmitting equipment and related radio-device standards. 发射设备技术指标 Transmitter Technical Indicators

National rules for amateur transmitting equipment specifically mandate frequency tolerance (频率容限) and spurious-domain emission power (杂散域发射功率) limits (A). While out-of-band emission (C) is regulated generally, the exam's paired correct phrase is "frequency tolerance and spurious emission power"; modulation deviation/index (B) and allotted band/necessary bandwidth (D) are not the stated mandatory pair.

The required indicators are that the equipment meets the frequency tolerance and that its spurious emission stays within the maximum permissible power level (A). The distractors wrongly say spurious emission should be "not lower than" (i.e., allowed to exceed) the limit, which is the opposite of the rule.