CRAC Class C (2025) — Questions 1151–1160

A window function (窗函数), such as Hamming (汉明), raised cosine (升余弦), or Blackman (布莱克曼), is multiplied with the sampled data before performing the FFT (a fast algorithm for the discrete Fourier transform). Its primary purpose is to taper the data block and reduce spectral leakage (谱泄漏) caused by the discontinuity at the block edges, so the true shape of the signal's power spectrum is revealed (A). A good window also sharpens the measured peaks, improving the precision of amplitude and frequency estimates and thus the observation accuracy (B). C is wrong because windowing does not change the display-window width — that is a screen/span setting; D is wrong because windowing is a general spectral-analysis tool, not a technique specific to keyed modes like CW or RTTY. 测量与仪表 Measurement and Instruments

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.

示波器的交流耦合 (AC coupling) inserts a series capacitor that blocks any DC component, so the displayed waveform is centered on zero — which is why the equal-amplitude (等幅) audio signal appeared symmetric about positive and negative. When the coupling is switched to 直流档 (DC coupling), the DC path is restored and the trace shifts vertically by an amount equal to the DC offset. A significant vertical shift therefore means a DC component is present in the signal itself (A). B is wrong because the oscilloscope does not add a bias — it merely stops removing the existing one; C (poor frequency response) and D (probe-to-ground stray capacitance) would not cause a simple steady vertical offset between AC and DC coupling.