CRAC Class B (2025) — Questions 1041–1050
U0637. Using a digital multimeter's DC voltage range to measure the voltage of a constant-amplitude audio signal between two points in a circuit, it is found that the reading is not zero. After swapping the two test leads and measuring again, the polarity of the reading reverses. The correct inference is:
用数字万用表的直流电压档测量电路两点间某个等幅音频信号 的电压，发现读数不为零。交换两支表笔之后再测，所得读数 的极性反转。正确的推论是：
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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.
U1109. Among the following charts, the one most easily used to express and explain the principle of PSK modulation is:
下列几种图表中，最容易用来表达和解释PSK调制原理的是：
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PSK (phase-shift keying, 相移键控) encodes data by changing the carrier phase, so a phase vector diagram / constellation (相 位矢量图) most directly shows the discrete phase states. Waveform/spectrum plots are less intuitive for phase states.
U1110. Among the following charts, the one most easily used to express and explain the principle of analog FM modulation is:
下列几种图表中，最容易用来表达和解释模拟FM调制原理的 是：
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Per the official answer key, a spectrum diagram (频谱图) is taken as the chart that most readily expresses and explains analog FM (频率调制) — it shows how the instantaneous frequency deviation spreads the signal into sidebands around the carrier. (Note: a time-domain waveform can also illustrate FM, but the key marks the spectrum as the intended answer.)
U1111. Among the following charts, the one most easily used to express and explain the principle of SSB modulation is:
下列几种图表中，最容易用来表达和解释SSB调制原理的是：
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SSB (single sideband, 单边带) is best shown on a spectrum diagram (频谱图): it visibly demonstrates that the carrier is suppressed and only one sideband is transmitted, compared with the double-sideband AM spectrum.
U1112. Among the following charts, the one most easily used to express and explain the principle of RTTY modulation is:
下列几种图表中，最容易用来表达和解释RTTY调制原理的是：
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RTTY (radioteletype, 无线电传) uses frequency-shift keying (FSK) — two tones shifting with the mark/space bits. A frequency–time curve (频率-时间曲线) most directly shows the frequency switching between the two states.
U1113. Among the following charts, the one most easily used to express and explain the principle of SSTV modulation is:
下列几种图表中，最容易用来表达和解释SSTV调制原理的是：
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SSTV (slow-scan television, 慢扫描电视) encodes each scan line as a tone whose frequency varies with the video brightness — a frequency-modulated audio sub-signal. A frequency–time curve (频率-时间曲线) best shows how brightness maps to frequency over time.
U1114. Among the following charts, the one most easily used to express and explain the difference in operating points of Class A, B, and
下列几种图表中，最容易用来表达和解释A、B、C类放大器工 作点的差别的是：
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Class A/B/C differ by conduction angle (360° / 180° / <180°). A waveform diagram (波形图) most directly shows how much of the input cycle the output follows — the clearest way to contrast the three classes.
U1115. Among the following charts, the one most easily used to express and explain the degree of temperature drift of an LC oscillator is:
下列几种图表中，最容易用来表达和解释LC振荡器温度漂移程 度的是：
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Temperature drift is a slow change of frequency over time. A spectrum waterfall diagram (频谱瀑布图) plots frequency vs. time with intensity, so a drifting LC oscillator shows a slanted trace — the best visualization of drift magnitude.
U1116. The illustrated circuit is a simple field-strength meter commonly used by amateur radio operators. The functions of the components are respectively:
图示电路为业余无线电爱好者常用的简易场强表。各部件的作 用分别为：
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In the classic diode field-strength meter, the antenna (A) picks up the wave, the L-C1-C2 tank performs resonant selection (谐 振选频), the diode (VD) detects/demodulates (检波) the RF to DC, C3 bypass-filters the reading, and the meter (M) indicates. Option A matches this signal chain; B/C/D misassign amplification/rectification roles a passive diode circuit cannot perform.
U1117. Using a small dipole antenna about 10 cm per arm in parallel with a crystal diode and a DC microammeter to make a simple RF field-strength meter. Regarding the choice of silicon versus germanium diode, the correct consideration should be:
用一副臂长约10cm的小偶极天线并联一个晶体二极管和直流微 安表，做一个简单的射频场强表。关于选用硅二极管还是锗二 极管，正确的考虑应当是：
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A germanium diode's lower forward voltage (~0.3 V) lets it begin conducting (detecting) at weaker RF fields than a silicon diode (~0.7 V), making the meter more sensitive. B is false (turn-on voltages differ); C's reverse-breakdown point is irrelevant to sensitivity; D is false — the simple circuit works.