CRAC Class B (2025) — Questions 791–800
U0533. The average power supplied to the antenna feeder by a radio transmitter over one RF cycle under no-modulation conditions is called:
无线电发信机在无调制情况下，在一个射频周期内供给天线馈 线的平均功率称为：
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Under no modulation, the unmodulated output is the carrier; its average power delivered to the feeder is the carrier power (载波功率). Average power (C) and PEP (D) are modulation- dependent measures; "useless power" (B) is not a standard term here. So A is correct.
U0534. If a voice transmitter has the same peak output power under different modulation modes, then with no voice input, the actual RF output power from large to small can be ranked as:
如果某话音发射机在不同调制方式下的峰值输出功率相同，则 在无语音输入时，实际射频输出功率由大到小可以排序为：
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With no audio: FM keeps full carrier power (constant envelope) → highest. AM's carrier is only part of total power (the rest is sidebands that vanish with no modulation) → lower than FM. SSB suppresses the carrier and one sideband, so with no modulation there is essentially no output → lowest. Ranking: FM > AM > SSB — option A. Keep FM, AM, SSB as-is.
U0963. The necessary bandwidth is: for a given emission class, the bandwidth just sufficient to ensure the required quality of information transmission at the corresponding rate and under the specified conditions. The necessary bandwidths of amateur single sideband (SSB) voice, low-speed Morse code (CW), frequency-modulation (FM) voice, and vestigial- sideband amateur television (VSB ATV) are respectively:
必要带宽（necessary bandwidth）是指：对给定的发射类别 而言，其恰好足以保证在相应速率及在指定条件下具有所要求 质量的信息传输的所需带宽。业余电台单边带话音通信SSB、 低速莫尔斯电码通信CW、调频话音通信FM和残余边带业余电 视VSB ATV的必要带宽分别是：
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The necessary bandwidth (必要带宽) for typical amateur emissions: SSB voice ≈ 3000 Hz (often 2.7 kHz channel), CW ≈ 400 Hz (narrow keying), FM voice ≈ 12.5 kHz (channel spacing), and vestigial-sideband ATV (残余边带业余电视) ≈ >5 MHz (wide video). The order in A matches SSB, CW, FM, ATV exactly.
U0964. For Morse-code (CW) communication at a speed of 25 WPM, accounting for propagation fading and similar factors, the necessary bandwidth is normally not greater than:
以CW方式进行速度为25WPM的摩尔斯电码通信，如果考虑传 播衰落等因素，必要带宽通常不大于：
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CW (等幅电报 / continuous wave) at 25 WPM has a very narrow necessary bandwidth — roughly keying rate dependent, typically about 100–200 Hz. Allowing for fading and practical filter skirts, it is normally within 200 Hz. 2700 Hz is an SSB voice figure; 6.25/12.5 kHz are digital/FM figures.
U0965. For RTTY communication using single sideband, at a speed of 50 baud with a frequency shift of 170 Hz, the necessary bandwidth is normally not greater than:
用单边带方式进行RTTY通信，速度为50波特，频偏为 170Hz，必要带宽通常不大于：
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RTTY (无线电传) at 50 baud / 170 Hz shift: the necessary bandwidth is approximately the shift plus twice the baud rate (170 + 2×50 ≈ 270 Hz) plus filter margins, commonly taken as about 450 Hz. So 450 Hz (A) is correct; 200 Hz is CW-like and too narrow, while 2700/12.5 kHz are voice/FM figures.
U0966. For single-sideband voice communication that intermittently also transmits RTTY messages at 50 baud with a 70 Hz shift, the necessary bandwidth is approximately:
用单边带方式进行话音通信并间或传输速度为50波特，频偏为 70Hz的RTTY消息，必要带宽约为：
Exam tips · 考试提示
Since the channel is primarily SSB voice, the necessary bandwidth is governed by the voice component (~2700 Hz). The occasional narrow 50-baud/70 Hz RTTY (≈70 + 2×50 ≈ 170 Hz) easily fits inside the SSB passband, so the overall necessary bandwidth is about 2700 Hz, not the narrow RTTY figure. Hence A.
U0967. For ordinary frequency-modulation (FM) voice communication, the necessary bandwidth is approximately:
用通常的调频方式进行话音通信，必要带宽约为：
Exam tips · 考试提示
Conventional FM (调频) voice, with ±5 kHz deviation and 3 kHz audio, occupies about 12.5 kHz of necessary bandwidth (the standard narrowband FM channel). SSB voice is 2700 Hz (B), CW ~200 Hz (C); 6.25 kHz (D) is a narrower digital channel. So 12.5 kHz is correct for typical FM.
U0968. A commercial amateur single-sideband transceiver has a voice channel of 300–3000 Hz, with the high audio frequencies strongly attenuated. If one wishes to modify the equipment to extend the signal bandwidth to 30–16000 Hz and greatly improve signal quality, the feasible approach is:
商用业余单边带电台的语音信号通道为300-3000Hz，高音频 部分衰减很大。如果希望对设备进行改装，以将信号带宽扩展 到30-16000Hz，大大提升信号质量。可行的方案是：
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The necessary bandwidth (必要带宽) for the amateur service is limited by regulation; widening SSB to 30–16000 Hz would exceed the allowed bandwidth and cause harmful interference (有害干扰) to adjacent channels. Therefore the correct approach is to abandon the modification (A). Options B–D are technical tweaks that would still violate the bandwidth limit.
U0535. Which of the following technical indicators describes a receiver's ability to reject adjacent-frequency interference?
下列哪项技术指标描述了接收机抗拒邻近频率干扰的能力？
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Adjacent-frequency rejection is governed by the receiver's selectivity (选择性), particularly the IF filter's ability to pass the wanted channel and reject nearby frequencies — hence "IF selectivity" (中频选择性). Sensitivity (B) is weak-signal ability; scan rate (C) and background noise (D) are unrelated. So A is correct.
U0536. A receiver's ability to reject interference signals near its operating frequency can be expressed by selectivity indicators, which are respectively:
接收机抗拒工作频率附近干扰信号的能力可以用选择性指标来 表示，分别为：
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Near-in-frequency rejection (adjacent-channel) is described by channel bandwidth, channel selectivity, and the filter's rectangular coefficient (shape factor) — option A. In-band ripple (B) describes passband flatness, image rejection (C) is for 2×IF signals, and front-end bandwidth (D) addresses far-away strong signals. So A is correct.