CRAC Class A (2025) — Questions 531–540
U0530. If an amateur radio station operates at a DC voltage of 13.8 V, with an FM RF output power of N watts and a power-supply efficiency of about 80%, then the operating current while transmitting is approximately:
若一部业余无线电台的工作电压为直流13.8伏，FM方式的射频 输出功率为N瓦，电源效率约为80%，则发射时的工作电流约 为：
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DC power drawn = RF output ÷ efficiency = N / 0.8 W. Current I = P_dc / V = (N/0.8) / 13.8 = N / 11.04 ≈ 0.0906 × N A ≈ 0.091 × N A — option A. (The 0.058 figure would correspond to 220 V AC, i.e., U0531.) So A is correct.
U0531. If an amateur radio station operates at an AC voltage of 220 V, with an FM RF output power of N watts and a power- supply efficiency of about 80%, then the operating current while transmitting is approximately:
若一部业余无线电台的工作电压为交流220伏，FM方式的射频 输出功率为N瓦，电源效率约为80%，则发射时的工作电流约 为：
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I = (N/0.8) / 220 = N / 176 ≈ 0.00568 × N A ≈ 0.0057 × N A — option A. (Compare with 13.8 V DC giving 0.091×N.) So A is correct.
U0532. If an amateur radio station transmits FM at an RF output power of N watts with a power-supply efficiency of about 80%, then the electrical energy consumed per 10 seconds of transmitting is approximately:
若一部业余无线电台以FM方式发射时的射频输出功率为N瓦， 电源效率约为80%，则每发射10秒钟所消耗的电能约为：
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DC power drawn = N/0.8 W. Energy over 10 s = (N/0.8) × (10/3600) Wh = N × 10 / 2880 Wh = N × 0.003472 Wh = N × 0.000003472 kWh ≈ 0.0000035 × N kWh — option A. So A is correct.
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.
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.
U0537. The flatness of a receiver's frequency response when receiving SSB voice signals can be expressed by selectivity indicators, which are respectively:
接收机接收SSB话音信号时的频响均匀程度可以用选择性指标 来表示，分别为：
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For SSB voice, what matters is that the passband is flat (uniform response) so the voice is not muffled or tinny — described by in-band ripple (带内波动, passband flatness) and channel bandwidth. Option A is correct. The shape- factor/selectivity set (B) is more about adjacent-channel rejection; image (C) and front-end (D) are different metrics. Keep SSB as-is.
U0538. A receiver's ability to reject strong interference signals far from its operating frequency can be expressed by selectivity indicators, which are respectively:
接收机抗拒相距工作频率较远的强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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For strong signals far from the operating frequency, the relevant rejection is set by the front-end bandwidth (前端带宽) — the RF/preselector passband that keeps out off-band blockers before they overload the receiver. So A is correct. (Near-channel = B/C set; image = D.)
U0539. A receiver's ability to reject strong interference signals at a frequency twice the IF away from its operating frequency can be expressed by selectivity indicators, which are respectively:
接收机抗拒相距工作频率两倍于中频的强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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A signal at 2×IF from the operating frequency is the image frequency (镜像频率 = f ± 2×IF), so its rejection is the image rejection ratio (镜像抑制比). Option A is correct. This closes the four-part series (near = B/C, far = front-end, image = this).