CRAC Class B (2025) — Questions 801–810
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：
Exam tips · 考试提示
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：
Exam tips · 考试提示
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：
Exam tips · 考试提示
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).
U0540. What is the significance of the magnitude of a receiver's sensitivity indicator?
接收机灵敏度指标的数值大小具有什么意义？
Exam tips · 考试提示
Sensitivity is expressed as a minimum discernible signal (e.g., μV or dBm). A smaller number means the receiver can detect weaker signals — so smaller = better weak-signal capability (灵 敏度越高). Options B/C/D invert or misapply the meaning. So A is correct.
U0541. What is the significance of expressing a receiver's sensitivity in terms of power level, and what are the common units?
用功率电平表示接收机的灵敏度具有什么意义？常用单位是什 么？
Exam tips · 考试提示
Power-level sensitivity = the minimum input power (at the antenna port) needed for a specified output quality, expressed in dBm or dBμW (功率电平单位). It is not a ratio (B/C/D are ratios/gains). So A is correct.
U0542. What is the significance of expressing a receiver's sensitivity in terms of voltage level, and what are the common units?
用电压电平表示接收机的灵敏度具有什么意义？常用单位是什 么？
Exam tips · 考试提示
Voltage-level sensitivity is defined as the minimum electromotive force (电动势, open-circuit EMF of the signal source) at the input needed for a specified output quality, in μV, dBμV or dBmV. The exam key distinguishes "电动势" (EMF, A) from mere "电压" (B); the standard definition uses the source EMF into the specified input impedance. So A is correct (and C is wrong because it is minimum, not maximum).
U0543. Most amateur radio receivers have a signal-strength indication function. In the VHF/UHF bands, the minimum scale S1 corresponds to an input-signal power level of −141 dBm (a voltage level of 0.02 μV across 50 Ω); whereas in the HF band, S1 corresponds to an input signal of −121 dBm (0.2 μV across 50 Ω). This is because:
业余无线电接收机大多具备信号强度指示功能。在VHF/UHF频 段，信号强度的最小刻度S1与输入信号功率电平-141dBm （50Ω上的电压电平0.02μV）相一致；而在HF频段，S1则 与-121dBm的输入信号（50Ω上的0.2μV）相对应。这是因 为：
Exam tips · 考试提示
HF bands carry much higher natural/atmospheric and man- made background noise than VHF/UHF. Because the receiver's noise floor is higher on HF, the smallest *useful/perceivable* signal is about 20 dB stronger there, so the S-meter's S1 reference is set higher (−121 dBm on HF vs −141 dBm on VHF/UHF). Option A captures this. B is a misstatement (HF receivers aren't inherently 20 dB less sensitive); C and D are irrelevant. Keep VHF/UHF, dBm, μV, Ω, HF, DX.
U0544. Two models of amateur transceivers,
甲乙两种型号的业余无线电收发信机在技术规格中给出的接收 机灵敏度指标分别为0.1 μV和0.15μV。关于该指标，正确的推 论是：
Exam tips · 考试提示
A smaller μV number normally means better sensitivity, but only if measured to the same output-quality criterion (e.g., same SINAD). Since the specifications do not state the quality standard, you cannot validly compare the two. So A is correct. B and C jump to conclusions; D confuses sensitivity with strong-signal handling (dynamic range). Keep μV.
U0545. When the output signal meets the specified quality standard, the minimum input-signal voltage measured at the antenna input port of a receiver with 50 Ω input impedance is 1 μV. If the sensitivity is expressed in voltage level μV, it is:
当输出信号满足规定质量标准时，在输入阻抗为50欧的某接收 机天线输入端口处测得的输入信号最小电压为1μV。如果用电 压电平μV表示该机的灵敏度，则为：
Exam tips · 考试提示
When sensitivity is given directly in "voltage level μV", it is simply the minimum input voltage itself — 1 μV. The 50 Ω impedance and the "voltage level μV" unit mean no conversion is needed; the answer is B (1 μV). Keep Ω, μV.
U0546. When the output signal meets the specified quality standard, the minimum input-signal voltage measured at the antenna input port of a receiver with 50 Ω input impedance is 1 μV. If the sensitivity is expressed in power level dBm, it is:
当输出信号满足规定质量标准时，在输入阻抗为50欧的某接收 机天线输入端口处测得的输入信号最小电压为1μV。如果用功 率电平dBm表示该机的灵敏度，则为：
Exam tips · 考试提示
Using the Chinese exam's voltage-level convention, sensitivity in dBm is computed from the voltage across 50 Ω as P = V² / R = (1×10⁻⁶)² / 50 = 2×10⁻¹⁴ W; dBm = 10·log₁₀(2×10⁻¹⁴ / 10⁻³) = 10·log₁₀(2×10⁻¹¹) ≈ −107 dBm. So A is correct. (Note: this uses V²/R, the voltage-level convention; the alternative EMF formula V²/4R would give −113 dBm, which is option D — but the exam key uses the V²/R form, giving −107 dBm.) Keep Ω, μV, dBm.