CRAC Class A (2025) — Questions 551–560
U0550. When the output signal meets the specified quality standard, the minimum input-signal voltage measured at the antenna input port of a receiver with a 50-ohm input impedance is 0.5 μV. If this receiver's sensitivity is expressed as a voltage level in dBμV, 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0.5μV。如果用 电压电平dBμV表示该机的灵敏度，则为：
Exam tips · 考试提示
A receiver's sensitivity is the minimum usable input signal; dBμV is a voltage level referenced to 1 μV (dBμV = 20·log10(V/1 μV)). The official answer key marks A. Note for study: 0.5 μV computes to 20·log10(0.5) ≈ −6 dBμV, which is not among the listed options; the marking follows the source key, so memorize the key's choice rather than the raw arithmetic here.
U0551. Given a receiver whose antenna input impedance is 50 Ω and whose sensitivity specification is 2 μV, the corresponding power level is:
已知某接收机的天线输入阻抗为50Ω，灵敏度指标为2μV，则 功率电平相应为：
Exam tips · 考试提示
Converting a voltage sensitivity to a power level uses P = V²/R and dBm = 10·log10(P/1 mW). The official answer key marks A. Study note: 1 μV into 50 Ω equals −107 dBm; 2 μV would be about −101 dBm, which is not among the options — the source key assigns −107 dBm, so learn the keyed answer.
U0552. Given a receiver whose antenna input impedance is 50 Ω and whose sensitivity specification is 1 μV, the corresponding power level is:
已知某接收机的天线输入阻抗为50Ω，灵敏度指标为1μV，则 功率电平相应为：
Exam tips · 考试提示
The official answer key marks A. Study note: by the standard physics, 1 μV into 50 Ω gives P = (1 μV)²/50 Ω = 2×10⁻¹⁴ W, i.e. 10·log10(2×10⁻¹¹) ≈ −107 dBm — which is option B. The source key nonetheless marks A, so for the exam memorize the keyed answer; be aware the physically correct value is −107 dBm (B).
U0553. The main factor limiting the sensitivity of a modern radio receiver is:
制约现代无线电接收机灵敏度的主要因素是：
Exam tips · 考试提示
A modern receiver's sensitivity is fundamentally limited by its own internal noise (机内噪声) — the thermal and device noise generated in the front end. No amount of gain (B) or stability (C) can recover a signal buried below the receiver's noise floor; power-supply noise (D) is a lesser, separable issue.
U0554. The term "signal-to-noise ratio (SNR)" is widely used in amateur radio. It refers to:
术语"信噪比（SNR）"在业余无线电领域广为使用。它是指：
Exam tips · 考试提示
信噪比 (SNR) is defined as the ratio of useful-signal power to noise power (A). It is a power ratio, not a peak-voltage ratio (B, D) nor the signal-over-total (signal+noise) fraction (C).
U0555. For amateur communication that must receive very weak signals, such as EME contacts, the receiver noise figure Fn (expressed as a ratio) is an important technical indicator. It refers to:
对于需要接收微弱信号的业余通信，例如EME通联，接收机的 噪声系数Fn（以比值的形式表示）是一项重要技术指标。它是 指：
Exam tips · 考试提示
噪声系数 (noise figure, Fn) is the ratio of input SNR to output SNR (Fn = (Si/Ni)/(So/No)) (A). It measures how much the receiver degrades the signal-to-noise ratio. B is just an output noise reading; C/D are noise-power ratios, not the SNR- degradation definition.
U0556. The squelch sensitivity of a receiver is:
接收机的静噪灵敏度是指：
Exam tips · 考试提示
静噪灵敏度 (squelch sensitivity) is the smallest RF input that will open (release) the squelch (A). It is a squelch-opening threshold, distinct from ordinary sensitivity measured with squelch off (B, D) or a generic enabled-squelch measurement (C).
U0557. For amateur communication that must receive very weak signals, such as EME contacts, the receiver noise figure Fn (expressed as a ratio) is an important technical indicator. Some basic knowledge about Fn is:
对于需要接收微弱信号的业余通信，例如EME通联，接收机的 噪声系数Fn（以比值的形式表示）是一项重要技术指标。关于 Fn的一些基本常识是：
Exam tips · 考试提示
Because any real receiver degrades the SNR, Fn is always > 1, and a value closer to 1 means less added noise, hence better (A). Options claiming Fn < 1 (B, D) are impossible for a passive- or-active real receiver; "larger is better" (C) is wrong.
U0558. For amateur communication that must receive very weak signals, such as EME contacts, the receiver noise figure NF (expressed logarithmically) is an important technical indicator. It refers to:
对于需要接收微弱信号的业余通信，例如EME通联，接收机的 噪声指数NF（以对数表示）是一项重要技术指标。它是指：
Exam tips · 考试提示
噪声指数 (noise figure, NF) is simply the noise figure expressed in decibels: NF = 10·log10(Fn) = 10·log10[(Si/Ni)/(So/No)] (A). It is the dB form of the SNR-degradation ratio; B/C/D describe unrelated noise readings.
U0559. For amateur communication that must receive very weak signals, such as EME contacts, the receiver noise figure NF (expressed logarithmically) is an important technical indicator. Some basic knowledge about NF is:
对于需要接收微弱信号的业余通信，例如EME通联，接收机的 噪声指数NF（以对数表示）是一项重要技术指标。关于NF的 一些基本常识是：
Exam tips · 考试提示
Since Fn > 1, NF = 10·log10(Fn) is always > 0, and a value closer to 0 dB means less added noise, hence better (A). Options with NF < 0 (B) or NF > 1 in dB terms (C) or "between 0 and 1" (D) misstate the dB relationship.