CRAC Class A (2025) — Questions 521–530
Unofficial study reference only. Not affiliated with the FCC, NCVEC or CRAC — official sources prevail. 非官方学习参考，与 FCC、NCVEC、CRAC 无隶属关系；以官方发布为准。
U0520. What is the role of the mixer commonly found in receivers and transmitters?
接收机和发射机中常见的混频器有什么作用？
Exam tips · 考试提示
A mixer (混频器 / frequency converter) multiplies two frequencies (the incoming signal and a local oscillator) to produce sum and difference frequencies — performing frequency conversion (频率变换). It is not audio mixing (B), motor speed control (C), or a multi-frequency amplifier (D). So A is correct.
U0521. In a superheterodyne transceiver, the mixer's operation can be divided into "up-conversion" and "down-conversion"; specifically:
在超外差式收发信机中，混频器的工作方式可分为"上变 频"和"下变频"两种，具体指：
Exam tips · 考试提示
Up/down conversion is defined by the relationship between the input (RF) frequency and the resulting IF: if the IF is higher than the input frequency, the signal was up-converted; if the IF is lower, it was down-converted. This is the convention used in the Chinese exam — A. Note this is defined by IF vs input, not by the local-oscillator relationship (C/D) or merely input vs IF (B). So A is correct.
U0522. Superheterodyne amateur transceivers often have a control on the panel for selecting the IF filter bandwidth. The interference suppressed by these IF filters can be classified as:
超外差式业余收发信机的面板上经常设有选择中频滤波器带宽 的控制部件。这些中频滤波器所抑制的干扰可以分类为：
Exam tips · 考试提示
The selectable IF filter bandwidth rejects signals close to (but outside) the wanted channel — i.e., adjacent-frequency (邻近频 率) interference. Image-frequency interference (B) is handled by the preselector before the mixer, not by the IF bandwidth; C and D are not what IF bandwidth selection addresses. So A is correct.
U0523. In a superheterodyne amateur transceiver, the component responsible for suppressing image-frequency interference is:
在超外差式业余收发信机中，负责抑制镜像频率干扰的部件 是：
Exam tips · 考试提示
The image frequency lies 2×IF away from the wanted signal and also mixes to the IF. The only stage that can reject it before mixing is the preselector (波段预选滤波器) ahead of the mixer (变频级); once both signals reach the mixer they are indistinguishable. The IF filter (B), limiter (C) and tone control (D) cannot remove the image. So A is correct.
U0524. In a superheterodyne receiver the received signal frequency is lower (or higher) than the local-oscillator frequency by one IF. However, another corresponding signal that is higher (or lower) than the local-oscillator frequency by one IF may also enter the IF channel through mixing, forming "image- frequency interference" or "mirror-frequency interference". If a handheld transceiver's specification gives the VHF receiver's first IF as 45.05 MHz, then image-frequency interference received at 145.00 MHz may come from:
超外差式收信机所收信号的频率要比本振频率低（或高）一个 中频。然而，比本振频率高（或低）一个中频的另一个遥相对 应的信号也可能经混频窜入中频通道，形成"镜像频率干 扰"或"镜频干扰"。如果某对讲机的技术规格书给出的VHF接收 机第一中频为45.05MHz，那么在145.00MHz收到的镜频干 扰可能来自：
Exam tips · 考试提示
The image frequency is f_image = f_signal ± 2×IF. With f_signal = 145.00 MHz and IF = 45.05 MHz, the image is at 145.00 ± 90.10 = 235.10 MHz or 54.90 MHz — exactly option A. (Choosing the LO: if LO = 145+45.05 = 190.05, image = 190.05+45.05 = 235.10; if LO = 145−45.05 = 99.95, image = 99.95−45.05 = 54.90.) So A is correct.
U0525. In a superheterodyne receiver the received signal frequency is lower (or higher) than the local-oscillator frequency by one IF. However, another corresponding signal that is higher (or lower) than the local-oscillator frequency by one IF may also enter the IF channel through mixing, forming "image- frequency interference". If a handheld transceiver's specification gives the first IF in NFM mode as 47.25 MHz, then image-frequency interference received at 145.00 MHz may come from:
超外差式收信机所收信号的频率要比本振频率低（或高）一个 中频。然而，比本振频率高（或低）一个中频的另一个遥相对 应的信号也可能经混频窜入中频通道，形成"镜像频率干 扰"或"镜频干扰"。如果某对讲机的技术规格书给出NFM方式 时的第一中频为47.25MHz，那么在145.00MHz收到的镜频 干扰可能来自：
Exam tips · 考试提示
Image = f_signal ± 2×IF = 145.00 ± (2×47.25) = 145.00 ± 94.50 = 239.50 MHz or 50.50 MHz — option A. (For reference, the LO would be 145.00+47.25 = 192.25 MHz, which is option B, but B is the LO, not the image.) So A is correct.
U0526. In a superheterodyne receiver the received signal frequency is lower (or higher) than the local-oscillator frequency by one IF. However, another corresponding signal that is higher (or lower) than the local-oscillator frequency by one IF may also enter the IF channel through mixing, forming "image- frequency interference". If a handheld transceiver's specification gives the first IF used for NFM reception as 47.25 MHz, then image-frequency interference received at 435.00 MHz may come from:
超外差式收信机所收信号的频率要比本振频率低（或高）一个 中频。然而，比本振频率高（或低）一个中频的另一个遥相对 应的信号也可能经混频窜入中频通道，形成"镜像频率干 扰"或"镜频干扰"。如果某对讲机的技术规格书给出NFM接收 所用的第一中频为47.25MHz，那么在435.00MHz收到的镜 频干扰可能来自：
Exam tips · 考试提示
Image = f_signal ± 2×IF = 435.00 ± (2×47.25) = 435.00 ± 94.50 = 340.50 MHz or 529.50 MHz — option A. So A is correct (the other options are distractors; D's lower value 236.25 is far off).
U0527. In a superheterodyne receiver the received signal frequency is lower (or higher) than the local-oscillator frequency by one IF. However, another corresponding signal that is higher (or lower) than the local-oscillator frequency by one IF may also enter the IF channel through mixing, forming "image- frequency interference". If a handheld transceiver's specification gives the UHF receiver's first IF as 58.525 MHz, then image-frequency interference received at 435.00 MHz may come from:
超外差式收信机所收信号的频率要比本振频率低（或高）一个 中频。然而，比本振频率高（或低）一个中频的另一个遥相对 应的信号也可能经混频窜入中频通道，形成"镜像频率干 扰"或"镜频干扰"。如果某对讲机的技术规格书给出的UHF接收 机第一中频为58.525MHz，那么在435.00MHz收到的镜频干 扰可能来自：
Exam tips · 考试提示
Image = f_signal ± 2×IF = 435.00 ± (2×58.525) = 435.00 ± 117.05 = 317.95 MHz or 552.05 MHz — option A. So A is correct.
U0528. The efficiency of an amateur radio transmitter means:
业余无线电发射机的效率是指：
Exam tips · 考试提示
Transmitter efficiency (效率) is RF output power to the antenna divided by DC power drawn from the supply (输出到天线的功率 / 电源消耗功率). It is a local power-conversion measure; it does not involve the distant receiver (B, C) nor spurious ratio (D, which is a spectral-purity measure). So A is correct.
U0529. The efficiency of an amateur radio transmitter is always significantly below 1. The lost portion of energy:
业余无线电发射机的效率总是明显低于1。所损耗的那部分能 量：
Exam tips · 考试提示
The power not delivered to the antenna is dominated by heat dissipation in the PA and other circuitry (I²R and switching losses); only a tiny fraction becomes spurious emissions (杂散发 射). Energy is never "lost" (D) nor mostly returned to the supply via mismatch (C); mismatch would mainly reflect, not vanish. So A is correct.