U1290. In the power supply for an amateur transceiver, a switching
power supply can be more compact and lightweight than a
transformer-step-down linear power supply. The reason is:
(Choose all that apply.)
在给业余收发信机供电的电源装置中，开关电源可以比变压器
降压的线性电源更为轻巧。原因是：
Exam tips · 考试提示
Switchers are lighter because (1) the high switching frequency shrinks the transformer core and turn count (A), and (2)
switching devices run efficiently so the heatsink is small (B). C is false (switchers have more parts), and D is false — PWM
edges generate noise and EMC filtering is required, not
omitted.
U1291. The general working process of a communication switching
power supply that converts 220 V AC input to 13.8 V DC
output is:
将220V交流输入变换为13.8V直流输出的通信开关电源的一般
工作过程是：
Exam tips · 考试提示
An off-line switcher rectifies AC→high-voltage DC, chops it with a switching circuit into HV pulses, uses a small high-
frequency transformer to step down to LV pulses, then rectifies/filters to LV DC (A). B puts the transformer before
chopping (wrong for off-line); C uses a bulky 50 Hz step-down
transformer (that's a linear supply); D misstates the chain.
U1292. Sometimes a local oscillator (LO) using a PLL (phase-locked
loop) frequency synthesizer adds phase noise to the signal
after mixing. Possible reasons are: (Choose all that apply.)
有时，使用锁相环频率合成器的本机振荡器（LO）会为混频之
后的信号增添相位噪声。可能的原因是：
Exam tips · 考试提示
PLL LO phase noise arises from: loop-filter delay limiting how
fast drift is corrected (A), the VCO's inherently higher noise versus a crystal reference (B), and multiple-VCO/multi-mix
schemes summing noise (C). D describes harmonic/spurious
generation by the divider, not the dominant phase-noise mechanism, so it is not selected.
U1293. Many amateur transceivers use a direct digital frequency
synthesizer (DDS) in their local-oscillator circuit. Its main
characteristics are:
很多业余无线电收发信机都在本机振荡电路中使用了直接数字
频率合成器（DDS）。它的主要特点是：
Exam tips · 考试提示
DDS offers a simple architecture, no capture-range limit, low phase noise, and very fast hopping (A). Its stability tracks the
reference (not better than PLL per se, B false), it needs fast
logic (C false), and its output contains images/spurs requiring an anti-alias filter (D false).
U1294. A resistor is a passive component, so why does it also produce
noise?
电阻是无源元件，为什么也会产生噪声？
Exam tips · 考试提示
All resistive elements exhibit Johnson–Nyquist thermal noise, proportional to resistance and absolute temperature (A). It is
fundamental, not a manufacturing defect (B false); no resistor
is noise-free (C false); near DC thermal noise is flat, not minimal (D false).
U1295. The noise of active devices mainly includes: (Choose all that
apply.)
有源器件的噪声主要包括：
Exam tips · 考试提示
Active-device noise includes shot noise and thermal noise. Note
that 散粒噪声 (A) and 散弹噪声 (B) are two written forms of the same term — shot noise — so both are marked correct, and C
(thermal noise) is also correct. D (switching noise) is not a fundamental active-device noise type in this context.
U1296. Why do an overly long coaxial cable, or a coaxial cable with
too high a VSWR (standing-wave ratio), add noise to a
receiver?
为什么过长的同轴电缆或是VSWR过高的同轴电缆都为收信机
增添噪声？
Exam tips · 考试提示
A long feed line or a high SWR (驻波比 / VSWR) raises ohmic loss; that loss adds its own thermal noise and raises the system
noise figure, degrading the receiver (A). B/C describe unrelated
electrostatic effects; D is false — VSWR does increase loss.
U1297. Why does the atmosphere also have noise?
为什么大气也有噪声？
Exam tips · 考试提示
Atmospheric (static) noise — 天电噪声, abbreviated QRN — comes from lightning and other atmospheric electrical
discharges (A). QRN actually decreases with frequency (B false);
mixing/QRM (C) is man-made interference, not atmosphere; D misstates the mechanism.
U1298. A poor-quality switching power supply adds electromagnetic
noise to a radio receiver. The main source is:
劣质开关电源会为无线电接收机增添电磁噪声。源头主要是：
Exam tips · 考试提示
The dominant noise from a bad switcher is the harmonic
radiation of its switching circuit (conducted and radiated), which spreads across the bands (A). B is a linear-supply (50 Hz
transformer) issue; C causes ripple, not broadband RFI; D is incidental arcing, not the main source. 数字逻辑门 Digital Logic Gates
U1299. The two-input AND gate is one of the simplest digital logic
circuits. If the two inputs are respectively 00, 01, 10, 11, the
corresponding outputs are: