U1280. The "cutoff frequency" of a low-pass filter refers to:
低通滤波器的"截止频率"是指：
Exam tips · 考试提示
截止频率 (cutoff frequency) is defined as the −3 dB point:
where the response falls 3 dB below the passband level (A). A real filter does not hard-cut above it (B), and it passes (not
cuts) below it (C); D confuses cutoff with distortion.
U1281. The "3 dB bandwidth" of a band-pass filter refers to:
带通滤波器的"3dB带宽"是指：
Exam tips · 考试提示
3 dB bandwidth is the width between the two −3 dB points
measured around the center frequency (A). B describes the
stopband/reject region, not the bandwidth; C inverts the sense; D misstates gain.
U1282. A filter has a parameter called "passband ripple." It refers to:
滤波器有一个称为"通带波动"的参数，是指：
Exam tips · 考试提示
通带波动 (passband ripple) is the variation (peak-to-peak) of
insertion loss/gain across the passband (A). B describes overall response shape; C/D describe unrelated time-domain or
random effects.
U1283. When describing a receiver's band-preselection filter and a
transmitter's output filter, some technical documents (e.g.,
service manuals) mention the "order" or "number of poles" of
an LC filter.
The order (pole count) of an LC filter equals the number of
independent reactive energy-storage elements after simplification — A gives the rigorous method and the correct
examples (a 3rd-order low-pass = 2 capacitors + 1 inductor; a 3rd-order band-pass = 3 resonant sections). The official answer
key also marks B ("the number of branching sections is the
order"), which in many ladder/lattice filter texts coincides with the reactive-element count. C and D are incorrect formulas.
U1284. Suppose an amateur repeater's receiver and transmitter share
one antenna, with uplink and downlink frequencies F1 and F2
respectively. When installing this station, to prevent the
transmitter's RF output from blocking (desensitizing) the
receiver's normal reception, the usable measures are:
The repeater receives on the uplink F1 and transmits on the downlink F2. To isolate them, the transmitter leg rejects the
receiver's frequency F1 (so incoming F1 isn't lost into the TX), and the receiver leg rejects the transmitter's frequency F2 (so
the strong TX output doesn't block the RX) — both using band-
reject (notch) filters (A). B uses band-pass (wrong sense), C/D use mismatched center frequencies.
U1285. To prevent the RF energy emitted by an HF amateur station
from interfering with a telephone placed near the antenna,
you can connect in series between the telephone and the
telephone line:
要防止HF业余电台发出的射频能量干扰布放在天线附近的电话
机，可在电话机与电话线之间串联：
Exam tips · 考试提示
HF RF (3–30 MHz) is getting onto the phone line. A low-pass filter with cutoff ≤1 MHz passes the audio band (<4 kHz) but
blocks the HF RF from entering the line (A). A high-pass (B) would pass the RF; a 3 kHz band-reject/band-pass (C/D)
addresses audio, not HF.
U1286. If the normal transmission of an HF amateur station interferes
with a VHF television near the antenna, a possible solution is:
如果HF业余电台的正常发射干扰天线附近的VHF电视机，可能
的解决方案是：
Exam tips · 考试提示
The HF signal (≤30 MHz) is overloading the TV's VHF tuner. A high-pass filter at the TV input, passing VHF (>30 MHz) but
rejecting HF, fixes it (A). A high-pass at the transmitter (B) would block the ham's own HF; a low-pass at the TV (C) would
block VHF; a low-pass at the TX (D) is wrong type/frequency.
U1287. If 50 Hz AC passes through a full-wave rectifier and is applied
to a resistive load, what is the waveform across the load?
如果50Hz交流电通过全波整流器加到电阻负载上，负载两端的
波形为？
Exam tips · 考试提示
Full-wave rectification flips both halves of the 50 Hz AC, so the
output ripple repeats twice per AC cycle — 100 Hz pulsating DC (A). Half-wave would give 50 Hz (B); a stable DC (C) needs
filtering; D is the original input.
U1288. In a rectifier-filter circuit, the role of the bleed resistor is:
在整流滤波电路中，泄放电阻的作用是：
Exam tips · 考试提示
泄放电阻 (bleed resistor) safely discharges the charged filter
capacitor after the supply is switched off, preventing a shock (A). B addresses flyback (a snubber's job); C is a permanent
load/resistor; D is a fuse's job.
U1289. When building a switching power supply, a relay-driving
circuit, or even an indicator-lamp-driving circuit, we want the
controlling transistor to operate in the switching state. This is
because:
In saturation/cutoff (switching), the transistor is either nearly a
short (low on-resistance) or open, so I×V across it stays tiny and power loss is low (A). Switching actually generates more
RFI (B false), is highly nonlinear (C false), and needs drive/charge (D false).