Test your knowledge of CW radar principles, applications, and signal processing techniques
1
What is the fundamental principle that enables CW radar to detect moving targets?
A. Pulse compression
B. Frequency modulation
C. Doppler effect
D. Phase array beamforming
Answer: C. Doppler effect
CW radar detects moving targets using the Doppler effect, which causes a shift in the frequency of the returned signal relative to the transmitted signal. The Doppler shift is proportional to the radial velocity of the target.
2
Which mathematical expression correctly represents the Doppler shift (fd) in a CW radar system?
A. fd = (2v × ft) / c
B. fd = (2v) / (ft × c)
C. fd = (v × c) / (2ft)
D. fd = (2v × c) / ft
Answer: A. fd = (2v × ft) / c
The Doppler shift is calculated as fd = (2v × ft) / c, where v is the radial velocity of the target, ft is the transmitted frequency, and c is the speed of light. The factor of 2 accounts for the two-way path of the radar signal.
3
What is the primary limitation of a basic unmodulated CW radar system?
A. Inability to detect stationary targets
B. Inability to determine target range
C. Limited maximum detection range
D. Poor angular resolution
Answer: B. Inability to determine target range
A basic unmodulated CW radar transmits a continuous wave at a constant frequency. Without modulation, it cannot measure the time delay of the returned signal, which is necessary to determine range. This type of radar can only detect target velocity through Doppler shift.
4
Which modulation technique is used in FMCW radar to enable range measurement?
A. Pulse-width modulation
B. Phase modulation
C. Linear frequency modulation
D. Amplitude modulation
Answer: C. Linear frequency modulation
Frequency Modulated Continuous Wave (FMCW) radar uses linear frequency modulation (typically a sawtooth or triangular pattern) to enable range measurement. The frequency difference between transmitted and received signals is proportional to the target's distance.
5
In an FMCW radar with triangular frequency modulation, what does the beat frequency during the up-ramp represent?
A. Range only
B. Velocity only
C. Range and velocity combined
D. Signal-to-noise ratio
Answer: C. Range and velocity combined
During the up-ramp, the beat frequency is fb_up = fr - fd, and during the down-ramp, it's fb_down = fr + fd, where fr is the range frequency and fd is the Doppler frequency. By measuring both beat frequencies, range and velocity can be separated.
6
Which component in a CW radar system is responsible for separating the transmitted and received signals?
A. Duplexer
B. Circulator
C. Mixer
D. Frequency discriminator
Answer: B. Circulator
In CW radar systems, a circulator is typically used to separate transmitted and received signals. It directs the transmitted signal to the antenna and the received signal from the antenna to the receiver, providing isolation between transmitter and receiver.
7
What is the primary purpose of the mixer in a CW radar receiver?
A. To amplify the received signal
B. To convert RF to baseband
C. To filter out noise
D. To modulate the carrier wave
Answer: B. To convert RF to baseband
The mixer combines the received RF signal with a sample of the transmitted signal to produce an intermediate frequency (IF) or baseband signal. This output contains the Doppler frequency shift for velocity measurement or the beat frequency for range measurement in FMCW systems.
8
Which of the following is a key advantage of CW radar over pulsed radar?
A. Higher peak power
B. Better range resolution
C. No minimum range limitation
D. Simpler signal processing
Answer: C. No minimum range limitation
CW radar has no minimum range limitation because it transmits and receives simultaneously. In contrast, pulsed radar has a blind zone at close ranges where the receiver is still blanked during transmission.
9
In FMCW radar, the range resolution is primarily determined by:
A. Transmitter power
B. Bandwidth of the frequency sweep
C. Center frequency
D. Antenna gain
Answer: B. Bandwidth of the frequency sweep
Range resolution in FMCW radar is given by ΔR = c / (2B), where c is the speed of light and B is the bandwidth of the frequency sweep. A wider bandwidth provides better range resolution.
10
What phenomenon causes ambiguity in Doppler measurement for CW radar when target velocity exceeds certain limits?
A. Range folding
B. Aliasing
C. Clutter
D. Multipath interference
Answer: B. Aliasing
Aliasing occurs when the Doppler shift exceeds half the sampling frequency (Nyquist limit), causing velocity ambiguity. In CW radar, this happens when the target's radial velocity is high enough that the Doppler frequency exceeds the system's ability to unambiguously measure it.
11
Which of the following applications is most suitable for CW radar?
A. Air traffic control at long ranges
B. Weather monitoring
C. Police speed detection
D. Satellite imaging
Answer: C. Police speed detection
CW radar is ideal for police speed guns because it accurately measures vehicle velocity (using Doppler shift) without needing range information. Its simple design, low cost, and ability to operate at close ranges make it perfect for this application.
12
What is the main challenge in designing CW radar systems compared to pulsed systems?
A. Achieving high peak power
B. Preventing transmitter-receiver leakage
C. Generating short pulses
D. Implementing complex modulation
Answer: B. Preventing transmitter-receiver leakage
Since CW radar transmits and receives simultaneously, even a small amount of signal leakage from transmitter to receiver can overwhelm the much weaker received echo. Careful design and high isolation components (like circulators) are required to minimize this leakage.