ECE 514E - Continuous Wave Radar Test

1. What is the fundamental principle that allows Continuous Wave (CW) radar to measure target velocity?
2. Which of the following is a primary limitation of simple unmodulated CW radar systems?
3. In Frequency Modulated Continuous Wave (FMCW) radar, how is target range determined?
4. What is the primary purpose of using a circulator or duplexer in a CW radar system?
5. In a Doppler CW radar system, if a target is moving directly away from the radar at 100 m/s and the transmitted frequency is 10 GHz, what is the approximate Doppler frequency shift? (Assume speed of light = 3×10^8 m/s)
6. Which of the following modulation schemes is commonly used in FMCW radar to determine both range and velocity?
7. What is the main advantage of CW radar over pulse radar systems?
8. In an FMCW radar system, what parameter determines the maximum unambiguous range that can be measured?
9. What phenomenon causes the "blind speed" problem in simple CW radar systems?
10. Which application is LEAST suitable for a simple unmodulated CW radar system?
Question 1: Correct answer: c) Doppler frequency shift
CW radar measures target velocity using the Doppler effect. When a target moves relative to the radar, the frequency of the reflected signal shifts proportionally to the relative velocity.
Question 2: Correct answer: a) Inability to measure range to a target
Simple unmodulated CW radar cannot determine range because it lacks timing markers in the continuous transmission. Without modulation, there's no way to distinguish when a particular part of the signal was transmitted versus received.
Question 3: Correct answer: b) By comparing the instantaneous frequency difference between transmitted and received signals
In FMCW radar, the transmitted frequency is continuously varied. The difference between the transmitted frequency at any instant and the frequency of the received signal (which was transmitted earlier) is proportional to the target's range.
Question 4: Correct answer: c) To separate transmitted and received signals using the same antenna
A circulator or duplexer allows a single antenna to be used for both transmission and reception by directing the high-power transmitted signal to the antenna while routing the weak received signal to the receiver, preventing transmitter power from damaging the sensitive receiver components.
Question 5: Correct answer: b) 6.67 kHz
The Doppler frequency shift is calculated as f_d = (2v/λ) = (2v*f)/c, where v is velocity, f is frequency, and c is the speed of light. f_d = (2*100*10^10)/(3*10^8) = 6.67×10^3 Hz = 6.67 kHz.
Question 6: Correct answer: a) Triangular wave modulation
Triangular wave modulation is commonly used in FMCW radar because it allows separation of range and velocity information. The beat frequency during the up-ramp and down-ramp portions of the triangle can be compared to extract both parameters.
Question 7: Correct answer: c) No minimum range limitation
Unlike pulse radar systems that have a minimum range determined by the pulse width and receiver recovery time, CW radar can detect targets at very close ranges since it transmits continuously and receives simultaneously.
Question 8: Correct answer: c) The modulation period
The maximum unambiguous range in FMCW radar is determined by the modulation period. If the round-trip time exceeds the modulation period, the frequency difference becomes ambiguous as it could correspond to multiple possible ranges.
Question 9: Correct answer: b) When the target velocity produces a Doppler shift that is an integer multiple of the modulation frequency
In modulated CW radar systems, blind speeds occur when the Doppler frequency is an integer multiple of the modulation frequency, causing the phase relationship between transmitted and received signals to repeat exactly after each modulation cycle, making velocity measurement ambiguous.
Question 10: Correct answer: b) Aircraft altimetry
Aircraft altimetry requires precise range measurement to the ground, which simple unmodulated CW radar cannot provide. This application typically uses FMCW or pulse radar systems that can accurately determine distance.