🔬 CONTINUOUS WAVE (CW) Virtual Laboratory

ECE 514E-Radar & Satellite Engineering, Department of Electrical & Communication Engineering

Undergraduate Electrical Engineering | Microwave & Radar Engineering

📚 Theory

1. Basic Principle

A Continuous Wave (CW) Radar transmits a constant frequency signal continuously and detects moving targets by measuring the Doppler frequency shift in the returned echo. Unlike pulsed radar, CW radar cannot measure target range (distance) but excels at measuring radial velocity with high accuracy.

2. Doppler Effect

When a target moves relative to the radar, the frequency of the reflected signal shifts due to the Doppler effect:

fd = (2 × vr × f0) / c = (2 × vr) / λ

Where:

  • fd = Doppler frequency shift (Hz)
  • vr = Radial velocity of target (m/s)
  • f0 = Transmitted frequency (Hz)
  • c = Speed of light (3 × 108 m/s)
  • λ = Wavelength (m)

3. System Architecture

A basic CW radar consists of:

  • RF Oscillator: Generates continuous wave at frequency f0
  • Transmitter Antenna: Radiates the CW signal
  • Receiver Antenna: Captures the reflected signal (often shared via circulator)
  • Mixer: Combines transmitted and received signals to extract beat frequency (fd)
  • Low Pass Filter: Removes high frequency components, passes Doppler signal
  • Signal Processor: Analyzes Doppler frequency to determine velocity

4. Key Characteristics

  • Velocity Sensitivity: Can detect minute movements (heartbeat monitoring)
  • Range Ambiguity: Cannot determine distance to target
  • Direction Discrimination: Cannot distinguish approaching vs. receding without quadrature detection
  • Minimum Detectable Velocity: Limited by system noise and filter cutoffs
Important Limitation: Standard CW radar cannot distinguish between targets at different ranges. If multiple targets exist at different distances but with the same radial velocity, their Doppler shifts will be identical and they cannot be resolved separately.

⚗️ Virtual Experiment

10 GHz
100 m/s
0.8
Wavelength (λ)
3.00 cm
Doppler Shift (fd)
6.67 kHz
Target Direction
Approaching
Time Domain Signals (Transmitted vs Received)
Frequency Spectrum (Doppler Detection)
Mixer Output (Doppler Beat Signal)
I-Q Diagram (Phase Detection)

📝 Experimental Procedure

1

Setup and Calibration

Set the radar frequency to 10 GHz (X-band, typical for radar applications). Verify that the wavelength calculation updates correctly (λ = c/f = 3 cm). Set target velocity to 0 m/s and confirm that Doppler shift reads 0 Hz.

2

Velocity Variation Study

Vary the target velocity from -200 m/s to +200 m/s in steps of 50 m/s. For each step:

  • Record the calculated Doppler frequency
  • Observe the direction indicator (approaching vs. receding)
  • Note the phase relationship in the I-Q diagram
  • Capture the spectrum showing the Doppler peak
3

Frequency Dependence Analysis

Maintain constant velocity at 150 m/s. Vary radar frequency from 1 GHz to 40 GHz (L to Ka band). Plot the relationship between transmitted frequency and Doppler shift. Verify linear proportionality: fd ∝ f0.

4

Signal Characterization

At f0 = 10 GHz and v = 100 m/s:

  • Analyze the beat frequency signal envelope
  • Measure the period of the Doppler signal from the time domain plot
  • Calculate fd = 1/T and compare with theoretical value
  • Observe the I-Q diagram rotation direction for positive vs negative velocities
5

Resolution and Limitations

Investigate the minimum detectable velocity by setting small velocities (1, 5, 10 m/s). Discuss:

  • System sensitivity requirements
  • Low-frequency noise considerations
  • Why CW radar cannot measure range (no time-of-flight measurement)

📋 Laboratory Report Guidelines

1. Title Page & Abstract (10%)

  • Title: "Characterization of Continuous Wave Doppler Radar"
  • Date, Student Name, ID, Group Number
  • Abstract (150-200 words): Briefly state objectives, methodology (frequency range, velocity range), key results (measured vs theoretical Doppler shifts), and main conclusion regarding linearity and system limitations.

2. Introduction & Theory (20%)

  • Explain the principle of CW radar and Doppler effect
  • Derive the Doppler frequency equation with clear assumptions
  • Discuss applications (speed guns, motion sensors, vital signs monitoring)
  • State the wavelength-Doppler relationship for X-band (10 GHz)

3. Simulation Setup (15%)

  • Diagram of CW radar block diagram (transmitter, receiver, mixer)
  • Table of equipment parameters: Frequency range, Velocity range, Sampling rate
  • Screenshot of initial setup (f0 = 10 GHz, v = 0 m/s)

4. Results & Analysis (35%)

Required Elements:

  • Table 1: Velocity vs Doppler Frequency (theoretical vs simulated) for 10 data points
  • Figure 1: Plot of fd vs v with linear regression (slope should be 2f0/c)
  • Figure 2: Time domain signals showing phase shift at v = 100 m/s
  • Figure 3: Spectrum showing Doppler peak at fd for various velocities
  • Figure 4: I-Q diagrams for approaching vs receding targets
  • Error Analysis: Calculate percentage error between theoretical and measured Doppler shifts
  • Discussion of why errors occur (sampling limitations, FFT resolution)

5. Discussion (15%)

  • Explain the significance of the slope in your fd vs v plot
  • Discuss the "blind" nature of CW radar to stationary targets (v = 0)
  • Compare X-band (10 GHz) vs K-band (24 GHz) for police radar applications
  • Explain how the I-Q diagram enables direction discrimination
  • Propose a modification to enable range measurement (Hint: FMCW)

6. Conclusion (5%)

  • Summarize key findings (linearity confirmed, Doppler directionality observed)
  • State the operational limits of the simulated system
  • Mention one practical application suitable for this technology

Submission Requirements

  • Maximum 10 pages (single-spaced, 12pt font, 1-inch margins)
  • Include all figures with captions (Figure 1, Figure 2, etc.)
  • Appendix: Raw data tables
  • Submit both PDF and .docx/.tex files
  • Due date: One week after lab session
Academic Integrity: All graphs must be generated from your own simulation runs. Identical datasets between students will trigger plagiarism review. Include your student ID in one of the parameter fields when taking screenshots for verification.