1 Laboratory Objectives
Understand the fundamental principles of digital bandpass modulation techniques: ASK, FSK, and PSK
Analyze time-domain characteristics of modulated signals with varying modulation indices
Investigate spectral efficiency and bandwidth requirements for each modulation scheme
Evaluate the effect of carrier frequency and bit rate on signal characteristics
Compare power spectral density (PSD) and identify main lobe widths
Study constellation diagrams for PSK and understand signal space representation
Determine bit error rate (BER) performance under AWGN conditions
Design appropriate modulation schemes for given bandwidth and power constraints
Theoretical Background
Amplitude Shift Keying (ASK)
The simplest digital modulation where digital data modulates the amplitude of a carrier wave.
where c(t) ∈ {0, 1}
- • Bandwidth: 2 × Bit Rate
- • Spectral Efficiency: 0.5 bps/Hz
- • Power Efficient: No
Frequency Shift Keying (FSK)
Information is encoded by shifting the frequency of the carrier between discrete values.
s(t) = A·cos(2πf0t) for '0'
- • Bandwidth: |f₁ - f₀| + 2×Rb
- • Spectral Efficiency: ~0.4 bps/Hz
- • Immunity to Amplitude Noise
Phase Shift Keying (PSK)
Phase of the carrier is changed to represent digital data while amplitude remains constant.
where φk = 2πk/M, k=0,...,M-1
- • BPSK: Δφ = 180°
- • QPSK: Δφ = 90°
- • Bandwidth: 2 × Bit Rate / log₂(M)
Digital Modulation Concepts
Modulation Index (Digital)
For FSK: h = 2Δf/Rb = |f₁ - f₀|/Rb
When h = 0.5: Minimum Shift Keying (MSK) with continuous phase
When h is integer: Orthogonal FSK
Signal Constellation
Geometric representation of signals in signal space:
• BPSK: 2 points on real axis (±√Eb)
• QPSK: 4 points at 45°, 135°, 225°, 315°
Distance between points determines error probability
ASK Modulation Experiment
Time Domain - ASK Signal
Power Spectral Density
Theoretical Analysis
FSK Modulation Experiment
Time Domain - FSK Signal
Frequency Domain
Theoretical Analysis
PSK Modulation Experiment
Time Domain - BPSK Signal
Signal Constellation
Power Spectral Density
Theoretical Analysis
Performance Comparison
| Parameter | ASK | FSK | PSK |
|---|---|---|---|
| Bandwidth (Null-to-Null) | 2Rb | |f1-f0| + 2Rb | 2Rb/log2M |
| Spectral Efficiency | 0.5 bps/Hz | 0.4-0.5 bps/Hz | 0.5-2 bps/Hz |
| BER (Eb/N0 = 10dB) | ~10-3 | ~10-4 | ~10-5 |
| Immunity to Noise | Poor | Good | Excellent |
| Complexity | Low | Medium | High |
BER vs Eb/N0 Curves
Laboratory Procedure
Experiment 1: ASK Analysis
- Set carrier frequency to 5 kHz and bit rate to 500 bps
- Input binary sequence '10101010' and observe the time-domain waveform
- Note the presence of carrier during '1' bits and absence during '0' bits
- Record the bandwidth from the frequency spectrum display
- Vary the bit rate and observe the spectral spreading effect
- Add noise and determine the SNR threshold for error-free detection
Experiment 2: FSK Characteristics
- Set mark frequency (f₁) = 6 kHz and space frequency (f₀) = 4 kHz
- Calculate theoretical modulation index h = Δf/Rb
- Generate alternating bit pattern and observe frequency transitions
- Verify that two distinct spectral peaks appear in frequency domain
- Calculate Carson's bandwidth and compare with measured result
- Adjust frequencies to achieve orthogonal FSK (h = 1)
Experiment 3: PSK & Constellation
- Select BPSK mode with carrier frequency 5 kHz
- Input sequence '1010' and observe 180° phase shifts
- Examine constellation diagram showing two antipodal points
- Switch to QPSK and observe four constellation points at 90° intervals
- Compare bandwidth efficiency between BPSK and QPSK
- Analyze effect of phase noise on constellation spreading
Data Collection Sheet
ASK Measurements:
• Null-to-null bandwidth at Rb = 500 bps: _______ Hz
• Spectral efficiency: _______ bps/Hz
• Effect of noise on envelope: ____________________
FSK Measurements:
• Carson's bandwidth (h=4): _______ Hz
• Frequency deviation (h=0.5): _______ Hz
• Minimum orthogonal spacing: _______ Hz
Laboratory Report Guidelines
Required Sections
- 1. Introduction: Principles of ASK, FSK, and PSK modulation
- 2. Simulation Setup: Parameters used for each experiment
- 3. Results: Time-domain plots, spectra, and constellations (include screenshots)
- 4. Analysis: Bandwidth calculations, efficiency comparisons
- 5. Conclusions: Advantages/disadvantages and application scenarios
Evaluation Criteria
Discussion Questions
Q1: Why does ASK have poor noise immunity compared to PSK?
Q2: Calculate the minimum frequency separation required for orthogonal FSK at 1000 bps.
Q3: Compare the power efficiency of BPSK and QPSK at the same bit error rate.
Q4: Explain why the constellation diagram is useful for analyzing PSK signals.
Q5: Design a modulation scheme for a system requiring 2 kbps over a 1.5 kHz bandwidth.
Submission Requirements
- • Report length: 8-12 pages maximum
- • Include all waveform screenshots with proper labels
- • Show all mathematical calculations
- • Submit both PDF and working simulation parameters
- • Deadline: 1 week after lab session