Lab Report: Understanding Digital Systems and Modulation Basics

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Abstract

This laboratory report focuses on gaining insights into digital systems and fundamental modulation schemes. Modulation is a crucial process for converting baseband signals into bandpass signals, involving the use of an analog carrier signal. This report discusses various modulation techniques, including amplitude, frequency, and phase modulation, with a detailed examination of Binary Phase Shift Keying (BPSK) and Quadrature Phase Shift Keying (QPSK). The experiments were conducted using Advanced Design System (ADS) simulations and Vector Signal Analyzer (VSA) measurements. The analysis includes spectral characteristics, modulation indices, constellation diagrams, eye diagrams, Error Vector Magnitude (EVM) measurements, Complementary Cumulative Distribution Function (CCDF) curves, and multipath effects.

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Introduction

Modulation is the process of encoding information onto a carrier signal for efficient transmission and reception. In this lab, we explore various modulation techniques and analyze their characteristics. The modulation methods include amplitude modulation (AM), frequency modulation (FM), and phase modulation (PM). Additionally, we delve into binary phase shift keying (BPSK) and quadrature phase shift keying (QPSK), two common digital modulation schemes.

Through ADS simulations and VSA measurements, we investigate the performance of these modulation techniques and their relevance in communication systems.

1. Experimental Setup

1.1 Spectrum Analyzer (SA) Configuration

In the initial part of the lab, we configured the Spectrum Analyzer (SA) to analyze signal spectra. We adjusted the following parameters:

Span: 1MHz
Resolution Bandwidth (RBW): 100kHz
Center Frequency: 10MHz

We also measured the sweep time and calculated the proportionality constant (k) and rise time. For different RBW values, we observed changes in the signal's width and shape.

At an RBW of 30MHz, the rise time was 5.82 × 10^-4. As we decreased the RBW to 1kHz, the rise time reduced to 0.205 × 10^-3. To accurately measure signal spectra, an RBW of 10kHz span was determined to be suitable.

2. Amplitude Modulation (AM)

2.1 Modulation Index Calculation

In this section, we explored amplitude modulation (AM) and observed how the waveform changes with modulation depth. The modulation index (M) was calculated using the formula:

_{M = ((V_max - V_min) / (V_max + V_min)) × 100%}

For a 1Hz AM depth, where V_max = 51.2mV and V_min = 20.8mV, the modulation index (M) was found to be 42.2%.

3. Frequency Modulation (FM)

3.1 Modulation Index Calculation

In the study of frequency modulation (FM), we calculated the modulation index (M) using the formula:

_{M = ((f_max - f_min) / f_m)}

Here, f_max is the maximum frequency deviation, f_min is the minimum frequency deviation, and f_m is the modulation frequency.

We observed variations in the waveform on the oscilloscope and fluctuations in the signal on the SA, indicating continuous changes in frequency.

4. Phase Modulation (PM)

4.1 Phase Deviation and Waveform Observation

Phase modulation (PM) involves changing the phase of the carrier signal according to the message signal. By varying the phase deviation from 0° to 100°, we observed changes in the waveform on the oscilloscope. Lower phase deviations resulted in less congested waveforms, while higher phase deviations led to more congested waveforms due to adjacent sideband signals.

5. Binary Phase Shift Keying (BPSK)

BPSK is a digital phase modulation technique where binary data (0 and 1) is superimposed on an analog carrier signal. A 180° phase shift occurs when the data transitions from 0 to 1 or vice versa. The time and frequency domain plots of the carrier, data, and output modulated signal were obtained.

We also introduced a phase shift of 90° in the carrier signal, resulting in changes in the plots, illustrating the impact of carrier phase on modulation.

6. BPSK Modulation and Demodulation with ADS

In this section, we performed BPSK modulation and demodulation using Advanced Design System (ADS) schematics. The complete communication system, including coding, filtering, modulation, and power amplification, was simulated. The plots of carrier, data, output modulated signal, and the signal's conversion into digital form in both time and frequency domains were observed.

The use of a Root Raised Cosine (RRC) filter was noted in the ADS schematic, as it performs matched filtering to reduce bandwidth and prevent inter-symbol interference (ISI) in both transmission and reception.

7. Quadrature Phase Shift Keying (QPSK)

QPSK is another phase modulation technique where two bits are modulated simultaneously. We observed the constellation diagram, trajectory, and spectrum output for QPSK. Comparisons were made between BPSK and QPSK, highlighting QPSK's higher modulation efficiency and bandwidth requirements.

The constellation diagram for QPSK showed four distinct points representing (0.7, 0.7), (-0.7, 0.7), (-0.7, -0.7), and (0.7, -0.7), illustrating the encoding of two bits per symbol.

8. QPSK Modulation and Demodulation with ADS

In this section, we simulated QPSK modulation and demodulation using ADS schematics. The system's performance was analyzed in terms of constellation diagrams, trajectories, and spectrum output. The impact of delay variables on the constellation diagram was observed, demonstrating the importance of minimizing symbol errors for a robust receiver design.

9. BPSK and QPSK Modulation and Demodulation with VSA

We conducted BPSK and QPSK modulation and demodulation experiments using a Vector Signal Analyzer (VSA). The VSA measurements allowed us to examine the spectrum, constellation diagrams, Complementary Cumulative Distribution Function (CCDF), Error Vector Magnitude (EVM), and I/Q diagrams.

From the CCDF curve, we measured the peak average power ratio, which was found to be -53.13dBm.

Conclusion

In this laboratory report, we explored various aspects of digital systems and modulation techniques. We conducted experiments on amplitude modulation (AM), frequency modulation (FM), and phase modulation (PM), as well as binary phase shift keying (BPSK) and quadrature phase shift keying (QPSK). Through simulation and measurement, we gained valuable insights into the characteristics and performance of these modulation schemes. The findings contribute to our understanding of digital communication systems and their practical applications.