Analog Electronic Circuits (EC2.103) Course Project
Spring 2025, IIIT Hyderabad
This project presents the complete design, simulation, and implementation of a Quadrature Down Converter (QDC) system commonly used in modern wireless receivers such as Bluetooth, Wi-Fi, and WLAN. The QDC performs frequency down-conversion of signals from high frequencies (pass-band) to baseband for easier processing.
A Quadrature Down Converter is a critical component in RF communication systems that translates high-frequency signals to lower intermediate frequencies (IF). The system processes signals in two orthogonal channels - In-phase (I) and Quadrature-phase (Q) components - using sine and cosine local oscillator signals with 90-degree phase difference. This quadrature operation eliminates image frequencies, preserves phase information, and enables superior demodulation accuracy without requiring high-quality analog filters.
The system consists of three main building blocks:
- Quadrature Oscillator - Generates sine and cosine waves at 100 kHz with 90-degree phase difference using Wien Bridge topology
- MOSFET Mixers - Perform frequency translation by multiplying input RF signal with local oscillator signals
- Low Pass Filters - Extract baseband I and Q components by removing high-frequency mixing products
The complete system demonstrates the fundamental principle of quadrature downconversion used in modern communication receivers.
- Wien Bridge Oscillator with buffer amplifiers for load isolation
- MOSFET-based mixers operating in triode region for signal multiplication
- Third-order filtering combining HPF (15 Hz) and three-stage LPF (5.94 kHz effective cutoff)
- Buffer stages between mixer and filters to prevent loading effects
- Quadrature signal processing maintaining 90-degree I-Q phase relationship
| Parameter | Value |
|---|---|
| Oscillator Frequency | 100 kHz |
| Oscillator Amplitude | 1 V peak-to-peak |
| Input Signal Amplitude | 100 mV |
| Input Frequency Range | 95-105 kHz |
| Theoretical LPF Cutoff | 15.9 kHz (third-order) |
| Actual LPF Cutoff | 5.94 kHz (with loading) |
| HPF Cutoff | 15 Hz (DC removal) |
| Filter Roll-off Rate | -60 dB/decade |
| Supply Voltage | +/- 15 V |
| MOSFET Technology | TSMC 180nm |
| Op-Amp Model | UA741 |
- Successfully generated sine and cosine waveforms at precisely 100 kHz
- Maintained accurate 90-degree phase difference between outputs
- Achieved stable 1 V peak-to-peak amplitude with minimal distortion
- FFT analysis confirmed clean spectral output with fundamental frequency at 100 kHz
- Effective frequency translation achieved for all test frequencies (95 kHz, 98 kHz, 99 kHz, 101 kHz, 102 kHz, 105 kHz)
- Output IF frequencies correctly generated as difference between input and oscillator frequencies
- MOSFET switching operation verified in triode region
- Both I and Q channel mixers demonstrated identical performance characteristics
- Third-order RC low-pass filter implemented with theoretical cutoff at 15.9 kHz
- Actual 3 dB cutoff frequency measured at 5.94 kHz due to filter stage loading
- High-pass filter with 15 Hz cutoff added to eliminate DC offset from mixer
- Successfully suppressed high-frequency mixing products with -60 dB/decade roll-off
- Extracted baseband I and Q components with minimal attenuation of desired signals
- I and Q output channels maintained 90-degree phase relationship throughout operation
- Complex baseband representation successfully recovered from downconverted signals
- FFT analysis confirmed proper frequency translation and image rejection
- System demonstrated effective quadrature downconversion for wireless receiver applications
The project successfully demonstrated the design and implementation of a complete Quadrature Down Converter system. The three-stage architecture combining Wien Bridge oscillator, MOSFET mixers, and third-order RC low-pass filters effectively performed frequency downconversion while preserving phase information through I-Q signal processing.
Key achievements include:
- Accurate 90-degree phase difference maintained between I and Q channels
- Successful frequency translation from RF to baseband frequencies
- Image frequency rejection through quadrature processing
- Superior attenuation performance with third-order filtering (-60 dB/decade roll-off)
- Effective DC offset removal using high-pass filter
- Close agreement between simulation and theoretical predictions
The QDC system proves suitable for modern wireless communication applications including Bluetooth, Wi-Fi, and software-defined radio receivers. The design methodology and component selection demonstrate practical implementation of RF circuit theory in real-world communication systems.
AEC_Project_Final/
├── README.md # Project documentation (This File)
├── main.tex # Complete LaTeX project report
├── TSMC_180nm.txt # MOSFET model file (required)
├── UA741.301 # Op-amp model file (required)
├── Final.asc # Complete QDC circuit
├── Final_with_buffers_at_LPFs.asc # QDC with buffer stages
├── Oscillator_Sine_and_Cosine.asc # Oscillator circuit
├── Mixer.asc # Mixer circuit
├── Mixer_and_Lpf.asc # Combined mixer and filter
├── Lpf.asc # Low-pass filter circuit
└── plots/ # Simulation results and figures
When opening any .asc circuit file in LTSpice, ensure that the following model files are present in the same directory:
UA741.301- Op-amp model fileTSMC_180nm.txt- MOSFET model file
Without these files, the simulation will fail with missing model errors. The circuits reference these models for accurate component behavior during simulation.
-
Chanda Akshay Kumar
- Roll No: 2024102014
- Department: ECE
- Email: chanda.kumar@students.iiit.ac.in
-
Jakku Chandini Gayathri
- Roll No: 2024102020
- Department: ECE
- Email: jakku.c@students.iiit.ac.in
-
Jonnalagadda N Ram H K
- Roll No: 2024102045
- Department: ECE
- Email: nagarama.j@students.iiit.ac.in
Institution: International Institute of Information Technology, Hyderabad
- Course: Analog Electronic Circuits (EC2.103)
- Semester: Spring 2025
- Instructor: Prof. Zia Abbas, CVEST, IIIT Hyderabad
For detailed mathematical derivations, circuit schematics, component calculations, and comprehensive analysis, please refer to the complete project report (main.tex).
- A. Abidi, "Direct-Conversion Radio Transceivers for Digital Communications", IEEE Journal of Solid-State Circuits, 1995
- Behzad Razavi, "RF Microelectronics", 2nd Edition
- Sedra and Smith, "Microelectronic Circuits", 7th Edition
Project submitted as part of academic coursework at IIIT Hyderabad.