As part of a broader exploration into RF and analog front-end circuit design, we individually designed and simulated a complete Quadrature Down Converter (QDC) using LTSpice. The QDC is a critical component in communication receivers, responsible for translating a high-frequency RF signal down to baseband while preserving both amplitude and phase information through the generation of in-phase (I) and quadrature-phase (Q) signals.
This project involved the discrete design and integration of the following major analog blocks:
- 🎯 Quadrature Oscillator: Generated two sinusoidal outputs with a 90° phase difference to drive the I and Q mixer paths.
- ⚙️ Double-Balanced Mixers: Multiplied the RF input with LO signals to produce both I and Q intermediate frequencies (IF).
- 🎚️ Low-Pass Filters (LPF): Removed high-frequency components from the mixer output to isolate the desired baseband signal.
- Phase Accuracy: Designed and tuned the quadrature oscillator to produce precise 90-degree phase-shifted outputs.
- Mixing Behavior: Implemented double-balanced mixers for superior isolation and improved linearity.
- Filtering: Tailored LPFs to suppress LO and image frequencies, improving overall signal clarity.
- System Integration: Verified signal path integrity from RF input through to baseband I/Q outputs using transient and FFT analysis.
- Low Phase Noise: Ensured oscillator stability to maintain signal purity and minimize jitter.
- LTSpice for schematic design and simulation
- Transient Analysis for observing time-domain waveforms
- FFT (Frequency Domain) analysis for validating frequency translation and filter behavior
This hands-on project deepened our understanding of:
- RF front-end architecture
- Frequency translation via mixing
- Analog signal chain design and debugging
- Practical implementation and simulation of theoretical concepts
- Moksha Choksi
- Dhanika Kothari
- Kavya Veer
In the final report the calculation part of Quadrature oscillator should be done without consideration of inductor