A 2.4-GHz Direct-Conversion Merged LNA-Mixer and QVCO in 90nm CMOS

This project, from the RFIC course at Texas A&M, presents the design and simulation of a 2.4GHz direct-conversion receiver frontend implemented in 90nm CMOS technology. The design features a fully differential merged LNA-mixer and a quadrature voltage-controlled oscillator (QVCO), optimized for low power consumption and high integration efficiency in modern wireless systems.

Full Project Report

project_report

Theoretical Background

In radio frequency (RF) receiver design, several architectures are commonly used — including heterodyne, image-reject, and direct-conversion types. The direct-conversion receiver (also known as zero-IF) eliminates intermediate frequency (IF) stages by downconverting the input RF signal directly to baseband:

fIF = fLO − fRF

Since fLO is nearly equal to fRF, the IF lies close to DC, enabling the use of low-pass filters instead of bulky off-chip bandpass filters. This enhances integration and simplifies the overall receiver chain.

A critical requirement for direct-conversion systems is >quadrature local oscillator (LO) generation — providing I and Q signals 90° apart to support complex demodulation and mitigate self-mixing effects. This project achieves that using a low-power Quadrature VCO (QVCO) with anti-phase coupling.

Power efficiency is further improved through the use of a merged LNA–mixer architecture, in which the current generated by the low-noise amplifier (LNA) directly feeds the mixer’s transconductance stage. While this approach minimizes power and area, it introduces a tradeoff with linearity, as the reduced headroom limits the achievable IIP3.

Topology and Design Highlights

The frontend employs a merged LNA-mixer topology where the low-noise amplifier and mixer share current flow, significantly improving power efficiency. The LNA uses a cascode common-source configuration with inductive source and gate degeneration for input matching, while the mixer is based on a Gilbert cell quadrature structure to support direct-conversion architecture.

A quadrature VCO (QVCO) with anti-phase coupling provides the required local oscillator signals, achieving low phase noise and wide tuning range. All inductors were EM-simulated and optimized for quality factor and resonance frequency.

Achieved Results

The proposed receiver demonstrates strong performance compared to similar architectures, achieving high conversion gain and competitive noise figure while maintaining low power operation.

  • QVCO Phase Noise: –122.9 dBc/Hz @ 1 MHz offset
  • QVCO Tuning Range: 2.12 – 2.75 GHz (±12%)
  • LNA-Mixer Gain: 28 dB
  • Noise Figure: 5.33 dB
  • IIP3: 0.312 dBm
  • Power: 9.23 mW
  • LNA-Mixer Realistic Implementation: Verified with EM-simulated inductors using Sonnet
  • Frontend Output: Clean quadrature IF outputs suitable for direct baseband processing

Conclusion

The 2.4GHz direct-conversion receiver successfully integrates the LNA, mixer, and QVCO into a compact, low-power design suitable for applications such as Wi-Fi, Bluetooth, and other ISM-band communication systems. The merged architecture demonstrates the feasibility of achieving excellent performance metrics within the power and voltage constraints of advanced CMOS processes.