The Challenge
The client needed to reliably detect and decode 2.4GHz wireless signals at distances from 1 meter up to 500 meters — a 500x dynamic range — using a software-defined radio (SDR) receiver. Off-the-shelf RF front ends were either too expensive, too large, or lacked the sensitivity required for the application.
The key constraints were demanding: noise figure below 3dB, gain of at least 20dB, and sufficient filtering to reject out-of-band interference from nearby WiFi and Bluetooth transmitters — all in a compact footprint suitable for a battery-powered enclosure.
The Process
The design was structured as a three-stage signal chain, each stage addressing a specific requirement:
- Stage 1 — Band-Pass Filter: A 5-pole LC BPF centered at 2440MHz with 190MHz bandwidth (2340–2530MHz). Designed in ADS to achieve >35dB rejection outside the passband, protecting the sensitive downstream components from strong out-of-band signals.
- Stage 2 — RF Limiter: A PIN diode-based limiter clamping input power below −5dBm to protect the LNA from large interferers. Chosen for its fast response time and minimal insertion loss in normal operation.
- Stage 3 — Low-Noise Amplifier: The LNA stage provides 22dB gain with a noise figure of 1.8dB. Impedance matching networks on input and output were tuned on a VNA after PCB assembly to hit target S-parameters.
The PCB was laid out in Altium with careful attention to RF grounding: a solid ground plane underneath the signal path, short transmission lines, and SMA connectors at input and output. A full link budget analysis confirmed the system could achieve the required 500m range with 6dB margin.
The Result
The fabricated board was brought up successfully on first spin. VNA measurements confirmed the BPF passband response matched simulation within 5MHz, and the LNA achieved 21.8dB gain with a measured noise figure of 1.95dB — both within specification.
The complete receiver chain enabled reliable long-range wireless detection at ranges far beyond what off-the-shelf RF modules could achieve, at roughly one-quarter the component cost. The design has since been integrated into a production PCB that is manufactured in volume.
- BPF passband: 2340–2530MHz, insertion loss <2dB
- LNA gain: 21.8dB measured, NF: 1.95dB
- Out-of-band rejection: >38dB
- Detection range: reliably 500m in open field