Introduction

In a Wi-Fi receiver, the low-noise amplifier at the front of the chain sets the sensitivity of the whole radio. Its noise figure and gain determine how weak a signal the radio can decode, and because it is first in the chain, no later stage can recover the sensitivity it loses. This application note explains how to design a 5 GHz receive chain around a Skyworks LNA such as the SKY65981-11, covering noise figure, matching, grounding and stability.

Why the First Stage Dominates

In a cascaded chain, the noise contribution of each stage is divided by the gain that precedes it. That is why the first stage dominates the overall noise figure: if the LNA has a noise figure of about 1.6 dB and a gain of 12 dB, the stages after it contribute very little to the total, even if their own noise figures are high. The practical rule is simple: put the best low-noise part first, and keep everything ahead of it as lossless as possible.

The Cost of Loss Before the LNA

Any loss before the LNA adds directly to the effective noise figure. A switch or a filter ahead of the amplifier can add a decibel or more, which may be acceptable but must be counted. This is why the SKY65981-11 is placed immediately after the antenna and the switch, and why the switch is chosen for low insertion loss.

Matching and Layout

The device is internally matched for the 5 GHz band, but the board still matters. Keep the input trace as short as possible, use a controlled impedance and a solid ground reference, and place the decoupling capacitors as close to the device as the layout allows. The input is the most sensitive node in the receiver, so keep it away from the power supply, the digital section and the transmit path, all of which can couple noise into it.

Grounding

A clean ground is essential. Use a solid ground plane under the RF path, provide multiple vias from the device ground pad to the plane, and avoid splitting the ground return under the amplifier. In our lab, poor grounding is one of the most common reasons a receiver's measured noise figure exceeds the data sheet.

Stability

An amplifier must be stable for every source and load impedance it sees, not just the nominal ones. The antenna impedance varies with frequency and environment, and the switch presents a different load in each state, so verify stability across those conditions and at the temperature extremes. A series resistor or a matching network at the output can stabilize a marginal design, at some cost in gain.

Supply and Decoupling

The SKY65981-11 runs from a single supply between 2.8 and 3.6 volts with low current, which suits portable products. Decouple the supply close to the device and keep the supply trace short, because noise on the supply modulates the amplifier and raises the effective noise figure. Use a clean regulator or an LDO for the RF supply where possible.

Bench Validation

Before production, measure the receive chain on the bench: inject a known signal, measure the gain and the noise figure of the whole chain, and compare it with the cascade calculation. Measure at the temperature extremes and with the switch in each state. BeiLuo's RF lab can perform these measurements and supply SKY65981-11 samples for validation.

Conclusion

The low-noise amplifier is the cheapest place to improve receiver sensitivity, but only if the layout supports it. Put the best part first, keep the input clean and well grounded, verify stability across conditions, and measure the finished chain; do those things and the receiver will reach the sensitivity the data sheet promises.