When you're designing RF receivers, it's not just about low noise or high gain. One thing that often gets overlooked is input linearity. And yet, it's absolutely central to how a system performs, especially in environments packed with competing signals.
RF design is rarely about maxing out a single spec. Take Noise Figure (NF) and input linearity, for example. Improving one usually makes the other worse. Lower NF gives better sensitivity, but often needs more gain. Higher input linearity, which we usually describe as IIP3, means pulling back on gain. It’s a balancing act.
This article walks through what input linearity is, why it matters, and how to think about trade-offs without getting caught up in one metric at the cost of another.
At its core, linearity is about how well an amplifier preserves the shape of its input signal. If it’s doing the job properly, the output will follow the input in a straight line. But once things start to bend off course, you get distortion. And not just distortion - you get intermodulation products, spectral regrowth, and a whole bunch of other things you’d rather avoid.
In practice, we’re often interested in linearity before the system gets into compression. That’s where intermodulation distortion (IMD) starts creeping in, and that’s where things get interesting from a system design perspective.
Most RF engineers use IP2 and IP3 as the go-to benchmarks for linearity. They tell us when distortion is going to become a serious issue.
IIP3 - that’s Input Third-Order Intercept Point - refers to the input power level at which third-order intermodulation products would reach the same power as the main output signal. OIP3 does the same at the output side. And they’re connected by a simple formula:
OIP3 (dBm) = IIP3 (dBm) + Gain (dB)
This intercept point isn’t where things break, but it’s a marker that shows how close you're getting to the non-linear region…
Read the full, original blog here. Explore more RF design resources, tools and technical insights at qorvo.com.
