RF design in the 21st century

August 03, 2016 // By Paul Dillien
My first job on leaving college was maintaining military radios. I had covered RF theory, but found that the practice was significantly different. The company’s detailed design work was performed at a remote location, and shrouded in mystery. RF design was a “black art” that only a few specialists could understand. I later moved into pure logic design, where the relative simplicity of 1s and 0s held fewer uncertainties. This ultimately led me into two decades of involvement with FPGAs.

It is striking that logic design methodologies have made huge advances in higher levels of abstraction, tool support, and productivity, while RF design has made comparatively slow progress – until now. I recently discovered a couple of new highly integrated and fully programmable wireless transceivers, and was struck by the similarities between these wireless solutions and FPGAs. Viewed from 30,000 feet, both product types are field programmable, highly flexible, and can be used in a wide range of applications.


The highly integrated wireless products are sometimes classified as field programmable radio frequency (FPRF) devices, which is the term I’ll use from now on. To better understand the impact of FPRF chips, let’s consider how a typical project might run today.


The building blocks of a wireless receiver would be generic with a low noise amplifier (LNA), followed by some filtering. The RF might then be mixed in a tunable superheterodyne (superhet) stage to convert it to an intermediate frequency (IF) followed by a further mixer stage to translate to baseband. Alternatively, direct conversion, also known as zero-IF, is possible using modern semiconductor technology and astute design techniques. The baseband signal would be amplified and filtered, prior to conversion to in-phase and quadrature (I&Q) digital bit streams. The transmit path converts I&Q data streams to analogue signals via DACs, followed by filtering and amplification before being mixed to modulate the RF carrier and on to an RF amplifier.


Conventional design style

A typical “conventional design style” process would use discrete semiconductor components, rather than the new highly integrated chips. The System Architect would define a black box for the RF subsystem detailing all the performance specifications such as frequency of operation, noise figure, dynamic range, output power, and interfaces. The RF Designer will consider architectural options including a superhet scheme using mixers to convert a received signal to a fixed IF, or direct conversion mixer (sometimes called zero-IF),