RF design in the 21st century: Page 2 of 7

August 04, 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.

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), which has seen big advances recently with the latest designs and semiconductor processes. Then the system budgeting begins for every element, where decisions are made about the gain and dynamic range required at each block, including voltages and currents to ensure a proper match and avoid any bottlenecks limiting the overall performance.

For a discrete implementation the designer conducts a scan of products available from a range of semiconductor vendors. The cost, performance specifications, interface levels and timing, availability and lead times, along with power supply requirements would be primary considerations for every function. An evaluation board must be designed, manufactured and populated, to test the transceiver design and identify and rectify any adverse performance. Second order effects like power supply ripple or parasitic oscillation may be difficult to pin down at this stage.

Further complications arise when more than one RF frequency is required, and considerably more complex if multiple bands or different bandwidths are needed. Different frequencies may require tunable components and an agile antenna. Design problems are compounded when the specification calls for complex modulation schemes such as orthogonal frequency division multiplexing (OFDM), wideband code division multiple access (WCDMA), quadrature phase-shift keying (QPSK), or quadrature amplitude modulation (QAM) to be supported. QAM modulation is sensitive to both gain and phase errors, so care must be taken of component matching as well as performance over voltage, temperature, and frequency.

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