Implementing a Direct RF Transmitter for Wireless Communications - Application Note - Maxim

The application note summarizes the RF transmitter architectures of zero-IF, complex IF, high (real) IF, and direct RF before detailing the benefits of the direct RF transmitter for wireless applications, which have increased with the rise in smartphone and tablet computer use. As the application note shows, the superiority of a direct RF architecture with a high-performance DAC results in reduced component count and lower power dissipation while synthesizing very wideband signals. A similar version of this article appears on Wireless Design & Development, March 29, 2012. Introduction Wireless radio transmitters have evolved over the years from real IF (intermediate frequency) transmitters, to complex IF transmitters, to zero-IF transmitters. However, there are still limitations associated with these commonly used architectures. A newer approach, a direct RF radio transmitter, can overcome the limitations of traditional transmitters. This article compares various radio transmitter architectures for wireless communications. The direct RF radio transmitter, enabled by a high-performance digital-to-analog converter (DAC), will be shown to have clear advantages over the conventional technologies. The direct-to-RF radio transmitter also has its own challenges, but it paves the way for a true software-defined radio transmitter. An RF DAC, such as the 14-bit 2.3Gsps MAX5879, is an essential component for the direct-to-RF architecture. This DAC achieves excellent spurious and noise performance for bandwidths as wide as 1GHz. It features a novel approach for transmitting in the second and third Nyquist zones so it can perform RF synthesis at output frequencies as high as 3GHz. Measurement results verify the DAC's performance. Traditional RF Transmitter Architectures Traditional transmitter architectures have been implemented over last few decades based on the super-heterodyne principle, where an intermediate frequency (IF) is generated using a local oscillator (LO) and a mixer. The mixer typically creates two images, known as sidebands, around the LO. The wanted signal is then obtained by filtering out one of the sidebands. Modern radio transmitters, specifically the ones used in wireless base transceiver stations (BTS), commonly use complex in-phase (I) and quadrature phase (Q) symbols at baseband for a digitally modulated signal.