RECENTLY, A STUDY REVEALED a large gap between the fundamental limits of signaling (Shannon capacity) and the limits that are achievable with baseband signaling. In some applications, notches in the frequency domain are part of the link channels' frequency response. Multitone (MT) signaling may then be employed to potentially reduce the gap between current link performance and Shannon capacity. Unfortunately, conventional MT techniques are not energy-efficient at the multi-gigabitper- second operating rates needed for highspeed links. A new technique, dubbed analog multi-tone (AMT), has now been developed by Rambus, Inc.'s Amir Amirkhany, Aliazam Abbasfar, Metha Jeeradit, Ravi T. Kollipara, and Mark Horowitz in conjunction with QUALCOMM's Jafar Savoj, SiTime's Bruno Garlepp, and Vladimir Stojanovic from the Department of Electrical Engineering at Massachusetts Institute of Technology (MIT).

The team's 24-Gb/s software-programmable transmitter was built with 90 nm CMOS. Aside from a digital linear equalizer, it employs a pattern generator and a 12-GSamples/s, 8-b, digital-to-analog converter (DAC). It supports two- and four-channel AMT and baseband transmission from 2 to 256 PAM.

The transmission mode can be selected by programming the appropriate tap coefficients into the equalizer. The transmitter dissipates 510 mW of power. It is fabricated over an area of 0.8 mm2. According to experimental results, clear eye diagrams have been confirmed at 28 Gb/s. The transmitter flaunts 16 effective feedforward- equalizer (FFE) taps, 10-b tap coefficients, and no constraints on the taps' dynamic range. See "A 24 Gb/s Software Programmable Analog Multi-Tone Transmitter," IEEE Journal of Solid-State Circuits, April 2008, p. 999.