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For cable applications at higher data rates, transmitters must have signal bandwidths spanning a few hundred megahertz to gigahertz with a distortion floor below -50 dBc. Although it is possible to use multiple conventional analog transmitters in parallel for such applications, they are likely to consume too much power while occupying too much space. Such transmitters also are driven by a complicated local-oscillator (LO) system and require dedicated analog circuitry for in-phase/quadrature (I/Q) mismatch calibration. A simpler solution would be to use digital-to-analog-converter (DAC) -based transmitters, as shown by recent work from Broadcom’s Silvian Spiridon, Johan van der Tang, Han Yan, Hua-Feng Chen, Davide Guermandi, Xiaodong Liu, Erol Arslan, Frank van der Goes, and Klaas Bult.

Their 40-nm, CMOS, DAC-based multimode transmitter (MMTx) provides a signal bandwidth to 2.2 GHz while exhibiting in-band IM3 below -58 dBc. The MMTx can provide output power to +11 dBm. It exhibits 20 dB of analog power backoff with 1-dB steps and precision beyond ±0.1 dB. Measuring 1.65 mm2, the transmitter consumes just 375 mW.

The current-steering DAC boasts digital sin(x)/x (sinc) equalization and rolloff compensation. It samples to 5 GHz, thanks to the combination of high-speed, feed-forward pipelined digital logic and a distributed decoder. The digital circuit-supply voltage regulators are distributed to curtail the influence of layout and package parasitics. Thus, wideband linearity can be achieved even in noisy system-on-a-chip environments. To ease the transition from the digital input data stream to the analog current cells, a pseudo-differential, custom-designed CMOS latch is implemented. See “A 375-mW Multimode DAC-based Transmitter with 2.2 GHz Signal Bandwidth and In-Band IM3<-58 dBc in 40-nm CMOS,” IEEE Journal Of Solid-State Circuits, July 2013, p. 1603.

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