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Solving the Cable TV Infrastructure Downstream Transmitter Challenge

Driven by demand for faster internet connectivity, the cable TV industry has developed new network architectures for the delivery of multigigabit services to subscribers. This fiber deep approach, using a remote PHY device (RPD), moves critical hardware closer to the users by using digital fiber. This is comparable with a remote radio head in wireless (cellular) networks and while this saves space and reduces heat dissipation in the headend, it creates new design challenges for remote equipment.

Although lower in absolute frequency, cable TV signals have much wider bandwidths than wireless, extending across several octaves from 108 MHz to 1218 MHz, with multiple in-band harmonics. RPDs have created a perfect storm for designers, where the RF and mixed-signal hardware must cover a wider frequency range, with higher RF powers, lower noise floor, and better linearity, while consuming less dc power. The downstream final stage RF amplifiers each typically draw 18 W, and with a 4-port system, this is around 50% of the 140 W to 160 W power budget that can typically be delivered to (and dissipated by) an RPD.

ADI’s cable digital predistortion (DPD) efficiency enhancement technology, applied to a DPD optimized power doubler (ADCA3992), combined with advances in high speed data converter technology allows a single DAC (such as the AD9162), and a single ADC (such as the AD9208), complemented by a highly integrated clocking solution (HMC7044) make full-band DPD a reality.

This article describes the evolution to remote PHY and how Analog Devices has solved the efficiency and linearity challenge, using a proprietary DPD, with ADI’s algorithms and IP core integrated within the OEM’s existing FPGA implementation.

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