By Richard A Primerano, Comcast; Rick Spanbauer, Comcast
RF signals propagating through the coaxial plant experience numerous impairments. These include natural attenuation, micro-reflections, ingress, and distortions. These impairments can be counteracted by the predistortion, equalization, and echo-cancellation components built into Data-Over-Cable Service Interface Specifications (DOCSIS®) devices. To assist in validating the performance of these DOCSIS devices, it is necessary to subject them to channels with varying levels of impairments. Today, impaired channels are created by building physical mockups of the hybrid fiber/coax (HFC) plant with cable, splitters, and attenuators. To date, changing micro-reflection levels, for example, requires physically swapping cables and attenuators. This physical approach to channel emulation can be impractical and time consuming when needing to test many scenarios. In this paper, we present a Field-Programmable-Gate-Array-based Coaxial Channel Emulator that allows engineers to emulate reflections, channel tilt, ingress, and distortions in an automated testing framework. We show network analyzer sweeps of an actual coaxial channel side-by-side with the emulator, demonstrating close agreement between the two. This approach to testing has several benefits over traditional techniques. Since it is all-digital, tests are fully repeatable, and the channel can be reconfigured in software. The emulator is capable of modeling channels with hundreds of meters of cable, tens of taps, and arbitrary tilt, ingress, and distortion. Replicating this with hardware would require a very large, expensive setup. The emulator provides 800 MHz of instantaneous bandwidth and has both a web-based user interface and a software application programming interface (API) allowing engineers to vary channel characteristics in real time. The system fits in a 2U rack enclosure, providing a compact alternative to cable-based emulators.