Executive Summary
Common mode disturbance is generated by power electronics in all active devices. These are typically in the frequency range from 150 kHz to 30 MHz, though higher frequency disturbances are possible. These disturbances can be radiated and cause interference with wireless signals, or can become differential mode and be coupled into the upstream signal path of the cable plant, cause degradations to upstream signal-to-noise.
This document is identical to SCTE 249 2018 except for informative components which may have been updated such as the title page, NOTICE text, headers and footers. No normative changes have been made to this document.
Scope
The purpose of this test is to determine the common mode disturbance generated by power electronics in active CPE equipment. Since conducted disturbances on the AC port is already a part of FCC testing requirements, this method focuses on measurements of the common mode disturbance on the coaxial port. Common mode disturbance from stand-alone power supplies are conducted through a common ground plane on the CPE device to the outer conductor of the coaxial port. Therefore stand-alone power supplies are also within the scope of this standard.
Benefits
Devices that exhibit excessive amount of common mode disturbance performance can cause potential problems such as:
- AM interference – common mode disturbances in the frequency range from 500 kHz to 1.5 MHz can radiate into the air and cause interference to AM radio.
- CATV upstream SNR (MER) – abrupt impedance changes (such as bad contact from a loose connector) causes the common mode disturbance to become differential mode, which is coupled into the coaxial cable. Common mode disturbance from 5 MHz to 42 MHz affects SNR (MER) of existing CATV upstream signals, causing dropped connections and excessive bit error ratio on a per node basis. (Note: This standard extends testing to 200 MHz to take into account higher frequency splits necessitated by DOCSIS 3.1 and Full Duplex DOCSIS.)
Ensuring that all active devices are tested according to this standard successfully mitigates the problems listed above, which in turns improves the resiliency of the DOCSIS network.
Intended Audience
The intended audience for this document are development engineers, quality assurance engineers, product managers, and technical operations engineers from both manufacturers and operators. Technicians and installers can benefit from this document as well to help troubleshoot issues in the field.