Executive Summary
Reliable operation of today’s two-way cable networks is critical, especially considering that upstream transmission in the return path includes telephone, high-speed data, and various telemetry and other signals. Common path distortion is a type of impairment that can affect the quality of those upstream signals. This document describes test procedures for the characterization of CPD in lab and field environments.
Scope
The purpose of this document is to establish the standard methodology used to measure common path distortion (CPD) in cable systems.
Please note that this procedure is a very unique procedure for measuring common path distortion and distinguishes itself from other similar procedures in the following ways:
- Single port measurement, measures reflected energy.
- Inject two forward carriers separated by the typical analog channel spacing frequency and measure return beat at the channel spacing frequency. For example, in North America, inject two forward carriers separated by 6 MHz and measure the return beat at 6 MHz.
- Used to verify performance of single port passives such as terminators or passives with all other ports terminated. Designed to be consistent with the mechanisms that cause common path distortion in outside plant.
- A procedure is included to extend the test method to a live cable network, involving the injection of forward carriers at certain frequencies in the headend/hub, and measuring a return beat (or beats) at a headend/hub upstream test point.
This procedure is valid for devices that are segmented into classifications as follows:
- Class 3: Products that meet specifications though 1794 MHz
- Class 2: Products that meet specifications through 1218 MHz
- Class 1: Products that meet specifications through 1002 MHz
Benefits
This document describes standard methodology that can be used for characterizing CPD in a lab and field environment. In particular, the lab method is unique in that it supports CPD characterization of single-port devices. An extension of the lab method intended for use in a live cable network can be thought of as treating the entire network as a single-port “black box” of sorts. The procedures in this document can augment other CPD test and measurement techniques used by the cable industry.
Intended Audience
The intended audience for this document includes technicians, engineers, and others with an interest in measuring CPD in a lab environment and/or in a live cable network.