October 30, 2007

Troubleshooting Digital

Knock Out the Bugs

By Monta Hernon

Just because the world is going digital, cable operators can't forget about analog signals and the "gremlins" that affect them. This was a common thread woven throughout "Troubleshooting Digital Services Node to Tap," a recent webinar sponsored by JDSU and Communications Technology. Click here to view the on-demand version.

"The things that make us do things right in the analog world are also things that will make us do things right in the digital world," said Frank Park, regional vice president of engineering for Bresnan Communications. RF distortions can affect QAM carriers, managing hum can improve QAM carrier quality, and carrier-to-noise ratio (CNR) can transmit to MER on QAM channels, he explained.

"We need to use proper tools to maintain signals and troubleshoot them if something goes on," said Ron Hranac, technical leader with Cisco Systems. "Analog signal levels are our standard with respect to setting the levels of the digital channels," he added, mentioning QAM analyzers, the "workhorse" spectrum analyzer, and boxes that combine both functions into a "nice convenient package" as test equipment to have in the arsenal.

Functionality

As for functionality, Hranac highlighted measuring digital channel power, MER, and bit error rate (BER) and testing for microreflections, channel frequency response, and group delay. "It is important to have access to test equipment and know how to use it when monitoring, maintaining and troubleshooting the digital signals on our network," he said.

Chuck Bublitz, senior marketing manager, JDSU, added hum to the list of analog measurements that can be carried over to the digital side. "Just like multiple analog channels can be affected by the same hum source, so can digital services such as high-speed data and digital video that are on the same network. Particularly if one sees the same approximate level of hum on multiple analog channels, chances are the source is somewhere in the RF distribution network and is also affecting the QAM channels," he said, noting that the JDSU DSAM meter can detect hum to the FCC limit of 1 kHz.

"(The display) provides a visual 'signature' of the hum source, beneficial in determining if the source is common across multiple channels or associated with one channel, thus indicating an issue not related to the common distribution network," Bublitz added.

The DSAM also has a new measurement, digital quality index, which Bublitz described as a "real-time in-service" measurement for detecting intermittent impairments in a downstream QAM carrier. "Since DQI samples the output of the demodulator in the instrument at a much faster rate than either MER or BER, intermittent events in the network are instantly seen. And since BER does not change from ideal until an actual error has occurred, DQI provides extra measurement margin," he said.

As for MER, Hranac recommended testing for unequalized MER as well as for equalized MER. With unequalized MER, there are crash points. For example, for QPSK the crash point is 8-10 dB, and for 64-QAM it is 22-24 dB. Operators should strive for 3-6 dB or more above the respective crash point, Hranac said.

- Monta Hernon