05.09.2005 11:18 AM
Spectrum analyzers play critical role in reining in COFDM interference

Editor’s note: The following is an excerpt from an article that appears in the May edition of Broadcast Engineering magazine by John Morrow, CEO of Morrow Technologies and Debbie Mucciolo, project manager, Morrow Technologies.

While COFDM is designed to work well in multi-path environments, interference can affect signal levels. COFDM signals are susceptible to the “Cliff Effect” and to over-saturation. Both result in the failure of the receive chain to produce a useable signal.

The “Cliff Effect” refers to the drop off in reception resulting from marginal signal levels. When analog signals degrade, the picture quality degrades with an increase of sparkles or snow. Digital signals lack the corresponding degradation of picture quality; when the signal drops below the threshold, the digital picture simply and abruptly disappears. Conversely, over-saturation occurs when the signal level exceeds the threshold.

The key to effectively managing COFDM signals lies in the use of a spectrum analyzer at the central receive site. This allows measurement of the signal’s power level and the carrier-to-noise ratio, ensuring optimal antenna alignment and reception power. When measuring carrier-to-noise, use of the averaging function will provide an accurate, quantifiable figure.

Monitoring a signal’s carrier-to-noise and power level provides a technician with conclusive information to assess antenna alignment.

The spectrum analyzer can also be used to identify interference. Prior to establishing a link, the analyzer will show the presence of any signals that may cause interference. If the interference occurs after the link has been established, the technician should see changes in the signal, which vary depending on the source of interference. Proactive monitoring gives a technician the opportunity to take corrective action before the interference causes the signal to drop.

To read the article in its entirety, read the May edition of Broadcast Engineering.

For more information, visit www.morrowcorp.com.

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