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"Influence of FM and GSM in the Terrestrial Band"
June 2009

Headend adjustments, weather analogue or digital, is a day-to-day activity for SMATV installers. But don´t let the regularity of the task fool you. To maintain the steady balance in level and power of the complete terrestrial band is not something to be taken ligthly. Tangible differences in adjacent channels can cause important degradation to the overall quality of the signal to be distributed throughout the building. 

Capture 1 - UHF Band with a 500MHz span

An elevated FM can be a potential enemy in any adjustment done to a channel in the UHF band. It could affect an analogue channel or a digital multiplex equally. It is a problem quite commonly observed and to troubleshoot it and to resolve it could be quite tricky and use up a lot of our allocated time to a particular installation job. A good field spectrum analyser can be our best friend. With traditional meters you had to open up the span at least to 500MHz to get a good glimpse of how your band is equalised. However opening up the span also meant a considerable reduction of any specific channels you needed to adjust. You had to continously switch between a wide and narrow span to see how the band was reacting to the specific tweaking done on a channel or multiplex. Even with the most user-friendy meters, the task took up some preciuos time.

Capture 2 – Negative effect on a B.E.R measurement due to an elevated FM influencing the digital multiplex

Capture 3 – Channel affected by the FM with a traditional span view

TWe can also find a negative influence of the GSM band on our UHF television channels and multiplexes. The GSM band oscillates around the 900MHz and strong receptions at the antenna could potentially affect our television distribution.

Capture 4 – GSM band is affecting the high end of the UHF band

So, how can we keep sight of how FM and GSM strong influences are affecting the equalisation of the band and still have a good detailed view of whatever channel or multiplex I am adjusting?

Spectrum Zoom. With this clever feature available on the H45 Advance model, we can simultaneously trace two separate spectrums in one screen. When activating the feature, the screen divides in two halves, having a fixed 20MHz span on the bottom half and a total configurable span between 5MHz and 2,5GHz on the top half.

The good news is that prior to the H45 incorporating Digital Processing, this type of feature was only available in professional laboratory spectrum analysers, but now we are able to find such functionality in a portable meter weighing only 2Kg.

Let´s see how the Zoom Spectrum could have helped us in the previous example:

Capture 5 - Spectrum Zoom of UHF band affected by high FM, with the affected channel tuned

Capture 6 - Spectrum Zoom of UHF band affected by a GSM signal with the affected channel tuned

By using the Spectrum Zoom, the installer could have tweaked channels 40 or 69 as depicted on the examples given and at the same time observe how the rest of the UHF band is behaving. No more toggles between narrow and wide spans. You can now have both in one screen.


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