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Rcvr multicoupler gain balance

This forum is for discussions regarding System Infrastructure and Related Equipment. This includes but is not limited to repeaters, base stations, consoles, voters, Voice over IP, system design and implementation, and other related topics.

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Rcvr multicoupler gain balance

Postby Bill_G » Sat Feb 04, 2017 8:23 pm

Whenever you use a preamp, it's important to set the gain correctly. It's doubly important when used in conjunction with a TTA (tower top amp). Too much or too little gain will affect your inbound call success. Achieving an overall receive system gain of 1-3 db is okay, but any more than that will probably overload the front end of the base station rcvr. This applies to any system, analog or digital, in any band. You want to overcome the losses in the transmission system, and no more.

Case and point is a system I worked on this past week. Direct sensitivity to the base stations was -120dbm for 12db sinad. All ten base stations could hit that number directly into their antenna ports. But, generating into the antenna port of the dbSpectra multicoupler shelf, they all came in at -113dbm for 12db sinad. And with the antenna connected, they came in at -109dbm for 12db sinad. That's more than 10db down from where they should be operating making their system tough for portables to access.

The front panel of this model of multicoupler has a binary switch for selecting attenuation - 1-2-4-8 - with a maximum of 15db attenuation possible with just the switches. The magic number for this system turned out to be 8db. By injecting -120dbm into the multicoupler antenna input, I watched the sinad meter as I increased the attenuation. Once I got into the neighborhood of 8db, the sinad numbers hit the 12db mark I was looking for. 2db either side drove the sinad back down. Then I put the antenna back on, did the test again, and again landed around 8db.

Not only was the shelf preamp overdriving the front ends of the receivers, it was also accentuating site noise from the antenna increasing the desense. Once the appropriate amount of attenuation was applied, the desense disappeared, and the overall bottom end performance of the site improved. There was an immediate improvement in the call success rate especially for portables.

More isn't always better. It's important to check the gain balance of your multicouplers and preamps so that you don't impair your system.

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