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VSAT technology and installation >> LinkStar Forum: VSAT hubs and terminals >> Explain 1dB compression test please ?
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Message started by imNAK on Jul 23rd, 2010 at 5:57pm

Title: Explain 1dB compression test please ?
Post by imNAK on Jul 23rd, 2010 at 5:57pm
can some one explain ..what is the 1dbm compression test .... related to tx..

thanks
Noor Ahmed
http://www.afsat.net

Title: Re: 1dB BUC compression test
Post by Forum Admin on Jul 23rd, 2010 at 8:18pm
Set the remote site to transmit in CW mode so you can measure the signal level accurately at the hub on a spectrum analyser. Choose a frequency where you are not interfering with anyone. Use a sufficiently narrow resolution bandwith so that the top of the carrier is well clear of the noise floor, so that you get accurate measurements. 20dB (C+N)/N is ideal.

Note the transmit power setting at the remote and the spectrum analyser level, Write them down, about halfway down the paper page.

Decrease the remote site power level by 1 dB and repeat the hub spectrum analyser measurement.  Write down each pair of measurements.  If you are obviously in the linear portion of the curve then go up in level cautiously.  Keep increasing and you will find that each 1 dB up step at the remote no longer causes a 1 dB step up at the hub.  The BUC is starting to be overloaded and the signal is getting distorted.

Once the measured level is 1 dB behind the transmit level the BUC has reached its -1dB gain compression point.  Don't go any higher.  Record the modem power setting and make sure the hub never tells the remote to go above this value during rain, for example.  The normal, clear sky, operating power will typically be about 6 dB below the -1dB compression point.  Set the final clear sky level to give the required Eb/No at the hub as specified in your system design. TDMA hubs may well do this automatically so that all incoming bursts are the same level as far as possible..

The -1 dB gain compression point is where the black double ended arrow is 1 dB long.

Best regards, Eric.

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