Home page of Satellite Internet and Information

Satellite Internet Forum.

Welcome, Guest.
Welcome to this satellite broadband discussion forum. Wherever you are and whatever your problem we are here to help each other. Connecting to the internet via satellite is not always easy but is critically important to those in remote places or with poor terrestrial infrastructure. Both service providers and customers are encouraged to contribute. Register at the bottom of the forum home page if you wish to contribute or ask question. Read the Forum rules.
      Satellite Internet Forum : Home Page          
Pages: 1

Explain 1dB compression test please ?

(Read 7042 times)
Ex Member
Ex Member

Jul 23rd, 2010 at 5:57pm  
can some one explain ..what is the 1dbm compression test .... related to tx..

Noor Ahmed
Back to top
« Last Edit: Aug 18th, 2011 at 3:38pm by Admin1 »  
IP Logged
YaBB Administrator

Personal text from: Profile,
Options, Top line

Posts: 903
Reply #1 - 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.
Back to top
« Last Edit: Jul 24th, 2010 at 7:36am by Admin1 »  
IP Logged
Pages: 1