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A new way of doing 1dB test?

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Ex Member
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Jan 5th, 2012 at 1:39am  
Hello, guys

I notice some field engineers are doing 1dB compression test by themselves, not together with satellite operator....

so basically, they just drive antenna a few degrees away, to somewhere they feel "safe" to shoot at, and increase the modem output, till the BUC reaches 1dB point.

Personally, I feel that is not right, since the 1dB test is not only for BUC itself, but also for the satellite reception end. But I am not sure about that.

Could someone comment on this "new method"? thx.

best regards,
sgl
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Eric Johnston
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Reply #1 - Jan 5th, 2012 at 7:45am  
If you have some means of measuring the output power of your BUC locally then it makes good sense to do the test of the BUC yourself. You are not trying to measure the satellite link overall.

At the teleport hub, output power measurement can be done using a high loss, calibrated cross-guide coupler and a broad band power meter head. Alternatively some BUC/HPA have their own built in power meter. Switch the output to a suitably rated waveguide dummy load or carefully point the dish away from the geo orbit. Ideally have redundant HPAs and do the tests locally using the offline HPA into a dummyload, then switch over. Annual tests like this are good, since an HPA may degrade slowly and if you are operating a 10W carrier on a 150W HPA you might be completely unaware that the actual max power capability has dropped over time to say 16W ! Be aware that if you have hub multicarrier operation then the the BUC must be rated perhaps twice the actual composite power.  e.g Two 5 watt carriers.  During rain, with 6 dB UPPC, these each become 20W carriers, total power = 40W.  BUC rating needs to be at least 80W to avoid causing unacceptable intermod interference.  

If you have small VSATs with no instrumentation then testing with cooperation of the NOC is the way to go.  

The purpose of measuring the -1 dB compression point is to determine the maximum allowable modem output power, in single carrier operation, and maybe verify the output power is similar to the rated value. Putting the max modem output power into the iDirect hub database will stop the system ever increasing the modem output power too far and possibly damaging the BUC, under extreme rain conditions for example.

When lining up a link, in clear sky, wait till C/N is nominal, record the modem output level and observe the difference between the modem output level and the -1dB compression point. More than 6 dB is good as this means thta you have 6 dB available for the uplink power control range used by the iDirect software to compensate for rain.
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Ex Member
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Reply #2 - Jan 5th, 2012 at 8:10am  
Thanks for your input, Eric.

So far, for the few BUC I have worked with, they all have a M&C interface, from which we can check the output power.

The reading may not be accurate, but usually within +/- 0.2 dBm.
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