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TX power very low

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May 9th, 2012 at 11:42am  
hello,

in our network we have two X5 satmodems with the same cables and BUC model: ALB-1282228 8W.
The first satmodem is able to reach a nominal upstream C/N approx 7.5 dB with a tx power -33 dBm.
The second one has upstream C/N approx 9.45 dB with tx power -40 dBm.
I just moved the warning "low tx power" from -35 dBm to -40 dBm in the iBuilder Warning properties, but -40 dBm (very low power) is still too high because the upstream C/N is higher than the nomina value.
What should be the cause of the difference?
The iDirect X5 Specification indicates: TX Out: Type-F, 950–1700 MHz, +7dBm/-35dBm.
Is the value -40 dBm too low, then not good for the satmodem health?

Regards,
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Reply #1 - May 9th, 2012 at 3:33pm  
First I suspect the difference between these two is caused by the one with lower C/N being slightly off the bird and in need of a Peak and Pol and a compression test.  Second, I am not familiar with ALB BUCs but I can say that Terrasat IBUCs will go into a alarm mode if the dB from the modem goes below -40 (this setting can be adjusted though). Third I am not aware of the low power causing any actual issues with the modem, and the iBuilder alarm shouldnt be cause for concern (its just that such a low reading isnt normal, but its caused by having a more powerful BUC that was actually needed to close the link). If your determined to make this situation disappear then I would add a 10dB attenuator to each TX IFL and recommission both antennas. 

Regards!
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Reply #2 - May 10th, 2012 at 6:44pm  
Hi superDave,

thank you.
I measured the linearity from -35 dBm and -40 dBm and the satmodem is working properly.
Also the first satmodem is working in linearity region, but I think that the cables have a higher loss.

Regards
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Reply #3 - May 10th, 2012 at 6:55pm  
I tried to calculate the link budget for this service (http://www.satsig.net/linkbugt.htm), using the following parameters:

Uplink frequency GHz: 14
Uplink antenna diameter m: 1.2
Uplink antenna, power at the feed W: 8
Bandwidth Hz: 500000
Satellite uplink G/T dB/K: 3

Result: Uplink C/N dB: 19.5953

--------------

Downlink frequency GHz: 12
Downlink receive antenna diameter m: 6.3
Downlink system noise temperature (antenna+LNA) K: 70
Downlink satellite EIRP dBW: 48

Result: Downlink C/N dB: 51.5166


Why?
The Downstream C/N of the satmodem is about 11 dB, the Upstream C/N is approx 7.5 dB with a tx power -33 dBm.

How I have to read the Link budget?

Thankyou.
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« Last Edit: May 10th, 2012 at 8:08pm by N/A »  
 
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Reply #4 - May 12th, 2012 at 10:46pm  
Reference to the link budget is leading us in the wrong direction. (Your transmit power is probably somewhat less than 8W if you are struggling to reduce your power so as not to put a too powerful carrier into the hub.  Also +48 dBW downlink EIRP probably refers to whole transponder, not to a small 500kHz carrier, which would be approx 22 dB lower)

You have an iDirect modem and rather powerful (8W) and high gain (+70 dB) BUC. The result is an excessively good signal at the hub under clear sky conditions, even with the iDirect modem operating at below its minimum output level as specified.

Modems should be operated within their specified level range. e.g 7dBm/-35dBm.   Above and below the range, some distortion or noise floor problem is to be expected. Try to keep 5 dB (or preferably 10 dB) clear from either end if possible, to allow for future adjustment.

I think there is 0 to 30 dB attenuator adjustment possible in the Terrasat iBUC. Check.  If so, reduce the gain by 15 or 20 dB.  Generally equipment works best with any gain settings neither at maximum or minimum.  

Record the actual power out from the IBUC.

When comparing sites, have consideration for the location of the sites in the uplink beam contour pattern (G/T and pfd). Also to the cable lengths and dish sizes.  Poor dish pointing will make the power out increase.  Also when comparing sites with Terrasat IBUCs, check the attenuator settings in the IBUCs and note the readings of power in and power out of the iBUCs, also DC supply volts and current.

Remember that the iDirect modem may increase its power out during heavy rain.  This must be possible without overdriving and damaging the BUC.

Best regards, Eric.
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Reply #5 - May 18th, 2012 at 11:54am  
Thank you Eric.

Speaking about a generic iDirect network with the following details:

Uplink frequency GHz: 14
Uplink antenna diameter m: 1.2
Uplink antenna, power at the feed W: 8
Bandwidth Hz: 500000
E36B uplink G/T dB/K: 3 (Italy)

Result: Uplink C/N dB: 19.5953

--------------

Downlink frequency GHz: 12
Downlink receive antenna diameter m: 6.3
Downlink system noise temperature (antenna+LNA) K: 70
E36B Downlink EIRP dBW: 48 (Italy)

Result: Downlink C/N dB: 51.5166

Can you please explain me how compare the results with my iDirect data about one satmodem?

Downstream C/N = 11 dB
Upstream C/N approx 7.5 dB with a tx power -33 dBm.

Why the LinkBudget report Downlink C/N dB: 51.5166 but the real value is 11 dB?

Regards
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Reply #6 - May 18th, 2012 at 12:34pm  
The figure you are using for satellite downlink EIRP refers to a whole transponder with a single carrier at saturation, such as is applicable for a whole transponder TV carrier.
viz: E36B Downlink EIRP dBW: 48 (Italy)

If your carrier is 500 kHz wide you will need to buy 1.4 x 500 kHz = 700 kHz transponder bandwidth.

If the transponder saturated single carrier EIRP is +48 dBW.

Then for multicarrier operation back off = -3.5 dB  *
Actual total downlink EIRP of all carriers = 48 - 3.5 = 44.5 dBW

Your purchased allocated bandwidth is 700 KHz wide. Transponder is 36 MHz wide. *

Your share of the power is 10 log (700 / 36000) = -17.1 dB

So your carrier downlink power is 44.5 - 17.1 = 27.4 dBW

Put +27.4 dBW as your downlink EIRP.

* use values provided by the satellite operator.

There is more to this than the above.  You should find out the uplink PDFsat with the current transponder gain setting and the multi-carrier input backoff.  From this you work out the exact relationship between uplink input PFD and the downlink power.  

Your 14 GHz, 1.2m dish and 8W gives a PFD of -111 dBW/m^2 at the satellite.  Add +17.1 dB for all the other carriers and the total transponder PDF is -111 + 17 = -94 dBW/m^2
Assuming -6 dB input back off then the PDFsat would be -88 dBW/m^2.  This is just guessing. The satellite operator will be able to tell you the actual PDFsat for the current gain setting and the input back off for multicarrier operation.

Best regards, Eric.
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« Last Edit: May 18th, 2012 at 2:03pm by Eric Johnston »  
 
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Reply #7 - May 31st, 2012 at 11:12am  
Thankyou
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