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signal modulation tx

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Posts: 2
Jul 2nd, 2010 at 3:44am  

i have problem to connect sinosat-1
i use linkstar s2 modem,  at hub cant recieve tx modulation from modem
but at hub can recieve cw signal.
i using 2.4 meter antena
buc 5 watt

EB/No  9-10
BER 0.0003xx
my modem is broken ?

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Eric Johnston
Senior Member

Posts: 2109
Reply #1 - Jul 2nd, 2010 at 7:25am  
I don't know if the internal power supply in the LinkStar-S2 is adequate for a 5W BUC.  Check the specifications for both items. There is an external BUC power supply option.

You say that your BER is 0.0003xxx   It is quite likely that you can get a good BER over quite a wide range of dish pointing, however the transmit beam is narrower than the receive beam and it is possible that you get a good receive BER but the transmit beam is not yet aimed at the satellite.  Make sure you really are in the centre of the beam. Degrade the elevation pointing till BER=0.01 and mark the nut position, then go to the other side and adjust for exact BER=0.01 also. Half the distance. Repeat in azimuth.  The middle of the receive beam is quite rounded and small movements make little difference, the BER is also varying from one reading to the next, so peaking up by just looking for the highest reading is not effective.  Mechanically halfing the angle between identical (poor) BERs either side gets perfect results.

The transmit level may be wrong. Either too low or too high.  Measure the CW level at the hub. Alter the drive level at the modem and plot the graph modem output power versus CW level at the hub. Then set the modem output level at the -1dB gain compression point of the BUC, or maybe 6 dB or more below this, whatever meets the hub requirement for clear sky Eb/No.

Try reducing the transmit output power from the modem.
Excessive power from the modem will cause the BUC to saturate and generate a distorted signal which will not be successfully demodulated at the hub. It will also cause interference to other sites and may damage your BUC.
Most BUCs take more DC power from the power supply as the drive and output level is increased as they use class B output power transistor stage.

If you get it working please tell us what fixed it.

Best regards, Eric.
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