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How many carriers per SSPA

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Posts: 18
Sep 28th, 2010 at 10:26am  
I want to know how to calculate the number of carriers on a single SSPA say one 350W and a second 400W. The modulation,FEC and other parameters are known. Is there a calculator to calculate and compare SSPAs for multicarrrier operation.
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Eric Johnston
Senior Member

Posts: 2109
Reply #1 - Sep 28th, 2010 at 3:40pm  
1. Look at the specifications of the SSPA and find the graph or table that shows input and output back off versus carrier to intermodulation ratio (C/IM). Ask for a graph that shows the input/output power curve plus the C/IM on the same graph. Look for back off values for C/IM=27dB, C/IM=30dB.

When operated single carrier you may operate the SSPA at its rated power. e.g. 400W or +26 dBW

When operated multi carrier you will operate the SSPA backed off several dB from its rated power. e.g.400W minus 3 dB output back off = 200W or +23 dBW.  Note the 200W is the total aggregate power of all the carriers, e.g. 10 carriers at 20W.

Some earth station SSPAs (and satellite transponders) incorporate linearisers which enable them to be operated nearer saturation without causing excessive intermodulation interference. The amount of back off needed is typically 1 to 4 dB, but it depends on the SSPA design and how the sales person describes it. The 'rated' power is preferably expressed as the -1dB gain compression point of the worst production device. In which case you are guaranteed to get that.  If the figure is 'typical' your 400W amplifier may give more or less, say +/- 1 dB (316W to 501W)  You must check. The difference between 350W and 400W is only 0.6 dB so maybe it is the same device ?

2. From the link budgets you can determine your required uplink transmit EIRP per carrier in clear sky conditions.  Subtract your antenna transmit gain referred to the SSPA flange. So how much power dBW or watts do you need per carrier ? e.g. +7 dBW or 5 watts.  This is the same as you would use if you were transmitting 1 carrier with smaller SSPA. Make allowance of uplink power control (e.g. 6 dB range). So max power per carrier = 20W

If you are proposing to use 8PSK or 16QAM modulation, linearity and AM/PM conversion is important. Make sure your SSPA is generously over rated.

Based on the total power the SSPA can be operated at without causing excessive interference plus the list of carrier powers enhanced by uplink power control, plus some safety margin, you can work out how many carriers are possible.

It is often cheaper to use a larger transmit dish and lower power SSPA. Take into account the cost of the electrical supply and air conditioning needed. For high power SSPAs you need a high loss cross-waveguide coupler on the output, Agilent power meter and Agilent spectrum analyser. The satellite operator will ask you to provide spectrum plots of your SSPA output spectrum to verify acceptably low intermod levels. A 400W dummy load on the spare SSPA will enable you to do tests without interfering with the satellite. Be especially careful with CW carriers.  Set up your IF combiner system with a high value fixed attenuator at input to the SSPA so that no one can accidentally increase a modem output level by 20 dB and jam the satellite and damage your SSPA. Put a key locks on the SSPA gain control, power supply and waveguide switches.

The above simply suggests a C/IM of 30 dB as being acceptable. The actual limit value depends on meeting the satellite operators intermod power spectral density limit for EIRP. Predicting this and modelling it is beyond the scope of text here.  Digitally modulated carriers with scrambling ON, produce smooth intermod products. CW and FM carriers without energy dispersal tend to cause unacceptable interference. If your carriers are in a transponder with adjacent weak low density carriers you will be very restricted on intermodualtion radiation.  If you have your own whole transponder then your worry will be interference into the adjacent transponders, and of course to your own services. 2 carriers will produce 3rd order intermods at frequencies of 2A-B and 2B-A, 3 carriers will generate 3rd order intermods at all permutations of A+B-C.  2nd order intermods exist e.g. 2A at say 2 x 6 GHz = 12 GHz, and should be blocked/absorbed in the SSPA output low pass filter.

I suggest you talk to the satellite operator about link budgets, your dish size and suggestions as to a suitable SSPA. I am not aware of any calculator.

This sort of forum topic is one where an input from an SSPA manufacturer would be helpful and appreciated.

Best regards, Eric
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