Subject says it all.  You think that 4W BUC can handle 2 modems?

Hi,
Unfortunately there's not enough info to answer the question.  It depends upon carrier sizing on the upstream, antenna sizing, basically your link budgets, you have to take into consideration a link budget is typically done for a VSAT to size the BUC accordingly for a single modem or carrier on the return leg, now trying to push a second carrier through the BUC may well drive it straight into saturation.  
I would look at the link budgets closely, work out how much power you are using from the BUC, look at your input power to the BUC and see if you have enough capacity to add another carrier.  in all honesty, adding a modem to another system that is built would typically take the original one over the edge, no-one typically over engineers a system by that much.

Physically yes it is possible, be sure to use suitable DC blocks on one of the modems cable runs if the other modem is providing the power (this will add loss but will save you a modem), be sure to use high grade combiner/splitter components, cheap combiners on a system like this is a sure way to induce errors - good combiners typically have the DC block and pass legs as well.  

you will have to be very careful with your Max Tx powers etc when looking at the UCP control from the hub, you need to remember you will have two modems stepping up their Tx power.

Unless of course both satellite routers are in the same network on the same hub and frequency hopping is disabled for their particular inroute. Then they will never burst at the exact same time and no output backoff is necessary and the LBA for a single remote will apply. This is called Deterministic TDMA.

If you are hanging 2 modems on a single IF/RF chain.....Yes....it can be done with splitters.  You can accomplish the same thing with a music box, but if you want to do it on the cheap, just use splitters.

Be careful.

Read what dot says above to avoid both modems transmitting bursts simultaneoulsy.

If you want to transmit bursts simultaneously the power rating of the BUC needs to be the aggregate of the max power required by each transmission plus about 3 dB, so if 0.25 watts is the normal clear sky power per uplink burst (1 watt when +6dB uplink power control is applied) then the total max transmitted power is 2 watts and the BUC power rating needs to be 4 watts.  You must not operate a BUC in multicarrier mode without adequate output back off to stop intermodulation products and distortion.  You will interfere with other return links on nearby frequencies.

In this instance the hub should put both modems into CW transmit mode on slightly different frequencies and measure the two 3rd order IMs at the hub.  Then calibrate the exact clear sky and UPPC operating points.

Best regards, Eric.

Great info all.  I didn't take into account the simultaneous bursts but I didn't think both would have exceeded 4W.  Maybe I can scrounge up a larger SSPA around here ;)  I just want to verify this...  Each remote would be on differnet nets.  But this talk about making sure the remotes don't burst at the same time.  This is being said solely because of the limitation of the BUC correct?


The plans are on hold for this project:)  Musicbox is a bit more than I'd like to spend Mike:)

When operating in single carrier mode a BUC may be operated up to its rated power output.  Don't try to go above this as the voltage spikes in the modulation may damage the output transistors.

When operated in multi-carrier mode, like two simultaneous burst transmissions, it must be operated well backed off by, like -3 dB.  So a 2 watt BUC could support two burst carriers, each at 0.5 watts, total aggregate power 1 watt.

Your hubs need to activate both modems simultaneously with unmodulated CW carriers and carefully measure the 3rd order IMs using a spectrum analyser.  The two 3rd order IMs will appear equally spaced either side of the two test carriers, so the CW test frequencies need to be chosen carefully to not cause interference.  Then adjust the output levels of the modems till the 3rd order IMs are about -25 dB and benchmark these levels as the maximum outputs from each modem. Then reduce the levels to the operating nominal level and set the uplink power control range accordingly.   e.g.  Operating clear sky single burst power = 0.125 watts.  Max single burst power with +6 dB UPPC active = 0.5 watts.  Total power of two carriers = 1 watt.  Plus 3 dB backoff gives suitable BUC rating = 2 watts.  If 0.125 watts per burst in clear sky in inadequate to give nominal C/N at the hub you will need a higher power BUC like 4 or 8 watts or to reduce your uplink burst bit rate.

When choosing a BUC for multicarrier operation ask the manufacturer is if it suitable and what is the recommended back off operating point.  Most cheap low power BUCs are provided for single carrier operation.  You may find that the initiation or termination of the second burst causes phase and amplitude hits on the existing burst and thus introduces errors on your return link bursts.  A highly stable DC supply voltage at the BUC DC power input is important so use low DC resistance coax cable.

Be very careful, any mistake and you will interfere with other peoples' services.

Best regards, Eric.

duckhead wrote on Apr 3^rd, 2009 at 9:32am:

Great info all.  I didn't take into account the simultaneous bursts but I didn't think both would have exceeded 4W.  Maybe I can scrounge up a larger SSPA around here ;)  I just want to verify this...  Each remote would be on differnet nets.  But this talk about making sure the remotes don't burst at the same time.  This is being said solely because of the limitation of the BUC correct? The plans are on hold for this project:)  Musicbox is a bit more than I'd like to spend Mike:)

Are your inroutes configed for freq hop or carrier grooming?