Post by Admin1 on Mar 28th, 2020 at 11:25am
Determining the correct output power setting from the modem that will cause your BUC to saturate is not easy.
The purpose is to define the maximum value for operational use, that will never be exceeded. The value varies accoring to your cable length.
You will normally set the operational power somewhat backed off from saturation. The exact value will be told to you by the satellite network operations centre (NOC) during the carrier line up.
Typically the process will involve you chnaging to a specific frequncy and using unmodulated CW carrier.
Your modem output power level will be reduced to minimum and then increased in steps of 1 or more dB, while the carrier level is measured at the NOC. Initially the measured level will go up 1 dB per 1 dB. This is called the linear operating range. As the BUC input power reaches a high value the measured signal will start to go up by smaller increments. If the NOC plots a graph of modem output versus measured satellite carrier the result will look something like this:
The key value is the -1 dB gain compression point, where the curved line falls 1 dB below an extension of the straight linear operation line.
If you are using a Comtech 570L then you may record the DC current and volts used by the BUC. Write these figures down as it becomes you only local measure of what the BUC is doing. (This assumes you are a VSAT and don't have a waveguide power tap on the BUC output and a Agilent power meter or similar. Teleports have output waveguide power monitoring and microwave spectrum analysers to measure any intermodulation when multiple carriers are used.)
BUC manufacturers normally tell you the guaranteed output power (watts) at the -1 dB compression point. The graph above shows a BUC specified at +3 dBW output but actually capable of +4 dBW, at it actual -1dB gain compression point.
I would still recommend using it only at +3dBW as this will give you a clean undistored signal and not exceed th manufacturers rating.
If you go above the -1 dB gain compression point the signal level will increase slightly as the BUC saturates, but the result will be interference to adjcent carriers and distortion of the wanted signal modulation and thus reduction in quality (increased error rate). Further increase in input power to the BUC may permanently damage the BUC, so be careful adjusting the modem output power. It is important to avoid interference to adjacent carriers.
If you are desiging a link budget it is prudent to use generously rated BUCs so that in normal service the power out is perhaps 6 dB below the -1dB compression point. This gives a good margin for uplink power control to mitigate rain fades. Also, as time goes by the satellite operator may ask you to increase power.
Best regards, Eric.
The purpose is to define the maximum value for operational use, that will never be exceeded. The value varies accoring to your cable length.
You will normally set the operational power somewhat backed off from saturation. The exact value will be told to you by the satellite network operations centre (NOC) during the carrier line up.
Typically the process will involve you chnaging to a specific frequncy and using unmodulated CW carrier.
Your modem output power level will be reduced to minimum and then increased in steps of 1 or more dB, while the carrier level is measured at the NOC. Initially the measured level will go up 1 dB per 1 dB. This is called the linear operating range. As the BUC input power reaches a high value the measured signal will start to go up by smaller increments. If the NOC plots a graph of modem output versus measured satellite carrier the result will look something like this:
The key value is the -1 dB gain compression point, where the curved line falls 1 dB below an extension of the straight linear operation line.
If you are using a Comtech 570L then you may record the DC current and volts used by the BUC. Write these figures down as it becomes you only local measure of what the BUC is doing. (This assumes you are a VSAT and don't have a waveguide power tap on the BUC output and a Agilent power meter or similar. Teleports have output waveguide power monitoring and microwave spectrum analysers to measure any intermodulation when multiple carriers are used.)
BUC manufacturers normally tell you the guaranteed output power (watts) at the -1 dB compression point. The graph above shows a BUC specified at +3 dBW output but actually capable of +4 dBW, at it actual -1dB gain compression point.
I would still recommend using it only at +3dBW as this will give you a clean undistored signal and not exceed th manufacturers rating.
If you go above the -1 dB gain compression point the signal level will increase slightly as the BUC saturates, but the result will be interference to adjcent carriers and distortion of the wanted signal modulation and thus reduction in quality (increased error rate). Further increase in input power to the BUC may permanently damage the BUC, so be careful adjusting the modem output power. It is important to avoid interference to adjacent carriers.
If you are desiging a link budget it is prudent to use generously rated BUCs so that in normal service the power out is perhaps 6 dB below the -1dB compression point. This gives a good margin for uplink power control to mitigate rain fades. Also, as time goes by the satellite operator may ask you to increase power.
Best regards, Eric.
