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Satellite Communications/Relations of ALC Output Power, C/N and C/N Margin


Hi Dave,

I'm try to understand the relationship of C/N and C/N margin when the transponder is operating in ALC mode.

It is known that the output power of TWTA at the spacecraft is consistent when it is operated in ALC mode. Thus, it will make the C/N recorded in the downlink path to be consistent as well, as long as it's operated within the ALC operational range.

But when the signal is pumped into our IRD, it shown the variation of C/N margin being recorded in that IRD. Why this happen? Hypothetically, the C/N margin should be the same.

Hi Sam,

Thanks for this question.

In a satellite link, the resulting C/N is the combination of the uplink C/N and the downlink C/N.

Now in case of uplink fading and the resulting increase in gain (in order to maintain the same downlink EIRP), the gain at the input of the transponder is increased. As a result the noise picked up is also increased. So the uplink C/N will decrease.
This will have a impact on the overall C/N, hence the C/N will slightly decrease even though the transponder operates in ALC.

I hope this quick summary answers your question,


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Dave Suffys


Questions related to design of satellite communication links.


Satellite communication, link budgets, DVB-S2, ACM


Telecommunications engineer, Sales Support Manager at Newtec

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