Calculate the Down-Link Budget of a satellite system based on the following: Inp
ID: 2080067 • Letter: C
Question
Calculate the Down-Link Budget of a satellite system based on the following: Input signal power 10mW Transmit Earth Station gain 40dB Transmit antenna diameter 15 m Uplink carrier frequency 6GHz Satellite receive antenna diameter 1 m Satellite transponder gain 100 dB Satellite Noise Temperature 200 degree Satellite transmit antenna diameter 1m Downlink carrier frequency 4 GHz Receive antenna diameter 6m LNA gain 40dB LNA noise temperature 120 degrees Receive earth station gain 40dB Given a two amplifier cascade with the following characteristics, find the total noise temperature. Amplifier 1: Gain = 15dB, T = 100 degree Kelvin Amplifier2: Gain 15 dB, T = 500 degree Kelvin Reverse the two amplifiers in the above example. Is there any difference in the noise temperature?Explanation / Answer
Qusetion 30.
The Noise Temperature at the output of an amplifier is the sum of the Noise Temperature of the source and the Noise Temperature of the amplifier itself multiplied by the Power Gain of the amplifier.
Tout = G * ( Tampl + Tsource )
Tout = Noise Temperature at amplifier output in degrees Kelvin.
G = Power Gain in linear scale not in dB.
Tampl = Noise Temperature of amplifier.
Tsource = Noise Temperature of source OR at Input of the Amplifier.
For first scenario where Amplifier 1 is first stage and Amplifier 2 is second stage.
Lets assume Tsource for first stage to be 293 degrees Kelvin.
Tout at output of first stage is: Tout(first)= G * ( Tampl + Tsource ) = 31.6*(100+293)= 12418.8 degrees Kelvin
Now Tout(first) serves as Tsource for second stage.
Tout at output of second stage is: Tout(second)= G * ( Tampl + Tsource ) = 31.6*(500+12418.8)= 408234.08 degrees Kelvin.
Toal Noise Temperature for first scenario is 408234.08 degrees Kelvin
Reversing the two amplifiers
For second scenario where Amplifier 2 is first stage and Amplifier 1 is second stage.
Lets again assume Tsource for first stage to be 293 degrees Kelvin.
Tout at output of first stage is: Tout(first)= G * ( Tampl + Tsource ) = 31.6*(500+293)= 25058.8 degrees Kelvin
Now Tout(first) serves as Tsource for second stage.
Tout at output of second stage is: Tout(second)= G * ( Tampl + Tsource ) = 31.6*(100+25058.8)= 792174.08 degrees Kelvin.
Toal Noise Temperature for second scenario is 792174.08 degrees Kelvin
Total Noise temperature of second scenario is greater than Total Noise temperature of first scenario
The first amplifier in a chain has the most significant effect on the total noise temperature than any other amplifier in the chain. The lower noise figure amplifier should usually go first in a line of amplifiers (assuming all else is equal).
If two amplifiers have the same Noise Temperature but different gains, the higher gain amplifier should precede the lower gain amplifier to achieve the best overall Noise Temperature.
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