Question

Compare the system noise figure of the following two receiving systems: • System A uses a Low Noise Block Down-converter mounted at the focal point of a 2 foot parabolic satellite dish for Ku-Band (12 GHz). The LNB has a noise figure of 3 dB, and has 50 dB gain, outputting a signal at 1 GHz into a 50 ft length of RG-11 coaxial cable that has 9 dB loss per hundred feet. • System B uses a Low Noise Amplifier, mounted at the focal point of the 2 foot parabolic satellite for Ku-band. The NA has a noise figure of .8 dB, and 25 dB gain. The same coaxial cable has about 41 dB loss per hundred feet at 12 GHz.

Answer 1

LNB is a receiving device mounted on satellite dish used for satellite TV reception.

It is a combination of low noise amplifier, mixer, oscillator, and immediate frequency. A typical LNB has a noise figure from 0.2 to 1 dB. its frequency ranges from 11.10 - 12.60 GHz.

Thus if the NF = 3dB then the noise temperature is given by,

= 288.6 K = 289 K

and noise factor is = 1.34 dB

But the typical LNB has a maximum noise factor = 1.01 dB

SYSTEM B: low noise amplifier is a device which amplifies a very low signal. It is mounted on the satellite dish. Its merit is that it amplifies without degrading the Signal to noise ratio.

here we know, noise factor = SNR_in/SNR_out

since noise factor

so lower the NF, smaller the degradation in SNR.

Thus NF of LNA should be very small and should have enough gain to amplify the very low signal.

To compare, the NF of LNA should not exceed 1dB since this affects the SNR. But if the NF of LNB is more than 1 dB then it increases the noise temperature.