KU EECS 622 - Noise Figure of Passive Devices (4 pages)

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Noise Figure of Passive Devices



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Noise Figure of Passive Devices

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Pages:
4
School:
University of Kansas
Course:
Eecs 622 - Microwav&Radio Transmsn Systms

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4 17 2005 Noise Figure of Passive Devices 1 4 Noise Figure of Passive Devices Recall that passive devices are typically lossy Thus they have a gain that is less than one we can define this in terms of device attenuation A A 1 G where for a lossy passive device G 1 therefore A 1 Q What is the equivalent noise temperature Te or noise figure F of a passive device i e not an amplifier A The equivalent noise temperature of a passive device can be shown to be approximately trust me Te A 1 T where T is the physical temperature of the passive device Typically we assume this physical temperature to be 290 K D so that Te A 1 290K D Jim Stiles The Univ of Kansas Dept of EECS 4 17 2005 Noise Figure of Passive Devices Tin A Te 2 4 Tout G Tin Te Tin Te A Thus we find that the output noise temperature of a passive device is Tout G Tin Te Tin Te A Tin A 1 290 K D A A Tin 290 K D 290 K D A A This result is very interesting and makes sense physically As attenuation A approaches the lossless case A 1 we find that Tout Tin In other words the noise passes through the device unattneuated and the device produces no internal noise Just like a length of lossless transmission line On the other hand as A gets very large the input noise is completely absorbed by the device The noise at the device output is entirely generated internally with a noise temperature Tout 290 K D equal to its physical temperature Jim Stiles The Univ of Kansas Dept of EECS 4 17 2005 Noise Figure of Passive Devices 3 4 Just like the output of a resistor at physical temperature T 290 K D Q So what is the noise figure F of a passive device Now we determined earlier that the noise figure of a twoport device is related to its equivalent noise temperature as F 1 Te 290 K o Therefore the noise figure of a passive device is F 1 A 1 290K D 290K D 1 A 1 A Thus for a passive device the noise figure is equal to its attenuation F 1 G A So for an active two port device e g an amplifier we find that two important and independent



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