ee4101 lectures 7 & 8 -- 2010

ee4101 lectures 7 & 8 -- 2010 - 1 EE4101 RF...

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Unformatted text preview: 1 EE4101 RF Communications Part B Prof TS Yeo 2 EE4101 RF Communications Part B – RF systems - Overview • Most operations are in linear (small-signal) region. • Extend of this region (defined either by input power or output power) is called the Dynamic Range. • Proportionality constant of input-output powers is called conversion loss/gain. 3 EE4101 RF Communications Part B – RF systems - overview • Non-zero noise power at output with zero input is called the noise floor. • Input power for output 1 dB below ideal output is called the 1 dB compression point. • Minimum Detectable Signal (MDS) is defined as the input power 3 dB above noise floor. • Dynamic range = 1 dB compression point - MDS 4 EE4101 RF Communications Part B – RF systems - noise oise generated by random motions of charges and charge carriers in evices and materials. oise sources: • Thermal noise: caused by thermal vibration of bound charges. • Shot noise: due to random fluctuations of charge carriers in electron tube and/or solid-state devices. • Flicker noise: occurs in electron tubes and/or solid-state devices. Noise power varies inversely with frequency, also called 1/f noise. • Plasma noise: due to random motion of charges in ionized gases and/or sparking electrical contacts • Quantum noise: due to quantized nature of charge carriers and photons. Generally insignificant compared to other noise sources. 5 EE4101 RF Communications Part B – RF systems – (equivalent) noise temperature • Random motion of electrons in a resistor at temperature T Kelvin produces random voltage fluctuations at resistor terminals. • Voltage has zero average value; non-zero rms value, v n = √ (4kTBR) • B: bandwidth; T: temperature in Kelvin; k (Boltzman’s constant): 1.38x10-23 J/ o K. R is the equivalent resistance 6 EE4101 RF Communications Part B – RF systems – (equivalent) noise temperature • Noisy resistor modeled by Thevenin equivalent consisting of noiseless resistor and noise generator. • Noise power delivered to the load over bandwidth B is • Maximum available thermal noise power from noisy resistor at temperature T is N = kTB kTB R R N v R v n n 4 2 2 ) 2 ( 7 EE4101 RF Communications Part B – RF systems – (equivalent) noise temperature • Arbitrary source of white noise can be modeled as equivalent noise source with equivalent noise temperature T e = N/(kB) • Cooler temperature => less noise • Smaller bandwith => less noise • Consider a noisy amplifier, bandwith B and gain G, matched to noiseless source and load resistors. Output noise power of amplifier N o is thus solely due to noise generated by amplifier itself. 8 EE4101 RF Communications Part B – RF systems – (equivalent) noise temperature • Equivalent noiseless amplifier which produces noise power N o at output due to input noise N i = N o /G = kT e B....
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ee4101 lectures 7 & 8 -- 2010 - 1 EE4101 RF...

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