Noise Figure, Noise Factor and Sensitivity - WINLAB
Noise Figure, Noise Factor and Sensitivity Wireless Systems Instructional Design Noise Any unwanted input Limits systems ability to process weak signals Sources: 1. 2. 3. 4.
Random noise in resistors and transistors Mixer noise Undesired cross-coupling noise Power supply noise Dynamic range capability of detecting weak signals in presence of large-amplitude signals Noise (contd.) noisiness of the signal measure = signal-tonoise ratio (frequency dependant) S ( f ) average signal power SNR N ( f ) average noise power
Random noise External Atmospheric Interstellar Receiver internal
Thermal Flicker noise (low frequency) Shot noise Sky Noise Noise factor IEEE Standards: The noise factor, at a specified input frequency, is defined as the ratio of (1) the total noise power per unit bandwidth available at the output port when noise temperature of the input termination is standard (290 K) to (2) that portion of (1) engendered at the input frequency by the input termination.
available output noise power F available output noise due to source Noise factor (cont.) ( NiG ( f ) Na ) Si Si No Na F 1 NiSo Ni So G ( f ) Ni It is a measure of the degradation of SNR SNRi 1 SNRo
due to the noise added Implies that SNR gets worse as we process the signal Na Spot noise factor F 1 kT The answer is the bandwidth F Noise factor (cont.) Quantitative measure of receiver performance wrt noise for a given bandwidth Noise figure NF 10 log( F ) Typically 8-10 db for modern receivers
Multistage (cascaded) system F2 1 F3 1 Fn 1 F F 1 ... G1 G1G 2 G1G 2 Gn 1 Sensitivity Minimum detectable input signal level for a given output SNR (also called noise floor) Not necessarily related to required output SNR Example
Link Budget Quick and dirty way of estimating RF link performance Prx Ptx Gtx Grx - L - Amisc Prx,Ptx received and transmitted power (dB) Grx,Gtx antenna gain (dBi) L path loss Amisc miscellaneous attenuation Link Budget (cont.)
Path loss (Friis formula): L(dB) 92.45 20log(F) 20log(d) Transmit power: L = 40 dB + 20log(d) @ 2.4 GHz L = 48 dB + 20log(d) @ 5.7 GHz 15 20 dBm (30 100 mW)
Antenna gain: given in decibels over an isotropic antenna (dBi) 0dBi (isotropic), 8 dBi (biquad), 15 dBi (helix), 24 dBi (parabolic) Link Budget (cont.) Received power (senitivity) Orinocco Aironet 350 SNR 11Mbps -82 dBm
-85 dBm 16 dB 5.5 Mbps -87 dBm -89 dBm 11 dB 2 Mbps -91 dBm -91 dBm 7 dB
1 Mbps -94 dBm -94 dBm 4 dB Link Budget (cont.) Amisc: Cables (@ 2.4 GHz)
RG 174: 2 [dB/m] RG 58: 1 [dB/m] RG 213: 0.6 [dB/m] IEEE 802.3: 0.3 [dB/m] LMR-400: 0.22 [dB/m] Connectors (BNC, N, SMA) 0.1 1 dB loss Project Link characterization with Network Analyzer
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