2003 Three Methods Noise Figure Measurement Three different
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BASESTATIONS WIRELESS INFRASTRUCTURE WIRELESS, CABLE
2003
Three Methods Noise Figure Measurement
Three different methods measure noise figure presented: Gain method, Y-factor method, Noise Figure Meter method. three approaches compared table.
Introduction
wireless communication systems, "Noise Figure (NF)," related "Noise Factor (F)," defines noise performance contributes receiver sensitivity. This application note describes this important parameter details ways measure
Noise Figure Noise Factor
Noise Figure (NF) sometimes ieferred Noise Factor (F). relationship simply: log10
Definition
Noise Figure (Noise Factor) contains important information about noise performance system. basic definition
From this definition, many other popular equations Noise Figure (Noise Factor) derived. Below table typical system Noise Figures:
Category
MAXIM Products
MAX2640 MAX2645
Noise Figure*
0.9dB 2.3dB
Applications
Cellular,
Operating Frequency
400MHz 1500MHz
System Gain
15.1dB
3.4GHz 3.8GHz 14.4dB 3.4GHz 3.8GHz -9.7dB
15.5dB
Mixer Mixer Receiver System Receiver System
MAX2684 MAX9982 MAX2700 MAX2105
13.6dB 12dB 3.5dB 19dB 11.5dB ~15.7dB
LMDS, Cellular, PCS, DBS,
3.4GHz 3.8GHz 825MHz 915MHz 2.0dB
1.8GHz 2.5GHz <80dB 950MHz 2150MHz <60dB
High Gain Mode, Gain Mode Measurement methods vary different applications. shown table above, some applications have high gain noise figure (Low Noise Amplifiers under mode), some have gain high noise figure (mixers LNAs under mode), some have very high gain wide range noise figure (receiver systems). Measurement methods have chosen carefully. this article, Noise Figure Meter well other popular methods "gain method" factor method" will discussed.
Using Noise Figure Meter
Noise Figure Meter/Analyzer employed shown Figure
Figure
noise figure meter, such Agilent N8973A Noise Figure Analyzer, generates 28VDC pulse signal drive noise source (HP346A/B), which generates noise drive device under test (DUT). output then measured noise figure analyzer. Since input noise Signal-to-Noise ratio noise source known analyzer, noise figure calculated internally displayed. certain applications (mixers receivers), signal might needed, shown Figure Also, certain parameters need Noise Figure Meter before measurement, such frequency range, application (Amplifier/Mixer), etc. Using noise figure meter most straightforward measure noise figure. most cases also most accurate. engineer measure noise figure over certain frequency range, analyzer display system gain together with noise figure help measurement. noise figure meter also limitations. analyzers have certain frequency limits. example, Agilent N8973A works from 10MHz 3GHz. Also, when measuring high noise figures, e.g., noise figure exceeding 10dB, result very inaccurate. This method requires very expensive equipment.
Gain Method
mentioned above, there other methods measure noise figure besides directly using noise figure meter. These methods involve more measurements well calculations, under certain conditions, they turn more convenient more accurate. popular method called "Gain Method", which based noise factor definition given earlier:
this definition, "Noise" effects. interference that comes input system form signals that differ from desired one. second random fluctuation carriers system (LNA, mixer, receiver, etc). second effect result Brownian motion, applies thermal equilibrium electronic device, available noise power from device Where Boltzmann's Constant (1.38*10-23 Joules/ Temperature Kelvin, Noise Bandwidth (Hz). room temperature (290 noise power density PNAD -174dBm/Hz. Thus have following equation: PNOUT -174dBm/Hz log10(BW) Gain equation, PNOUT measured total output noise power. -174dBm/Hz noise density 290°K ambient noise. bandwidth frequency range interest. Gain system gain. noise figure DUT. Everything equation scale.
make formula simpler, directly measure output noise power density dBm/Hz), equation becomes: PNOUTD 174dBm/Hz Gain "Gain Method" measure noise figure, gain needs predetermined. Then input terminated with characteristic impedance most applications, video/cable applications). Then output noise power density measured with spectrum analyzer. setup Gain Method shown Figure
Figure example, measure noise figure MAX2700. specified gain setting VAGC, gain measured 80dB. Then, device show above, terminate input with termination. read output noise density 90dBm/Hz. stable accurate reading noise density, optimum ratio (resolution bandwidth) (video bandwidth) RBW/VBW=0.3. Thus calculate -90dBm/Hz 174dBm/Hz 80dB 4.0dB. "Gain Method" cover frequency range, long spectrum analyzer permits. biggest limitation comes from noise floor spectrum analyzer. shown
equations, when Noise Figure (sub 10dB), (POUTD Gain) close -170dBm/Hz. Normal gain about 20dB. that case, need measure noise power density 150dBm/Hz, which lower than noise floor most spectrum analyzers. example, system gain very high, thus most spectrum analyzers accurately measure noise figure. Similarly, Noise Figure very high (e.g., over 30dB), this method also very accurate.
Factor Method
Factor method another popular measure Noise Figure. factor method, (Excess Noise Ratio) source needed. same thing noise source mentioned earlier "Noise Figure Meter" section. setup shown Figure
Figure head usually requires high voltage supply. example, HP346A/B noise sources need 28VDC. Those heads works very wide band (e.g.10MHz 18GHz HP346A/B) they have standard noise figure parameter their specified frequencies. example table given below. noise figures frequencies between those markers extrapolated. Table Example Noise Heads
HP346A HP346B Frequency (Hz) (dB) (dB)
5.39 5.28 5.11 5.07 5.07 15.05 15.01 14.86 14.82 14.81
Turning noise source turning voltage), engineer measures change output noise power density with spectrum analyzer. formula calculate noise figure
which number given table above. normally listed heads. difference between output noise power density when noise source off. equation comes from following: noise head provides noise source "noise temperatures": T=TH (when voltage applied) cold T=290°K. definition noise head
excess noise achieved biasing noisy diode. consider ratio power from amplifier (DUT) from applying cold T=290°K, followed applying T=TH inputs: This factor, from which this method gets name. terms Noise figure, F=Tn/290+1, noise factor (NF=10*log(F)) Thus, Y=ENR/F+1. this equation, everything linear regime, from this equation above. Again, let's MAX2700 example measure noise figure with Y-factor method. show above Figure Connect HP346A noise head input. Connect supply voltage noise head. monitor output noise density spectrum analyzer. Turning then turning power supply, noise density increased from -90dBm/Hz -87dBm/Hz. Y=3dB. Again stable accurate reading noise density, RBW/VBW 0.3. From Table 2GHz,
ENR=5.28dB. Thus calculate 5.3dB.
Summary
this article, three methods measure noise figure devices discussed. They each have advantages disadvantages each suitable certain applications. Below summary table pros cons. Theoretically, measurement results same device should identical, limitations equipment (availability, accuracy, frequency range, noise floor, etc), have carefully choose best method correct results.
Suitable Applications Noise Figure Super Meter
Advantage
Convenient, very accurate when measuring super (0-2dB) Easy setup, very accurate measuring very high suitable frequency range
Disadvantage
Expensive equipment, frequency range limited Limited Spectrum Analyzer noise floor. Can't deal with systems with gain
Gain Method
Very high Gain very high
Factor Method
Wide range
measure wide range When measuring Very high frequency regardless error could large. gain
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