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Spectrum Analyzer CW Power Measurements and Noise 2012 NCSL International Workshop and Symposium Spectrum Analyzer CW Power Measurements and Noise 1

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Overview Signal + Noise Model Bias & Variance of Noise and Signal + Noise Measurements Variance of Averaged Measurements Spectrum Analyzer Block Diagram and Measurement Recommendations 2012 NCSL International Workshop and Symposium Spectrum Analyzer CW Power Measurements and Noise 2

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Signal + Noise Model 2012 NCSL International Workshop and Symposium Spectrum Analyzer CW Power Measurements and Noise 3

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Noise Only Measurements 2012 NCSL International Workshop and Symposium Spectrum Analyzer CW Power Measurements and Noise 4 Noise voltage follows a Rayleigh distribution

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Spectrum Analyzer Averaging Methods Averaging Types: Voltage Power Log Power 2012 NCSL International Workshop and Symposium Spectrum Analyzer CW Power Measurements and Noise 5

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Noise – Measurement Bias AveragingBias (dB)Notes Power0by definition Voltage-1.05 Log Power NCSL International Workshop and Symposium Spectrum Analyzer CW Power Measurements and Noise 6 = Euler’s constant: …

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Noise – Standard Deviation of Discrete Measurements AveragingStd Dev (dB)Notes Power4.34 Voltage4.54 Log Power5.77Monte Carlo 2012 NCSL International Workshop and Symposium Spectrum Analyzer CW Power Measurements and Noise 7

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Signal + Noise – Measurement Bias 2012 NCSL International Workshop and Symposium Spectrum Analyzer CW Power Measurements and Noise 8 AveragingBias (dB) Power Voltage Log Power m = signal-to-noise ratio (W/W) = Euler’s constant: …

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Signal + Noise – Measurement Bias 2012 NCSL International Workshop and Symposium Spectrum Analyzer CW Power Measurements and Noise 9

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Signal + Noise – Standard Deviation of Discrete Measurements 2012 NCSL International Workshop and Symposium Spectrum Analyzer CW Power Measurements and Noise 10 AveragingStd Dev (dB) Power Voltage Log Power m = signal-to-noise ratio (W/W)

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Signal + Noise – Standard Deviation of Discrete Measurements 2012 NCSL International Workshop and Symposium Spectrum Analyzer CW Power Measurements and Noise 11

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Standard Deviation of Averaged Measurements 2012 NCSL International Workshop and Symposium Spectrum Analyzer CW Power Measurements and Noise 12 Filter TypeApplicationNBW/RBW 4-pole syncMost SAs analog pole syncSome SAs analog1.111 FFT/digitalFFT/digital IF swept SAs1.056

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Spectrum Analyzer Block Diagram 2012 NCSL International Workshop and Symposium Spectrum Analyzer CW Power Measurements and Noise 13

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Spectrum Analyzer Block Diagram – Input Attenuator and Preamplifier 2012 NCSL International Workshop and Symposium Spectrum Analyzer CW Power Measurements and Noise 14 Attenuation = 0 dB Preamplifier On but… don’t overdrive the mixer!

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Spectrum Analyzer Block Diagram – Preselector 2012 NCSL International Workshop and Symposium Spectrum Analyzer CW Power Measurements and Noise 15 set based on impact to noise floor

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Spectrum Analyzer Block Diagram – Frequency Span & Sweep Time 2012 NCSL International Workshop and Symposium Spectrum Analyzer CW Power Measurements and Noise 16 span = zero sweep time = based on noise uncertainty

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Spectrum Analyzer Block Diagram – RBW 2012 NCSL International Workshop and Symposium Spectrum Analyzer CW Power Measurements and Noise 17 Signal + Noise: set RBW to give at least 8 dB SNR. Reducing RBW beyond this does not give any benefit (assumes log power averaging)

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Spectrum Analyzer Block Diagram – RBW (Signal + Noise) 2012 NCSL International Workshop and Symposium Spectrum Analyzer CW Power Measurements and Noise 18 SNR > 8 dB produces negligible bias when using log power averaging

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Spectrum Analyzer Block Diagram – RBW (Signal + Noise) 2012 NCSL International Workshop and Symposium Spectrum Analyzer CW Power Measurements and Noise 19

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Spectrum Analyzer Block Diagram – RBW (S+N) 2012 NCSL International Workshop and Symposium Spectrum Analyzer CW Power Measurements and Noise 20 For time-averaged measurements, changing RBW does not change uncertainty due to noise

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Spectrum Analyzer Block Diagram – Envelope Detector 2012 NCSL International Workshop and Symposium Spectrum Analyzer CW Power Measurements and Noise 21 detector = average

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Spectrum Analyzer Block Diagram – Video Filter 2012 NCSL International Workshop and Symposium Spectrum Analyzer CW Power Measurements and Noise 22 video filter = “wide open”

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Spectrum Analyzer Block Diagram – Averaging Type 2012 NCSL International Workshop and Symposium Spectrum Analyzer CW Power Measurements and Noise 23 averaging type noise: power signal + noise: log power

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Spectrum Analyzer Block Diagram – Trace Averaging & Trace Points 2012 NCSL International Workshop and Symposium Spectrum Analyzer CW Power Measurements and Noise 24 trace averages = 1 trace points = any function: average entire trace

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Spectrum Analyzer Block Diagram – Summary of Recommendations FunctionNoiseSignal + Noise Input Attenuation 0 Preamplifier On Preselector based on impact to noise floor Frequency Span Zero Resolution Bandwidthwide narrow enough for SNR > 8 dB Detector average Video Bandwidth wide Averaging Type powerlog power Trace Averages 1 Trace Points n/a Function average entire trace 2012 NCSL International Workshop and Symposium Spectrum Analyzer CW Power Measurements and Noise 25

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