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![Page 1: Spectrum Analyzer CW Power Measurements and Noise 2012 NCSL International Workshop and Symposium Spectrum Analyzer CW Power Measurements and Noise 1.](https://reader036.fdocuments.net/reader036/viewer/2022062407/56649cce5503460f9499a075/html5/thumbnails/1.jpg)
Page 1
Spectrum Analyzer CW Power Measurements
and Noise
2012 NCSL International Workshop and Symposium
2012 NCSL International Workshop and Symposium
Spectrum Analyzer CW Power Measurements and Noise
![Page 2: Spectrum Analyzer CW Power Measurements and Noise 2012 NCSL International Workshop and Symposium Spectrum Analyzer CW Power Measurements and Noise 1.](https://reader036.fdocuments.net/reader036/viewer/2022062407/56649cce5503460f9499a075/html5/thumbnails/2.jpg)
Overview
• Signal + Noise Model
• Bias & Variance of Noise and Signal + Noise Measurements
• Variance of Averaged Measurements
• Spectrum Analyzer Block Diagram and Measurement Recommendations
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Signal + Noise Model
Signal
Noise = Ni + jNq
Signal+Noise
Ni distribution
Nq distribution
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Spectrum Analyzer Averaging Methods
• Averaging Types:
• Voltage
• Power
• Log Power
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Voltage Power Log Pwr(mV) (mW) (dBm)
Meas 1 16 0.01 -22.81Meas 2 79 0.12 -9.09Meas 3 84 0.14 -8.48Meas 4 762 11.62 10.65Meas 5 178 0.64 -1.97Meas 6 817 13.36 11.26Meas 7 964 18.60 12.70Meas 8 648 8.39 9.24Meas 9 923 17.02 12.31Meas 10 541 5.85 7.67Average 501 7.57 2.15Voltage 501 615 286Power 5.02 7.57 1.64Log Pwr 7.01 8.79 2.15
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Noise – Measurement Bias
Averaging Bias (dB) Notes
Power 0 by definition
Voltage -1.05 -
Log Power -2.51 -
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= Euler’s constant: 0.577216…
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Noise – Standard Deviation of Discrete Measurements
Averaging Std Dev (dB) Notes
Power 4.34
Voltage 4.54
Log Power 5.77 Monte Carlo
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Signal + Noise – Measurement Bias
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Averaging Bias (dB)
Power
Voltage
Log Power
m = signal-to-noise ratio (W/W) = Euler’s constant: 0.577216…
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Signal + Noise – Measurement Bias
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Signal + Noise – Standard Deviation of Discrete Measurements
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Averaging Std Dev (dB)
Power
Voltage (model based on Monte Carlo)
Log Power (model based on Monte Carlo)
m = signal-to-noise ratio (W/W)
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Signal + Noise – Standard Deviation of Discrete Measurements
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Standard Deviation of Averaged Measurements
• Averaging Discrete Measurements – reduced by
• N = number of samples.
• Time Averaged Measurements – reduced by
• tint = integration (measurement) time
• NBW = noise bandwidth
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Filter Type Application NBW/RBW
4-pole sync Most SAs analog 1.128
5-pole sync Some SAs analog 1.111
FFT/digital FFT/digital IF swept SAs 1.056
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Spectrum Analyzer Block Diagram
Inputsignal
InputAttenuator
Preamplifier
Preselector
Mixer
Local Oscillator
Sweep Generator
Processing and Display
IF (Resolution Bandwidth) Filter
Video Filter
Envelope Detector
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Spectrum Analyzer Block Diagram – Input Attenuator and Preamplifier
Inputsignal
InputAttenuator
Preamplifier
Preselector
Mixer
Local Oscillator
Sweep Generator
Processing and Display
IF (Resolution Bandwidth) Filter
Video Filter
Envelope Detector
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Attenuation = 0 dBPreamplifier On
but… don’t overdrive the mixer!
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Spectrum Analyzer Block Diagram – Preselector
Inputsignal
InputAttenuator
Preamplifier
Preselector
Mixer
Local Oscillator
Sweep Generator
Processing and Display
IF (Resolution Bandwidth) Filter
Video Filter
Envelope Detector
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set based on impact to noise floor
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Spectrum Analyzer Block Diagram – Frequency Span & Sweep Time
Inputsignal
InputAttenuator
Preamplifier
Preselector
Mixer
Local Oscillator
Sweep Generator
Processing and Display
IF (Resolution Bandwidth) Filter
Video Filter
Envelope Detector
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span = zerosweep time = based on noise uncertainty
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Spectrum Analyzer Block Diagram – RBW
Inputsignal
InputAttenuator
Preamplifier
Preselector
Mixer
Local Oscillator
Sweep Generator
Processing and Display
IF (Resolution Bandwidth) Filter
Video Filter
Envelope Detector
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Signal + Noise: set RBW to give at least 8 dB SNR. Reducing RBW beyond this does not give any benefit (assumes log power averaging)
Noise: set RBW wide
(power averaging)
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Spectrum Analyzer Block Diagram – RBW (Signal + Noise)
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SNR > 8 dB produces negligible bias when using log power averaging
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Spectrum Analyzer Block Diagram – RBW (Signal + Noise)
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SNR approximated by
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Spectrum Analyzer Block Diagram – RBW (S+N)• Standard Uncertainty of discrete measurements
approximated by:
•
• m = SNR (W/W) =
• For time-averaged measurements:
• But
• Therefore: (time-averaged, SNR > 8 dB)
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For time-averaged measurements, changing RBW does not change uncertainty due to noise
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Spectrum Analyzer Block Diagram – Envelope Detector
Inputsignal
InputAttenuator
Preamplifier
Preselector
Mixer
Local Oscillator
Sweep Generator
Processing and Display
IF (Resolution Bandwidth) Filter
Video Filter
Envelope Detector
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detector = average
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Spectrum Analyzer Block Diagram – Video Filter
Inputsignal
InputAttenuator
Preamplifier
Preselector
Mixer
Local Oscillator
Sweep Generator
Processing and Display
IF (Resolution Bandwidth) Filter
Video Filter
Envelope Detector
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video filter = “wide open”
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Spectrum Analyzer Block Diagram – Averaging Type
Inputsignal
InputAttenuator
Preamplifier
Preselector
Mixer
Local Oscillator
Sweep Generator
Processing and Display
IF (Resolution Bandwidth) Filter
Video Filter
Envelope Detector
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averaging typenoise: powersignal + noise: log power
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Spectrum Analyzer Block Diagram – Trace Averaging & Trace Points
Inputsignal
InputAttenuator
Preamplifier
Preselector
Mixer
Local Oscillator
Sweep Generator
Processing and Display
IF (Resolution Bandwidth) Filter
Video Filter
Envelope Detector
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trace averages = 1trace points = anyfunction: average entire trace
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Spectrum Analyzer Block Diagram – Summary of Recommendations
Function Noise Signal + Noise
Input Attenuation 0
Preamplifier On
Preselector based on impact to noise floor
Frequency Span Zero
Resolution Bandwidth widenarrow enough for SNR > 8 dB
Detector average
Video Bandwidth wide
Averaging Type power log power
Trace Averages 1
Trace Points n/a
Function average entire trace
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