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Guide to Calibrating Spectrum Analyzers 1
RF / MICROWAVEFUNDAMENTALS
GUIDE TO CALIBRATINGSPECTRUM ANALYZER
RF MICROWAVE FUNDEMENTALS - Guide to Calibrating Spectrum Analyzers 2
ARCHITECTURE OF SPECTRUMA ANALYZER
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RF MICROWAVE FUNDEMENTALS - Guide to Calibrating Spectrum Analyzers 3
KEY ELEMENTS OF SPECTRUMANALYZER
• RF Input Attenuator
– Reduces the input signal to prevent mixer saturations• Mixer
– Down-converting the input signal to a fixed IF signal
• IF Gain – Variable Gain Amplifier at IF frequency to ensure the reference level at the top of
the display to corresponds to the required input signal level
• IF Filter – Tunable IF bandwidth or resolution bandwidth
– Determines how well input signals with small frequency differences can bedistinguished from each other
– Determines the Noise floor
RF MICROWAVE FUNDEMENTALS - Guide to Calibrating Spectrum Analyzers 4
KEY ELEMENTS OF SPECTRUMANALYZER (CON’T)
• Detector/Log Amplifier – Converting RF signal into corresponding Analog Voltages to be displayed on the
screen
• Video Filter
– Uses LPF to average and smooth the displayed traces• Display
– Shows the Spectrum of the measured input signal
• Sweep Generator – Controls the frequency of the Local Oscillator to ensure fixed IF frequency during
the sweep
• Local Oscillator – For down-converting input signal into a fixed IF signal via the Mixer
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RF MICROWAVE FUNDEMENTALS - Guide to Calibrating Spectrum Analyzers 5
LIST OF CALIBRATIONTESTS
• Display Linearity
• Noise Floor • Input Attenuator
• Frequency Response and Absolute Amplitude Accuracy
• Filter Bandwidth and Shape
• Noise Sidebands
• Residual FM
• Calibration Amplitude Accuracy
• Marker Count Accuracy
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LIST OF CALIBRATION TESTS(CON’T)
• Filter Bandwidth Accuracy
• IF Gain Uncertainty
• Filter Bandwidth Switching
• Reference Level Accuracy
• Frequency Span Accuracy• Frequency Readout Accuracy
• Sweep Time
• Harmonic Distortion
• Third-Order Intermodulation Intercept
• Gain Compression
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DISPLAY LINEARITY
• Calibrates the detector/log amplifier, video filter and display components
of the Spectrum Analyzer
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DISPLAY LINEARITY (CON’T)
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DISPLAY LINEARITY (CON’T)
•Test Setup
RF MICROWAVE FUNDEMENTALS - Guide to Calibrating Spectrum Analyzers 10
DISPLAY LINEARITY (CON’T)
• Verifies Amplitude linearity over a wide dynamic range
• Requires fixed input attenuator and reference levelsettings
• Requires signal source with high-precision attenuationlinearity as a reference standard
• Factors affecting display linearity calibration – Accuracy of reference step attenuators
– Mismatches
– Display resolution of Spectrum Analyzer
– Signal-to-noise ratio of the measurement
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RF MICROWAVE FUNDEMENTALS - Guide to Calibrating Spectrum Analyzers 11
NOISE FLOOR TEST
• Has significant bearing on the Spectrum Analyzer’s ability to
measure low-level signals
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NOISE FLOOR TEST (CON’T)
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NOISE FLOOR TEST (CON’T)
•Test Setup
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NOISE FLOOR TEST (CON’T)
• Need to apply a passive 50-ohm terminator to the SpectrumAnalyzer’s input
• Need to perform Display Linearity and Reference LevelAccuracy Calibration prior to this calibration
• Mean Noise Level, PM=PR + AR in dBm• Factors affecting Noise Floor Calibrations
– Resolution of Spectrum Analyzer Display
– Display Linearity of the Spectrum Analyzer
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INPUT ATTENUATOR TEST
•Calibrates the attenuator circuits of the spectrum
analyzer
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INPUT ATTENUATOR TEST (CON’T)
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INPUT ATTENUATOR TEST (CON’T)
•Test Setup
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INPUT ATTENUATOR TEST (CON’T)
•Stepped Attenuator is important as it prevent Mixer from Saturations and damage
•Monitor ∆MKR while increasing external stepattenuator and decreasing the Analyzer’s inputattenuation accordingly
•Attenuator Accuracy,ACC = (ATTNOM – ATTEXT ACTUAL) - ∆MKR
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INPUT ATTENUATOR TEST (CON’T)
•Factors affecting Input Attenuator Calibrations – Accuracy of the external reference step attenuators
– Mismatch between the 1dB step attenuator and the SpectrumAnalyzer input
– Display resolution of the Spectrum Analyzer
– Display linearity of the Spectrum Analyzer
– Signal-to-noise ratio of the measurement
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ABSOLUTE AMPLITUDE ACCURACYAND FREQUENCY RESPONSE TEST
•Calibrates the attenuator and mixer components of the Spectrum Analyzer
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ABSOLUTE AMPLITUDE ACCURACY ANDFREQUENCY RESPONSE TEST (CON’T)
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ABSOLUTE AMPLITUDE ACCURACY ANDFREQUENCY RESPONSE TEST (CON’T)
•Test Setup
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ABSOLUTE AMPLITUDE ACCURACYAND FREQUENCY RESPONSE TEST(CON’T)
• Measures Amplitude error of the Analyzer as a function of frequency
• Verifies Analyzer’s amplitude response at a fixed levelover its entire frequency
• Requires Power Meter and power sensors
• Compares the Spectrum Analyzer Absolute Power reading and the Power Meter reading at given frequency
• Performed over 25 to 40 frequency points
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ABSOLUTE AMPLITUDE ACCURACYAND FREQUENCY RESPONSE TEST(CON’T)
• Factors affecting Calibration Accuracy – Frequency response of reference power sensor
– Mismatch between the power splitter and the power sensor and betweenthe power splitter and the Spectrum Analyzer
– Resolution of the Spectrum Analyzer display
– Display linearity of the Spectrum Analyzer
– Harmonic content of the signal generator output
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RESOLUTION BANDWIDTH ANDSELECTIVITY TEST
• Calibrates the bandwidth and bandwidth selectivity of theSpectrum Analyzer’s individual resolution filter
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RESOLUTION BANDWIDTH ANDSELECTIVITY TEST (CON’T)
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RESOLUTION BANDWIDTH ANDSELECTIVITY TEST (CON’T)
•Test Setup
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RESOLUTION BANDWIDTH ANDSELECTIVITY TEST (CON’T)
• Bandwidth selectivity (Shape Factor) measures the ability of the Analyzer to resolve closely-spaced signals of unequalamplitude
• Bandwidth Selectivity = ∆f 60dB/ ∆f 3dB
• Factors affecting Bandwidth Selectivity Test – Frequency Span/delta-marker
– Uncertainty of the span per division
– Display linearity of the Spectrum Analyzer
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FILTER BANDWIDTH SWITCHINGTEST
•Calibrates the gain and filter circuits of the Spectrum
Analyzer
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FILTER BANDWIDTH SWITCHINGTEST (CON’T)
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FILTER BANDWIDTH SWITCHINGTEST (CON’T)
•Test Setup
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•Verifies that the filter gain remains constant for different filter bandwidth and span settings
•Factors affecting Filter Bandwidth Switching Test – Display Linearity of the Spectrum Analyzer
– Resolution of the Spectrum Analyzer Display
FILTER BANDWIDTH SWITCHINGTEST (CON’T)
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REFERENCE LEVEL ACCURACY
•Calibrates the IF Amplifier and gain switching
circuits
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REFERENCE LEVEL ACCURACY(CON’T)
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REFERENCE LEVEL ACCURACY(CON’T)
•Test Setup
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REFERENCE LEVEL ACCURACY (CON’T)
•Factors affecting Reference Level Accuracy – Accuracy of the Step Attenuator
– Mismatches
– Resolution of the Spectrum Analyzer Display
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NOISE SIDEBAND TEST
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NOISE SIDEBAND TEST (CON’T)
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NOISE SIDEBAND TEST (CON’T)
•Test Setup
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NOISE SIDEBAND TEST (CON’T)
• Verifies that the noise levels at frequency offsets above andbelow the carrier signal drop off rapidly enough to fall withinthe specified limits
• Important to ensure that the signal generator used in this test
is known to have low phase noise• Can be expressed in dBc or dBc/Hz which shows thesideband noise level normalized to a rectangular 1-Hzbandwidth
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RF MICROWAVE FUNDEMENTALS - Guide to Calibrating Spectrum Analyzers 41
NOISE SIDEBAND TEST (CON’T)
• Factors affecting Noise Sideband Test – Display linearity of the Spectrum Analyzer
– Resolution of the Spectrum Analyzer display – Amplitude modulation noise of the Spectrum Analyzer
– Uncertainty of the 3dB filter bandwidth
– Amplitude modulation noise of the generator signal
– Noise sideband of the signal generator
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RESIDUAL FM TEST
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RESIDUAL FM TEST (CON’T)
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RESIDUAL FM TEST (CON’T)
•Test Setup
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RESIDUAL FM TEST (CON’T)
• measures the residual FM caused by inherent short-term
instability of the analyzer’s local oscillator system• uses the linear part of the analyzer’s IF filter response asan FM demodulator
• Factors affecting Residual FM Test – Display linearity of the spectrum analyzer
– Resolution of the spectrum analyzer display
– The residual FM of the signal generator
– Uncertainty of the frequency span as tested
– Nonlinearity of the IF filter response
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FREQUENCY SPAN TEST
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FREQUENCY SPAN TEST(CON’T)
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FREQUENCY SPAN TEST(CON’T)
•Test Setup
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FREQUENCY SPAN TEST(CON’T)
•determines the accuracy of the analyzer’s frequencyspan
•test the linearity of the analyzer display’s frequencyaxis
•Factors affecting Frequency Span Test – Resolution of the spectrum analyzer display and maker readout
– Frequency accuracy of the synthesizer
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SWEEP TIME ACCURACY TEST
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SWEEP TIME ACCURACY TEST(CON’T)
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SWEEP TIME ACCURACY TEST(CON’T)
•Test Setup
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SWEEP TIME ACCURACY TEST(CON’T)
•Sweep Time = (1/f FG )• (number of cycles of
waveform)•Factors affecting Sweep Time Accuracy Test
– Resolution of the spectrum analyzer display
– Frequency accuracy of the function generator
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HARMONIC DISTORTION TEST
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HARMONIC DISTORTION TEST(CON’T)
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HARMONIC DISTORTION TEST(CON’T)
•Test Setup
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HARMONIC DISTORTION TEST(CON’T)
• verifies that any unwanted harmonic distortion generatedwithin the analyzer is within the specified limits
• Factors affecting Harmonic Distortion Test – Harmonic content of the test signal
– Uncertainty of the reference level
– Display linearity of the spectrum analyzer
– Resolution of the spectrum analyzer display
– Signal-to-noise ratio
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THIRD ORDER INTERMODULATIONINTERCEPT TEST
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THIRD ORDER INTERMODULATIONINTERCEPT TEST (CON’T)
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THIRD ORDER INTERMODULATIONINTERCEPT TEST (CON’T)
•Test Setup
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THIRD ORDER INTERMODULATIONINTERCEPT TEST (CON’T)
• the mixer generate unwanted intermodulation if two signalsare present simultaneously at the spectrum analyzer input
• amount of intermodulation produced is described by theIntermodulation Intercept figure
• Together with the results of the noise floor test, theintermodulation intercept test will determine the dynamicrange of the spectrum analyzer
• third order intermodulation intercept, TOI = RL+(IR3/2)
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THIRD ORDER INTERMODULATIONINTERCEPT TEST (CON’T)
•Factors affecting Third Order IntermodulationIntercept Test
– Resolution of the spectrum analyzer display
– Display linearity of the spectrum analyzer
– Uncertainty of the reference level – Intermodulation rejection of the signal generators combined with the
power coupler
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GAIN COMPRESSION TEST
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GAIN COMPRESSION TEST(CON’T)
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GAIN COMPRESSION TEST(CON’T)
•Test Setup
GAIN COMPRESSION TEST(CON’T)
• Determines how well the spectrum analyzer can measurelow amplitude signals in the presence of high amplitudesignals
• Require two signal generators, a power meter to leveleach source and a coupler to introduce both signalssimultaneous to the spectrum analyzer input
• Factors affecting Gain Compression Test – Resolution of the spectrum analyzer display
– Display linearity of the spectrum analyzer
– Uncertainty of the reference level
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