calibrating spectrum analyzer

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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




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


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




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


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




DISPLAY LINEARITY

• Calibrates the detector/log amplifier, video filter and display components

of the Spectrum Analyzer


DISPLAY LINEARITY (CON’T)





•Test Setup



• 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




NOISE FLOOR TEST

• Has significant bearing on the Spectrum Analyzer’s ability to

measure low-level signals


NOISE FLOOR TEST (CON’T)





•Test Setup



• 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




INPUT ATTENUATOR TEST

•Calibrates the attenuator circuits of the spectrum

analyzer


INPUT ATTENUATOR TEST (CON’T)





•Test Setup



•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





•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


ABSOLUTE AMPLITUDE ACCURACYAND FREQUENCY RESPONSE TEST

•Calibrates the attenuator and mixer components of the Spectrum Analyzer




ABSOLUTE AMPLITUDE ACCURACY ANDFREQUENCY RESPONSE TEST (CON’T)


ABSOLUTE AMPLITUDE ACCURACY ANDFREQUENCY RESPONSE TEST (CON’T)

•Test Setup




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


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


– Harmonic content of the signal generator output




RESOLUTION BANDWIDTH ANDSELECTIVITY TEST

• Calibrates the bandwidth and bandwidth selectivity of theSpectrum Analyzer’s individual resolution filter


RESOLUTION BANDWIDTH ANDSELECTIVITY TEST (CON’T)





•Test Setup



• 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





FILTER BANDWIDTH SWITCHINGTEST

•Calibrates the gain and filter circuits of the Spectrum

Analyzer


FILTER BANDWIDTH SWITCHINGTEST (CON’T)





•Test Setup


•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





REFERENCE LEVEL ACCURACY

•Calibrates the IF Amplifier and gain switching

circuits


REFERENCE LEVEL ACCURACY(CON’T)




REFERENCE LEVEL ACCURACY(CON’T)

•Test Setup


REFERENCE LEVEL ACCURACY (CON’T)

•Factors affecting Reference Level Accuracy – Accuracy of the Step Attenuator

– Mismatches

– Resolution of the Spectrum Analyzer Display




NOISE SIDEBAND TEST


NOISE SIDEBAND TEST (CON’T)





•Test Setup



• 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





• 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


RESIDUAL FM TEST




RESIDUAL FM TEST (CON’T)



•Test Setup





• 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


FREQUENCY SPAN TEST




FREQUENCY SPAN TEST(CON’T)



•Test Setup





•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


SWEEP TIME ACCURACY TEST




SWEEP TIME ACCURACY TEST(CON’T)



•Test Setup





•Sweep Time = (1/f FG )• (number of cycles of

waveform)•Factors affecting Sweep Time Accuracy Test


– Frequency accuracy of the function generator


HARMONIC DISTORTION TEST




HARMONIC DISTORTION TEST(CON’T)



•Test Setup





• 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


– Signal-to-noise ratio


THIRD ORDER INTERMODULATIONINTERCEPT TEST




THIRD ORDER INTERMODULATIONINTERCEPT TEST (CON’T)



•Test Setup





• 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)



•Factors affecting Third Order IntermodulationIntercept Test



– Uncertainty of the reference level – Intermodulation rejection of the signal generators combined with the

power coupler




GAIN COMPRESSION TEST


GAIN COMPRESSION TEST(CON’T)





•Test Setup


• 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


– Uncertainty of the reference level

calibrating spectrum analyzer

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