Jeff ThomasTom HolmesTerri HightowerLearn RF Spectrum Analysis Basics
Page 2Agenda• Overview: Spectrum analysis and its measurements• Theory of Operation: Spectrum analyzer hardware• Frequency Specifications• Questions and Answers break• Amplitude Specifications• Summary• Questions and Answers break
Page 3Learning Objectives• Name the major measurement strengths of a swept-tuned spectrum analyzer• Explain the importance of frequency resolution, sensitivity, and dynamic range in making analyzer measurements• Outline the procedure making accurate distortion measurements
Page 48563ASPECTRUM ANALYZER 9 kHz - 26.5 GHzOverview: What is Spectrum Analysis?
Page 5.ModulationModulationDistortionDistortionNoiseNoiseTypes of Tests Made
Page 6timeAmplitude(power)frequencyTime domainMeasurementsFrequency DomainMeasurementsFrequency Versus Time Domain
Page 7f1f2Fourier Spectrum Analyzer Fourier analyzer transforms a signal over time into a frequency spectrum DisplayAmplitudeFrequency
Page 8ff1f2Swept-Tuned Spectrum AnalyzerFilter “sweeps” over a frequency rangeDisplayAmplitudeFrequency
Page 9Agenda• Overview: Spectrum analysis and its measurements• Theory of Operation: Spectrum analyzer hardware• Frequency Specifications• Questions and Answers break• Amplitude Specifications• Summary• Questions and Answers break
Page 10FilterFrequencyReferenceLogAmpRF InputAttenuator MixerIF FilterDetectorVideoFilterLocalOscillatorSweepGeneratorIF GainInputDisplaySpectrum Analyzer Block Diagram
Page 11MIXERRFLOIFThe Mixer: Key to a Wide Frequency RangeInputRF = Radio frequencyLO = local oscillatorIF = intermediate frequencyfLOfin000fLO-finfLO+finfLO
Page 12IF FILTERInputIntermediate Frequency (IF) Filter• IF Bandwidth: also known as resolution bandwidth and RBW• Provides shape of frequency domain signal fDisplayedActualf
Page 13DETECTORNegative detection: smallestPositive detection: largestSample detection: lastAmplitudeInputDetectorValues Displayed
Page 14VIDEO FILTERInputWithout video filteringWith video filteringVideo Filter
Page 15LCD DISPLAYSWEEP GENLOfrequencyLocal Oscillator and Sweep Generator• Provides swept display
Page 16IF GAINRF INPUT ATTENUATORInput Attenuator and IF Gain Circuits• Protects input circuits• Calibrates signal amplitude• Keeps signal display position constant
Page 17Agenda• Overview: Spectrum analysis and its measurements• Theory of Operation: Spectrum analyzer hardware• Frequency Specifications• Questions and Answers break• Amplitude Specifications• Summary• Questions and Answers break
Page 18What Spectrum Analyzer Specifications are Important?• Frequency Range• Frequency and Amplitude Accuracy• Frequency Resolution• Sensitivity• Distortion• Dynamic Range
Page 19Frequency RangeLow frequencies for baseband and IFHigh frequencies for harmonics and beyond
Page 20Getting the Frequency Range You NeedLOMixerIF signal, fiffinfin Range01LO Range02fLO-finfLO+finfLO3fif1230 frequencyThe input signal is displayed when fLO -fin = fIF
Page 21Sweep generatorLOGetting the Frequency Range You Need0finfLO-fin fLO+finfLOfin Range01LO Range0234DisplayfifLO FeedthroughIF filter
Page 22LOGetting the Frequency Range You Need0finfLO-finfLO+finfLOfif34finfin Range01LO Range02Input signal displayed fLO -fin = fIF
Page 23Getting the Frequency Range You Need• Lower frequency limited by LO feedthrough• Upper frequency limited by LO range and IF frequency• Microwave frequency measurement usesharmonic mixing
Page 24AbsoluteAmplitudein dBmRelativeAmplitudein dBRelativeFrequencyFrequencyFrequency and Amplitude Accuracy
Page 25Frequency and Amplitude Accuracy•Frequency accuracy: −Internal/external frequency reference−Use of internal counter•Amplitude accuracy:−Not as good as a power meter−Dependent upon measurement procedure−Excellent relative measurements
Page 26Resolution BandwidthResidual FMNoise SidebandsWhat Determines Resolution?RBW Type and SelectivitySignal Resolution
Page 273 dB3 dB BWLOMixerIF Filter/Resolution Bandwidth Filter (RBW)SweepDetectorInputSpectrumDisplayRBWIF Filter Bandwidth
Page 283 dB10 kHz10 kHz RBWResolving Two Equal-level Signals
Page 293 dB60 dB60 dB BW60 dB BW3 dB BW3 dB BWSelectivity =Resolving Two Unequal-level Signals•3 dB bandwidth•Selectivity (filter shape)
Page 3010 kHzDistortionResolving Two Unequal-level Signals• For a RBW of 1 kHz and a selectivity of 15:1, the 60 dB bandwidth is 15 x 1 kHz = 15 kHz...• …so the filter skirt is 7.5 kHz away from the filter’s center frequency7.5 kHz60 dB
Page 31Residual FM"Smears" the SignalResidual FM
Page 32Noise Sidebands can prevent resolution of unequal signalsPhase NoiseNoise Sidebands (Phase Noise)
Page 33Penalty For Sweeping Too FastIs An Uncalibrated DisplaySwept too fastSweep Rate
Page 34SPAN 3 kHzRES BW 100 HzTypical SelectivityAnalog 15:1Digital 5:1Analog versus Digital Resolution BandwidthsAnalog FilterDigital Filter
Page 35Rules to Analyze By:Use the Analyzer’s Automatic Settings Whenever Possible • When using the analyzer in its preset mode, most measurements will be easy, fast, and accurate• Automatic selection of resolution bandwidth, video bandwidth, sweep time and input attenuation• When manuall...
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Page 37Agenda• Overview: Spectrum analysis and its measurements• Theory of Operation: Spectrum analyzer hardware• Frequency Specifications• Questions and Answers break• Amplitude Specifications• Summary• Questions and Answers break
Page 38SweepLOMixerRFInputRES BWFilterDetectorA spectrum analyzer generates and amplifies noise just like any active circuit.Sensitivity and Displayed Average Noise Level
Page 3910 dBAttenuation = 10 dBAttenuation = 20 dB signal levelDisplayed noise is a function of RF input attenuationSignal-to-noise ratio decreases as RF input attenuation is increasedRF Input Attenuator Effects
Page 40Decreased BW = Decreased Noise100 kHz RBW10 dBDisplayed noise is a function of IF filter bandwidthIF Filter (Resolution Bandwidth) Effects10 kHz RBW1 kHz RBW10 dBBest sensitivity = narrowest RBW
Page 41Video BW smoothes noise for easier identification and measurement of low-level signalsVideo Bandwidth Effects
Page 42Signalequalsnoise~2.2 dBSensitivity - the smallest signal that can be measured
Page 43Rules to Analyze By:Getting the Best Sensitivity Requires Three Settings• Narrowest resolution bandwidth• Minimum RF attenuation• Sufficient video filter to smooth noise (VBW < 0.01 Resolution BW)
Page 44Resulting signalMixers Generate DistortionWhere is Distortion Generated?Signal to be measuredFrequency translated signalsMixer-generated distortion
Page 45Most Influential Distortion is the Second and Third OrderTwo-Toned IntermodHarmonic Distortion< -50 dBc< -50 dBc< -40 dBc
Page 46f2f3fPowerin dBDistortion Increases as a Function of the Fundamental’s Power∆ =1 dB ∆=2 dB∆=3 dBFundamental2nd Harmonic3rd HarmonicFor every dB fundamental level change,the 2nd changes 2 dB and the 3rd changes 3 dB.
Page 47How Distortion Amplitudes ChangeSince distortion changes relative to the fundamental, a graphical solution is practical.f2f3f1 dB/dBfund2 dB/dBfund∆ =1 dB ∆=2 dB∆=3 dB
Page 480-20-40-60-80-100-60-300+30.Third Order Intercept - TOISecondOrderThirdOrderPlotting Distortion as a Function of Mixer Level+5Distortion, dBcPower at the mixer =Input level minus the attenuator setting, dBm
Page 49Is the distortion from the signal or from the analyzer?No change in amplitude -distortion is part of input signal (external)Change Input Attenuation by 10 dB1Watch Signal on Screen:Change in amplitude - at least some of the distortion is being generated inside the analyzer (internal)IF GAI...
Page 50DynamicRangeDynamic Range -Optimum Amplitude Difference Between Large and Small Signals
Page 51.Displayed Noise Limits Dynamic RangeDistortion, dBc0-20-40-60-80-100-60-300+30Noise at 10 kHz RBWDisplayed average noise level can be plotted like distortion. 1 kHz RBWPower at the mixer =Input level minus the attenuator setting, dBm
Page 52..Maximum 2nd Order Dynamic RangeMaximum 3rd Order Dynamic RangeDynamic Range as a Function ofDistortion and Noise LevelDistortion and Signal-to-Noise, dBc0-20-40-60-80-100Power at the mixer =Input level minus the attenuator setting, dBm-60-300+30
Page 53Displayed Noise LevelClose-in Dynamic Range Limited by Noise Sidebands100 kHz to 1 MHzSideband NoiseDynamic Range Limited By Noise Sidebands, dBc/HzDynamic Range Limited By Distortion and Displayed Noise
Page 54Rules to Analyze by:Determining Dynamic Range• Internal second and/or third order distortion• Displayed noise level• Noise sidebands when close to large signalsYour spectrum analyzer’s dynamic range is dependent upon:
Page 55+30 dBm Maximum Power Level+10 dBm-45 dBm Second-order DistortionMixer Compression-35 dBm Third-order DistortionSignal/Noise Range 105 dBMeasurement Range 145 dBThird Order Distortion~80 dB0 dBc Noise SidebandsDynamic Range is Defined by Your ApplicationSecond Order Distortion~70 dBNoise S...
Page 56Summary• The RF spectrum analyzer is a heterodyne receiver• Offers a narrow resolution capability over a wide frequency range• Measures small signals in presence of large signals• Remember to:—Adjust the measurement procedure for specific application —Test for internal distorti...
Page 57Agilent Spectrum Analyzer Product Families - Swept TunedPSA SeriesHighest performance SA!3 Hz to 50 GHzPre-selection to 50 GHzWorlds best accuracy (0.24dB) 160 RBW settingsPhase noise optimizationFFT or swept at any RBWComplete set of detectorsFastest spur searchVector signal analysis.856X...
Page 58Agilent Vector Signal Analyzer Product Families89600 SeriesMulti-Format & Flexible vector signal analysisDC – 6.0 GHzBandwidth: 36 MHz RF, 40 MHz BasebandRF and modulation quality of digitalcommunications signals including WLAN.Spectrum & Time (FFT) AnalysisOFDM Analysis (802.11a...
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