Superheterodyne Spectrum Analyzer and …

1 Superheterodyne Spectrum Analyzer andSpectrum AnalysisShimshon Levy&Harel MualemAugust 2006 CONTENTSI Superheterodyne Spectrum Analyzer and Spectrum NecessaryBackground:.. Prelab Exercise .. 52 Background FrequencyandTimeDomain .. Spectrum Analyzer Block Diagram and Theory of Operation .. Input Mixer .. IFFilterandSelectivity .. SignalsofUnequalAmplitude .. Sensitivity .. RequiredEquipment .. ReadingAmplitudeandFrequency .. SettheFrequency.. SettheSpan.. SettheAmplitude.. SettheMarker .. ResolvingTwoSignalsofEqualamplitude .. Notice!.. Measuring Signals using Logarithmic and Linear mode, Absolute Using the Maximum -hold Function to View the Frequency Measuring Low-Level Signals Using Attenuation, Video Bandwidth, MeasuringaSignalVeryClosetotheNoiseFloor .

2 Final 20 Part ISuperheterodyne SpectrumAnalyzer and Spectrum Analysis3 Chapter 1 INTRODUCTIONThis experiment deals with Spectrum Analyzer basic operation. The student will learnthe basic concept of frequency domain measurements also called spectral the experiment the student examine it using different kind of ObjectivesUpon completion of the experiment the student will: Get acquainted with the front panel of Spectrum Analyzer . Understand the function of each block of the Spectrum Analyzer . Making basic measurements with a Spectrum Analyzer . Know- how to operate the Spectrum Analyzer . Possess the necessary tools to evaluate signals in frequency Necessary Background:The student needs to have an understanding of Fourier Prelab Exercise1. Using Matlab, draw a graph of squarewave, frequency 1 MHz, amplitude 1Vp,intime domain and frequency domain (magnitude only), compare your graph to theo-retical Matlab, draw a graph of repetitive exponent fall signal (one periody=e t/T) in time domain, and frequency domain (magnitude only)frequency 100kHz, amplitude 1Vp,assume T= Draw block diagram of a Spectrum Analyzer and explain each of the followingblocks (Attenuator, amplifier,LPF, IFfilter,envelope detector, videofilter,display,LO,Rampgenerator).

3 4. Draw a graph in frequency domain of sine wave, square wave, triangle waveFrequency 150kHz, amplitude 10 volt,consists offirstfive 1 Repetitive one side exponent fall References1. ROBERT A. WITTE " Spectrum and Network Measurements" . New Jersy ,Prentice Hall,19912. CYDE F. COOMBS, Jr. "Electronic Instrument Handbook". McGraw-Hill, second edition C. Rauscher: "Fundamental of Spectrum analysis" Rhode&Schwarz Agilent Company: " Spectrum Analysis Basics". Application Note 150,January 2 BACKGROUND Frequency and Time DomainTraditionally, when you want to look at a periodic or non-periodic signal, you use anoscilloscope with or without memory capability to see how the signal varies with is very important information; however, it doesn t give you the full picture. Tofully characterized the performance of your device or system, you are required toanalyze the components of the signal(s) inthe frequency-domain.

4 This is a graphicalrepresentation of the signal s amplitude as a function of frequency. The SpectrumAnalyzer () is a dedicated tool for analyzing and measuring in the frequency domain,such as the oscilloscope is to the time domain. Figure-1 shows three sinewave signalsin both the time and frequency domains. In the time domain, all frequency compo-nents of the signal are summed together and displayed as one signal. In the frequencydomain, complex signals (that is, signals composed of more than One frequency) areseparated into their frequency components, and the level at each frequency is domain measurements have several distinct advantages. For exam-ple, let s say you re looking at a signal on an oscilloscope that appears to be a puresine wave. A pure sine wave has no harmonic distortion. If you look at the signalon a Spectrum Analyzer , you mayfind that your signal is actually made up of severalfrequencies.

5 What was not discernible on the oscilloscope becomes very apparenton the Spectrum Analyzer . From this view of the Spectrum , measurements of fre-quency, power, harmonic content, modulation, spurs, and noise can easily be the capability to measure these quantities, we can determine total harmonicdistortion, occupied bandwidth, Signal stability, output power, intermodulation dis-tortion, power bandwidth, carrier-to-noise ratio, and other measurements, using justa Spectrum Spectrum Analyzer Block Diagram and Theory of OperationThe main components of Spectrum Analyzer are an RF input attenuator, input am-plifier, mixer, IF amplifier, IFfilter, envelope detector, videofilter, CRT display, LO,ramp generator(see Fig - 2 ). Lets describe each component individuallySpectrum Analyzer Block Diagram and Theory of Operation7 FrequencyTimeFrequency domainAmplitudeTime domainFigure 1 Time and Frequency DomainAttenuatorAmpLPFM ixerIFFilterEnvelopeDetectorVideoFilterC RTD isplayRampGeneratorVCOLOI nput sectionFigure 2 simplified block diagram of Heterodyne Spectrum analyzerInput Section8fLOIFRFfRFfLOfffRFfLO -fRFfLO +fRFfLOFigure 3 Mixer Input SectionThe input to the Spectrum Analyzer block diagram has a step attenuator, followedby an amplifier.

6 The purpose of this input section is to control the signal levelapplied to the rest of the instrument. If the signal level is too large, the analyzercircuits will saturate the mixer and distort the signal, causing distortion productsto appear along with the desired signal. If the signal level is too small, the signalmay be masked by noise present in the Analyzer . Either problem tends to reducethe dynamic range of the measurement. The new instruments provide an autorangefeature, which automatically selects an appropriate input attenuation. The inputcircuitry of a typical Analyzer is very sensitive and will not withstand much attention should be paid to the allowable signal level at the input, particularlyfor microwave analyzers. Some instruments tolerate DC voltages at their inputs, butothers require that no DC be applied, or be restricted to small values.

7 The front endof Spectrum Analyzer is made with wide open on the basis that we have no idea howmany signals are involved in our measurement, in order to control somehow on themeasured Spectrum it is necessary to add LPF for image MixerA mixer (Fig-3) is a device that converts a signal from one frequency to another. Itis therefore sometimes called a frequencyconverterdeviceThe output of a mixer consists of the two original signals f{LO} and f{RF} aswell as the sum f{LO}+f{RF} and difference f{LO}-f{RF} frequencies of these IF Filter and SelectivityThe IFfilter is a Band Pass Filter (BPF) which is used as the window for detectingsignals. Its bandwidth is also called the Resolution Bandwidth (RB, RBW) of theSignals of Unequal Amplitude9 AmplitudeFrequencyInput signalIFBandwidthDisplayFigure 4 IFfilter width and resolutionanalyzer and can be changed via the front panel of the Analyzer .

8 By giving you a broadrange of variable resolution bandwidth settings, the instrument can be optimized forthe sweep and signal conditions, letting you trade-offfrequency selectivity (the abilityto resolve signals), signal-to-noise ratio (SNR), and measurement of thefirst things to note is that a signal cannot be displayed as aninfinitely narrow line such as mathematics ( ) delta function. It has some widthassociated with it. This shape is the Analyzer s tracing of its own IFfilter shape as ittunes past a signal. Thus, if we change thefilter bandwidth, we change the width ofthe displayed response. This concept enforces the idea that the IFfilter bandwidthand shape determines the resolvability between measuring two signals of equal-amplitude, (Fig-5) the value of the se-lected BW tells us how close together they can be and still be distinguishable fromone another (by a 3 dB dip ).

9 However, with wider BWs, the two signals may ap-pear as one. In general then, two equal-amplitude signals can be resolved if theirseparation is greater than or equal to the 3-dB bandwidth of the selected Signals of Unequal SensitivityOne of the primary uses of a Spectrum Analyzer is to search out and measure low-levelsignals. The sensitivity of any receiver is an indication of how well it can detects smallsignals. A perfect receiver would add no additional noise to the natural amount ofthermal noise present in all electronic systems, represented bySensitivity10 AmplitudeFrequency-3dB10kHz10kHzFigure 5 Resolving two signal with equal amplitude60dBBabndwidth-60dB-3dB BandwidthBWdBBWdBfactorShape360 =Figure 6 Iffilter shape and selectivitySensitivity11 Attenuation10dBAttenuation20dBSignal level10dBFigure 7 SNR decrease as input attenuation N is the noise power, k is Boltzman s constant, T = temperature inKelvin degree, and B is the bandwidth of thesystem in Hz.

10 In practice, all receivers,including Spectrum analyzers, add some amount of internally generated analyzers usually characterize this by specifying the displayed aver-age noise level in dBm, with the smallest RBW setting. An input signal below thisnoise level cannot be detected. Generally, sensitivity is on the order of -90 dBm to-145 dBm depending on quality of Spectrum Analyzer . It is important to know thesensitivity capability of your Analyzer in order to determine if it will measure yourlow-level signals. One aspect of the Analyzer s internal noise that is often overlookedis its effective level as a function of the RF input attenuator setting (Fig-7). Sincethe internal noise is generated after the mixer (primarily in thefirstactiveIFstage),the RF input attenuator has no effectontheactualnoiselevel.