Guide to Spectrum and Signal Analysis

Guide to Spectrum and Signal Analysis
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Guide to Spectrum and Signal Analysis

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    Guide to Spectrum and Signal Analysis

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    2 | Guide to Spectrum and Signal AnalysisCONTENTSINTRODUCTION ........................................................................................................................................4Frequency Domain / Time Domain ................................................................

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    www.anritsu.com | 3APENDIX ASpectrum Analyzer Conversion Factors ............................................................................................ 42SWR – Reflection Coefficient – Return Loss .......................................................................................

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    4 | Guide to Spectrum and Signal AnalysisINTRODUCTIONEngineers and technicians involved in modern RF or microwave communications have many measuring instruments at their disposal, each designed for specific measurement tasks. Among those available are:a) The Oscilloscope – primarily deve...

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    www.anritsu.com | 5In order to visualize these ‘domains’ refer to Figure 1.This represents an electromagnetic signal as a 3 dimensional model using:(i) a time axis (t)(ii) a frequency axis (f) and(iii) an amplitude axis (a)Observing from position X produces an amplitude time display whe...

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    6 | Guide to Spectrum and Signal AnalysisViewing the model in Figure 1 from position Y, however, produces an amplitude vs. frequency display showing each component of the signal in the complex waveform. Observation in this frequency domain permits a quantitative measurement of the frequency...

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    www.anritsu.com | 7Signal AnalyzersSignal analyzers sample a range of frequencies simultaneously, thus preserving the time dependency and phase between signals. This technique allows both transient and periodic / random signals to be displayed (Figure 5). Signal Analyzers and Spectrum analy...

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    8 | Guide to Spectrum and Signal AnalysisThis is the basic tuning equation that determines the frequency range of a spectrum/signal analyzer. Using the super heterodyne technique enables high sensitivity through the use of intermediate frequency (IF) amplifiers and extended frequency range ...

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    www.anritsu.com | 9It should be noted however, that as the IF bandwidth is reduced so the time to sweep a given frequency range increases since the charge time of the IF filter increases. This means that the sweep time is increased to allow the IF filter to respond and therefore present an ...

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    10 | Guide to Spectrum and Signal AnalysisThe IF bandwidth is normally specified by Δf at 3 dB (Figure 9). From this it can be seen that the narrower the filter bandwidth the greater the frequency resolution. However, as mentioned earlier, as the IF band width is reduced so the charge time...

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    www.anritsu.com | 11When measuring close in spurious components, the shape of the IF filter becomes important. The filter skirt inclination is determined by the ratio of the filter bandwidth at –60 dB to that at –3 dB (Figure 11).This skirt inclination is known as the ‘shape factor’...

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    12 | Guide to Spectrum and Signal AnalysisA spectrum analyzer’s ability to resolve two closely spaced signals of unequal amplitude is not only dependent on the IF filter shape factor. Noise sidebands can reduce the resolution capabilities since they will appear above the skirt of the filt...

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    www.anritsu.com | 13When comparing spectrum analyzer specifications it is important that sensitivity is compared for equal bandwidths since noise varies with bandwidth.An alternative measure of sensitivity is the noise factor FN:where S = Signal and N = NoiseSince the noise factor is a dime...

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    14 | Guide to Spectrum and Signal AnalysisWhen the signal power is added to the average noise power, the resultant signal power displayed will be 3 dB greater (Figure 14). This 3 dB difference is sufficient for low level signal identification.Signal Display RangeThe signal display range of ...

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    www.anritsu.com | 15Distortion products are produced in the analyzer whenever a signal is applied to the input. These distortion products are usually produced by the inherent nonlinearity of the mixer. By biasing the mixer at an optimum level internal distortion products can be kept to a mi...

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    16 | Guide to Spectrum and Signal AnalysisDynamic RangeThe dynamic range of a spectrum/signal analyzer is determined by four key factors.i. Average noise level. This is the noise generated within the spectrum analyzer RF section, and is distributed equally across the entire frequency range...

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    www.anritsu.com | 17Signal AnalyzersSignal analyzers incorporate a wide bandwidth digitizer in the IF to capture a time block of spectrum for analysis. Frequency, time and phase relationships of signals can be analyzed within the bandwidth and time limits of the captured spectrum. Digital m...

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    18 | Guide to Spectrum and Signal AnalysisFigure 19 shows a signal analyzer display of QPSK modulation in polar display format. The polar display is called a constellation or vector diagram. APPLICATIONSAs stated in the introduction, spectrum analyzers are used to display the frequency and ...

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    www.anritsu.com | 19The spectrum analyzer display enables accurate measurement of three key AM parameters.• Modulation Factor m.• Modulation Frequency fm.• Modulation DistortionFigure 20 shows the time domain display of a typical AM signal. From this the modulation factor, m, can be e...

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    20 | Guide to Spectrum and Signal AnalysisFor low levels of modulation it is more convenient to use the analyzers logarithmic display as in Figure 22.The relationship between the sideband level and the percentage modulation is shown in table 1.Figure 22Table 110010.01.00.10.010M [%]Ec – E...

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    www.anritsu.com | 21As an example, consider a case in which the carrier frequency Fc = 1000 MHz, and the modulation frequency fm = 1 kHz.Figure 23 shows the result of observation using an oscilloscope. From the envelope, %M = 50% (m = 0.5).Figure 24 shows the same signal displayed on the li...

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    22 | Guide to Spectrum and Signal AnalysisIf m = 0.05 (%M = 5%), then for the same conditions the sideband level will be 0.165 mV for a carrier level of 6.6 mV. Clearly for low modulation factors the logarithmic display is better suited (Figure 25).Modulation Frequency fmAs stated earlier, ...

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    www.anritsu.com | 23If the modulation factor is high enough, we can use the spectrum analyzer as a fixed tuned receiver as follows:a) set the carrier to the center of the display.b) ensure that the resolution bandwidth and the video bandwidth are sufficiently wide enough to en compass the ...

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    24 | Guide to Spectrum and Signal AnalysisModulation DistortionDistortion of an amplitude modulated carrier wave is commonly due to either or both of the following:a) second and subsequent harmonics of the modulation signal and,b) over modulation of the carrier wave. i.e. %M>100%.Measuri...

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    www.anritsu.com | 25Now consider Figure 29. This shows an over-modulated 100 MHz carrier with fm = 1 kHz. From the time domain display (Figure 30) we can see that the carrier is cut off when the modulation frequency is at a minimum. From the corresponding frequency domain display, the first...

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    26 | Guide to Spectrum and Signal AnalysisBy definition, the information transmitted by amplitude modulation is carried not by the carrier but via the sidebands. Thus varying the composite AM waveform varies only the sideband amplitude. If the carriers component is suppressed, then the over...

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    www.anritsu.com | 27Note that the carrier component J0 and the various sidebands JN go to zero amplitude for specific values of m. From these curves we can determine the amplitude of the carrier and the sideband components in relation to the unmodulated carrier. For example, we find for a m...

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    28 | Guide to Spectrum and Signal AnalysisFor voice communication a higher degree of distortion can be tolerated; that is, we can ignore all side bands with less that 10% of the carrier voltage (20 dB). We can calculate the necessary bandwidth B using the approximation:So far our discussion...

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    www.anritsu.com | 29FM Measurements with a Spectrum AnalyzerThe spectrum analyzer is a very useful tool for measuring Δf and m and for making fast and accurate adjustments of FM transmitters. It is also frequently used for calibrating frequency deviation meters.A signal generator or transm...

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    30 | Guide to Spectrum and Signal AnalysisThe spectrum analyzer can also be used to monitor FM transmitters (for example, broadcast or communications stations) for occupied bandwidth. Here the statistical nature of the modulation must be considered. The signal must be observed long enough t...

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    www.anritsu.com | 31As with AM, it is possible to recover the modulating signal. The analyzer is used as a manually tuned receiver (zero span) with a wide IF bandwidth. However, in contrast to AM, the signal is not tuned into the passband center but to one slope of the filter curve as illus...

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    32 | Guide to Spectrum and Signal AnalysisAM Plus FM (Incidental FM)Although AM and FM are different methods of modulation, they have one property in common; they always produce a symmetrical sideband spectrum.Figure 37 illustrates a modulated carrier with asymmetrical sidebands. One way th...

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    www.anritsu.com | 33PULSE AND PULSE MODULATED SIGNALSWhen a perfectly rectangular pulse waveform is transformed from the time domain to the frequency do main (Figure 38), the resulting envelope follows a function of the form:Figure 39 shows the spectral plot resulting from rectangular ampli...

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    34 | Guide to Spectrum and Signal AnalysisWe know from single tone AM how the sidebands are produced above and below the carrier frequency. The idea is the same for a pulse, except that the pulse is made up of many tones, thereby producing multiple sidebands which are commonly referred to a...

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    www.anritsu.com | 35A pulse repetition rate equal to the resolution bandwidth is the demarcation line between a true Fourier series spectrum, where each line is a response representing the energy contained in that harmonic and a pulse of the Fourier transform response.Pulse Spectrum A pulse...

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    36 | Guide to Spectrum and Signal AnalysisIntermodulation distortion is even generated in the spectrum analyzer itself and this distortion component is determined by the mixer input level. Consequently, when measuring intermodulation distortion using a spectrum analyzer, it is necessary to ...

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    www.anritsu.com | 371) Averaging noise powerSince a spectrum analyzer has a peak hold circuit in front of the A/D converter, when noise is measured, the maximum power of the noise over the sampling period is displayed. Generally, noise is evaluated as the average value of the power against ...

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    38 | Guide to Spectrum and Signal Analysisb) N% methodThe occupied frequency bandwidth is calculated as the bandwidth containing N% of the power trans mitted where N can be between 1% and 99%. A typical example is shown in Figure 44.Figure 44

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    www.anritsu.com | 39Adjacent Channel Leakage PowerAnother common transmitter measurement is that of adjacent channel leakage power. This is defined as the ratio of the amount of leakage power in an adjacent channel to the total transmitted power. In order to calculate the upper and lower ad...

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    40 | Guide to Spectrum and Signal AnalysisBurst Average PowerTime domain spectrum analysis is a vital tool for analyzing pulsed or burst signals. One important measurement is burst average power which computes the average power within the burst “on” time (Figure 46). Using the same meas...

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    www.anritsu.com | 41Error Vector MagnitudeAs indicated earlier, signal analyzers are used to measure digital modulation. Vector and constellation diagrams are used to display the results. Error Vector Magnitude (EVM) is a measure used to quantify the quality or performance of a modulated si...

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    42 | Guide to Spectrum and Signal AnalysisAPPENDIX A Spectrum Analyzer Conversion Factors 50 Ω Input InpedanceTo →From ↓dBmdBVdBmVdBµVdBm0–13+47+107dBV+130+60+120dBmV–47–600+60dBµV–107–120–60075 Ω Input InpedanceTo →From ↓dBmdBVdBmVdBµVdBm0–11.25+48.7+108.7dBV...

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    www.anritsu.com | 43 SWR – Reflection Coefficient – Return LossSWRReflectionCoefficientReturn Loss(dB)SWRReflectionCoefficientReturn Loss(dB)17.3910.891311.05800.0282318.72420.794321.05150.0251325.84800.707931.04850.0224334.41940.631041.04070.0200343.56980.562351.03620.0178353.00950.501...

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    44 | Guide to Spectrum and Signal AnalysisPower MeasurementdBm+40+30+20+1000–10–20–30–40(Watts vs. dBm)Power Ratio dBm – mW – W1 mW10 mW100 mW1 W10 W100 nW1 W10 W100 W1 mW

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    www.anritsu.com | 45APPENDIX BAmplitude ModulationEcEminEmax%M = x 100 (Emax – Emin)(Emax + Emin)%M = x 100 2ELSBECFrequencyLinear AmplitudeUpperSidebandEUSBCarrierECModulatingFrequencyLowerSidebandELSB

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    46 | Guide to Spectrum and Signal AnalysisEc%ModulationSide LevelBelow Carrier(dB)146240192620203016.5401450126010.4709.1807.9906.91006.0SidebandLeverl BelowCarrier(dB)%Modulation10632020306.3402.0500.63600.2700.063800.02

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    www.anritsu.com | 4710010.01.00.10.010M [%]Ec – Esb [dB]–10–20–30–40–50–60–70

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    48 | Guide to Spectrum and Signal AnalysisAPPENDIX CCarrier1st Sideband2nd3rd4th5thCarrier & Sideband AmplitudeDeviation Ratio1.90.80.60.40.200.20.41234567896th7thModulatingCarrier

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    www.anritsu.com | 49Bessel FunctionsCarrierBassel NULLNumberM = ∆F/ƒ1st2.40482nd5.52013rd8.65314th11.79155th14.93096th18.07117th21.21168th24.35259th27.493510th30.6346Where M = modulation index ∆F = deviation ƒ = modulating frequency

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    50 | Guide to Spectrum and Signal Analysis1st SidebandBassel NULLNumberM = ∆F/ƒ1st3.832nd7.023rd10.174th13.325th16.476th19.627th22.768th25.909th29.05Where M = modulation index ∆F = deviation ƒ = modulating frequency

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    www.anritsu.com | 51APPENDIX DPulse Modulation1008060402010864218642110080604020108642186421Pulse Width (tpw) - sBandwidth (RBW) - kHzOptimum RBW as a Function of Pulse WidthTt = oEpTofft1T= PRFTeff = Width of Rectangular Pulse of same height and area as pulse applied to analyzer =∫op(T) ...

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    52 | Guide to Spectrum and Signal AnalysisPulse Width

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    www.anritsu.com | 53APPENDIX EIntermodulation Distortion / Intercept PointsCalculating Intercept Points requires knowledge of:1) the order (normally 2nd or 3rd) of the distortion product.2) input drive level in dBm (example: –30 dBm).3) the desired or specified suppression of inter-modul...

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    54 | Guide to Spectrum and Signal Analysis

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