lecroy 14_LTXXX-OM-E_Ch10 电路图.pdf

124ISSUED: January2002LTXXX-OM-E Rev B CH A P T E RTE N:UseAdvancedMathTools You have seenhowtouse Waverunnermathtools. Nowlookdeeperintowaveform processing and applythe scopes advanced mathfeatures. Inthis chapter, see how Toprocess extrema waveforms Toperformenhanced resolutionfiltering Torescale yourwaveform Todomore withFFT Touse a mathfunction Toplot parametertrends RadioFans.CN 收音机爱 好者资料库 LTXXX-OM-E Rev BISSUED: January2002125 CH A P T E RTE NUseAdvancedMathTools ComputeExtremaWaveforms Useextrematoviewatraceenvelopeof numerous sweeps. Your Waverunner scoperepeatedlycompares the maxima(roof)andminima(floor)values of newwaveforms withthoseof alreadyaccumulatedextrema records. Whenever theoscilloscopefinds that agivendatapoint of anewwaveform is greater thanthe correspondingroof recordvalue, or less thanthefloor recordvalue, it replaces that recordvaluewiththenew one. TheWaverunner thus accumulates themaximum andtheminimum envelopeof all waveform records. 1.Press 1 toselect CHANNEL 1 anddisplaytheWaverunner basic menus. 2.Press thebuttonto 3.Press A toselect andset upTraceA. 3.Press thebuttonfor 5.Press thebuttontoselectanddisplaytheSETUP OF A menus. 6.Thenpressthebuttontoselect “Extrema” fromtheMathTypemenu. T TI IP P: : You canchange the viewof the envelope at anytime without affecting the accumulated data. 7.Press thebuttontoselect Envelope shows theentireenvelope, whileFloorandRoofshowonlythelower andupper parts of the envelope. Changingtheselimits will not restart theanalysis. 8.Turntheupper knob toset thenumber of sweeps: RadioFans.CN 收音机爱 好者资料库 PA R TTW O:L O O K I N GD E E P E R 126ISSUED: January2002LTXXX-OM-E Rev B 9.Press thebuttontoselect thesourcetrace: TO DOEXT RE MA Your Waverunner scope will stop accumulating whenever the selected maximum number of sweeps is reached. You can interrupt this process by changing the trigger mode from N ORMA L to STOP (by pressing the STOP button), or by turning off the function trace. A ccumulation will continue when you perform the reverse action. Reset the currently accumulated extrema waveform by either pressing CL E A R SWE E PS, or changing a parameter such as gain, offset, coupling, trigger condition, or your timebase or bandwidth limit. Waverunner displays the number of currently accumulated waveforms in the displayed trace label of the zoom trace on which the extrema function is performed. You can display roof and floor records either individually or together. Whenever the maximum number of sweeps is reached, you can accumulate an even larger number simply by changing the value in the SE TUP for menu. However, leave the other parameters unchanged, or the calculation will be restarted. T TI IP P: : Waverunneravoids being slowed downbycomputing a particularmath functiononlywhenthat functions trace is turned on. Despite this, waveform processing cantake some time whenthere are manydata points. Cut this delayby limiting the numberof data points used inthe computation. Waverunnerwill process the entire waveformbytaking everynthpoint where ndepends onthe timebase and the desired maximumnumberof points. The first point takenis always the data value at the left-hand edge of the display. RadioFans.CN 收音机爱 好者资料库 CH A P T E RTE N:UseAdvancedMathTools LTXXX-OM-E Rev BISSUED: January2002127 RescaleandAssignUnits This advancedmathtool allows youtoapplyamultiplicationfactor (a), andadditiveconstant (b), toyour waveform. Youcandoit intheunit of your choice, dependingonthetypeof application. 1.Followthesteps for settinguptodomathonTraceA, B, C or D. 2.Pressthebuttontoselect RescalefromtheMathTypemenu. 3.Select a or b from thenext menu, belowMathType. 4.Press thebuttontoselect themantissa, exponent, or number of digits; turntheknobtoset its value. 5.If youwish, go backandselect units. A newmenu, called“units =” will thenappear andallowyouto defineoneof awiderangeof units for your chosenvalue, includingAmps, Celsius, Hertz, decibels, Kelvin, Ohms, Volts, andWatts. 6.Press thebuttontoselect thesourcetracefor filtering: TO DOAVE RAGIN G: SUMME D VSCON T IN UOUS(OPT ION) Summed A veraging is the repeated addition, with equal weight, of successive source waveform records. If a stable trigger is available, the resulting average has a random noise component lower than that of a single-shot record. Whenever the maximum number of sweeps is reached, the averaging process stops. A n even larger number can be accumulated simply by changing the number in the menu. However, the other parameters must be left unchanged or a new averaging calculation will be started. You can interrupt the averaging by changing the trigger mode from N ORM to STOP, or by turning off the active trace. Waverunner resumes averaging when you perform the opposite action to these. Reset the accumulated average by pushing the CL E A R SWE E PS button or changing an acquisition parameter such as input gain, offset, coupling, trigger condition, timebase, or bandwidth limit. The number of current averaged waveforms of the function, or its zoom, is shown in the displayed trace label. When summed averaging is performed, the display is updated at a reduced rate about once every 1.5 s in order to increase the averaging speed (points and events per second). Continuous Averagingis the repeated addition, with unequal weight, of successive source waveforms. It is particularly useful for reducing noise on signals that drift very slowly in time or amplitude. The most recently acquired waveform has more weight than all the previously acquired ones: the continuous average is dominated by the statistical fluctuations of the most recently acquired waveform. The weight of old waveforms in the continuous average gradually tends to zero (following an exponential rule) at a rate that decreases as the weight increases. PA R TTW O:L O O K I N GD E E P E R 128ISSUED: January2002LTXXX-OM-E Rev B EnhanceResolution ERES (EnhancedResolution)filteringincreases vertical resolution, allowingyoutodistinguishcloselyspaced voltagelevels. Waverunner ERES is similar tosmoothingthesignal withasimple, moving-averagefilter. However, it is moreefficient, bothinterms of bandwidthandpass-band. UseERES onsingle-shot waveforms, or wherethedatarecordis slowlyrepetitive whenyoucant useaveraging. Useit toreducenoisewhenyour signal is noticeablynoisy, but youdont needto perform noisemeasurements. Useit, too, whenyouperform high-precisionvoltagemeasurements: zoomingwithhighvertical gain, for example. 1.Followthesteps for settinguptodomathonTraceA, B, C, or D. 2.Then press thebuttontoselect Enh. Res from theMathTypemenu: 3.Press thebuttontoselect 1.5 bits, for example, from This menuallows youtochooseafilter that will enhancetheresolutionof thedisplayedsignal byfrom oneto threebits, in steps of 0.5 bits. 4.Press thebuttontoselect thesourcetracefor filtering: Result: Seeillustrationnext page. T TI IP P: : Depending onyoursampling speed, Waverunnerdigital filters canaffect bandwidth. If you need bandwidthat slow timebases, use Averaging withrepetitive sampling. CH A P T E RTE N:UseAdvancedMathTools LTXXX-OM-E Rev BISSUED: January2002129 The glitch in the waveform displayed on the top grid has been clearly eliminated by E RE S: the result is the waveform on the lower grid. Trace Bs label indicates this as the filtered waveform. A nd the information field below the grids tells you that Trace B is an E RE S function of Channel 1, that the waveform has been enhanced by 1.5 bits, and that filtering has reduced the number of points from 250 to 242 (see N N O OT TE E on page131) and the bandwidth to 30 MHz. HOWWAVE RUN N E REN H AN CE SRE SOL UT ION The Waverunners enhanced resolution feature improves vertical resolution by a fixed amount for each filter. This real increase in resolution occurs whether or not the signal is noisy, or your signal is single-shot or repetitive. The signal-to-noise ratio (SN R) improvement you gain is dependent on the form of the noise in the original signal. The enhanced resolution filtering decreases the bandwidth of the signal, filtering out some of the noise. PA R TTW O:L O O K I N GD E E P E R 130ISSUED: January2002LTXXX-OM-E Rev B The Waverunners constant phase FIR (Finite Impulse-Response) filters provide fast computation, excellent step response in 0.5 bit steps, and minimum bandwidth reduction for resolution improvements of between 0.5 and 3 bits. E ach step corresponds to a bandwidth reduction of a factor of two, allowing easy control of the bandwidth resolution trade-off. The parameters of the six filters are given in the following table. RE SOL UT ION IN CRE ASE D BY 3DB BAN DWIDT H ( NYQUIST) FIL T E RLE N GT H (SAMPL E S) 0.50.52 1.00.2415 1.50.12110 2.00.05824 2.50.02951 3.00.016117 With low-pass filters, the actual SN R increase obtained in any particular situation depends on the power spectral density of the noise on the signal. The improvement in SN R corresponds to the improvement in resolution if the noise in the signal is white evenly distributed across the frequency spectrum. If the noise power is biased towards high frequencies, the SN R improvement will be better than the resolution improvement. The opposite may be true if the noise is mostly at lower frequencies. SN R improvement due to the removal of coherent noise signals feed-through of clock signals, for example is determined by the fall of the dominant frequency components of the signal in the passband. This is easily ascertained using spectral analysis.The filters have a precisely constant zero-phase response. This has two benefits. First, the filters do not distort the relative position of different events in the waveform, even if the events frequency content is different. Second, because the waveforms are stored, the delay normally associated with filtering (between the input and output waveforms) can be exactly compensated during the computation of the filtered waveform. The filters have been given exact unity gain at low frequency. E nhanced resolution should therefore not cause overflow if the source data is not overflowed. If part of the source trace were to overflow, filtering would be allowed, but the results in the vicinity of the overflowed data the filter impulse response length would be incorrect. This is because in some circumstances an overflow may be a spike of only one or two samples, and the energy in this spike may not be enough to significantly affect the results. It would then be undesirable to disallow the whole trace. The examples on the following page illustrate how you might use the Waverunners enhanced resolution function. CH A P T E RTE N:UseAdvancedMathTools LTXXX-OM-E Rev BISSUED: January2002131 In low-pass filtering:The spectrum of a square signal before (above left) and after (above right) enhanced resolution processing. The result clearly illustrates how the filter rejects high-frequency components from the signal. The higher the bit enhancement, the lower the resulting bandwidth. To increase vertical resolution:In the example at right, the lower (“inner”) trace has been significantly enhanced by a three-bit enhanced resolution function. To reduce noise:The example below shows enhanced resolution of a noisy signal.The original trace (below left) has been processed by a two-bit enhanced resolution filter. The result (below right) shows a “smooth” trace, where most of the noise has been eliminated. N NO OT TE E: : Enhanced resolutioncanonlyimprove the resolutionof a trace; it cannot improve the accuracyorlinearityof the originalquantization. The pass-band willcause signalattenuationfor signals nearthe cut-off frequency. The highest frequencies passed maybe slightlyattenuated. Perform the filtering onfinite record lengths. Data willbe lost at the start and end of the waveform: the trace willbe slightlyshorterafterfiltering. The numberof samples lost is exactlyequaltothe lengthof the impulse response of the filterused betweentwoand 117samples. Normallythis loss just 0.2% of a 50000point trace is not noticed. However, you might filtera record soshort therewould be no data output. Inthat case, however, the Waverunnerwould not allowyou touse the ERES feature. PA R TTW O:L O O K I N GD E E P E R 132ISSUED: January2002LTXXX-OM-E Rev B DoMorewithFFT InPart One(Chapter 5, “Use Math Tools”)welookedat howtouseFast Fourier Transform (FFT)to displayandmeasuresignals in thefrequencydomain. Nowseehowtousetheoptional FFT Averagefeature, andhowtoset upanFFT spantoimproveresolution. Valuablehints onhowtoget evenmorefrom this tool arealsogivenhere. DO FFT AVERAGE 1.Followthesteps for settinguptodomathonTraceA, B, C, or D. 2.Pressthebuttontoselect FFT AVG fromtheMathTypemenu. 3.Pressthebuttontoselect anFFT functiontoaverage.Select PowerSpect, for example, andyoucandefinea functionas thepower averageof FFT spectra, computedbyanother FFT function. 4.Press CLEAR SWEEPS toreset FFT averageandshowthenumber of currentlyaccumulatedwaveforms inthe displayedtracefieldof themathtraceor its zoom. When FFT or FFT A veraging is used, the memory status field beneath the grid shows the parameters of the waveform descriptor. DO ADDITIONAL PROCESSING Youcan perform other mathandwaveform processingfunctions, suchas averagingor arithmetic, beforedoingFFT. For example: if a stabletrigger wereavailableyoucouldperform time-domainaveragingtoreducerandom noise inthesignal. T TI IP P: : Toincrease the FFT frequencyrange the Nyquist frequency raise the effective sampling frequencybyincreasing the maximumnumberof points orusing a fastertime base. Toincrease the FFT frequencyresolution, increase the lengthof the time-domainwaveformrecord by using a slowertimebase. USE CURSORS WITH FFT Movetheabsolutetimecursor intothefrequencydomaintoreadtheamplitudeandfrequencyof adatapoint. Dothis bymovingit beyondtheright-handedgeof atime-domain waveform. Then movetherelativetime cursors intothefrequencydomaintosimultaneouslyindicatethefrequencydifferenceandtheamplitude differencebetweentwopoints oneachfrequency-domaintrace. Usetheabsolutevoltagecursor toreadthe CH A P T E RTE N:UseAdvancedMathTools LTXXX-OM-E Rev BISSUED: January2002133 absolutevalueof apoint inaspectrum intheappropriateunits. Andadjust therelativeVoltagecursors to indicatethedifferencebetweentwolevels oneachtrace. N NO OT TE E: : The following FFT-related errormessages mayappearat the topof the screen: “Incompatible input record type” FFT Average done ona functionnot defined as FFT. “Horizontalunits dont match” FFT of a frequency-domainwaveformis not available. “FFT source data zerofilled” If there areinvalid data points inthe source waveform(at the beginning orat the end of the record), these are replaced byzeros before FFT processing. “FFT source data over/underflow” Thesource waveformdata has beenclipped inamplitude, eitherinthe acquisition gaintoohighorinappropriate offset orinprevious processing. The resulting FFT contains harmonic components that would not be present inthe unclipped waveform