lecroy 14_LTXXX-OM-E_Ch10 电路图.pdf

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1、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

2、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(r

3、oof)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 t

4、hus 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.Thenpressthebuttontoselec

5、t “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 th

6、eanalysis. 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 s

7、weeps 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 pressin

8、g 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 di

9、splay 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

10、: : 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 t

11、he 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 Resc

12、aleandAssignUnits 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 RescalefromtheMathT

13、ypemenu. 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 chosenva

14、lue, 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 tri

15、gger 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 parame

16、ters 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 b

17、y 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 pe

18、rformed, 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

19、 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 g

20、radually 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

21、 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 signa

22、l 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.Pre

23、ss 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 onyour

24、sampling 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

25、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 n

26、umber 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 th

27、e 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

28、 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 corr

29、esponds 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

30、.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 dist

31、ributed 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-

32、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 t

33、he 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 filter

34、ed 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

35、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 exampl

36、es 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 proce

37、ssing. 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

38、 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

39、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

40、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 i

41、s 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”)welook

42、edat 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 settinguptodoma

43、thonTraceA, 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 averageandshowth

44、enumber 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 processingfunction

45、s, 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 maximumnumb

46、erof 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 beyondther

47、ight-handedgeof atime-domain waveform. Then movetherelativetime cursors intothefrequencydomaintosimultaneouslyindicatethefrequencydifferenceandtheamplitude differencebetweentwopoints oneachfrequency-domaintrace. Usetheabsolutevoltagecursor toreadthe CH A P T E RTE N:UseAdvancedMathTools LTXXX-OM-E R

48、ev 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” FF

49、T 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