U1562pdf,Keysight U1562A无源探头可兼容U1600A系列手持式数字示波器。03 Keysight Eight Hints for Better Scope Probing- Application NcteHint #1Passive or active probe?For general-purpose mid-to-low-frequencyYellow: Signal before probedYellow: Signal before probedGreen: Signal after probedGreen: Signal after probed(less than 600-MHz)measurements.Purple: Output of probePurple: Output of probepassive high-impedance resistor dividerprobes are good choices. These rugged andinexpensive tools offer wide dynamic range(greater than 300 V)and high input resistance to match a scopes input impedanceHowever, they impose heavier capacitiveloading and offer lower bandwidths thanFigure 1-1. Comparison of passive and active probe measuring a s gnal that has alow-impedance(z0) passive probes or600 psec rise timeactive probes. All in all, high-impedance Keysight N2873A 500-MHz passivepassive probes are a great choice for gener- with 15-cm alligator ground leag ve probe Keysight N2796A 2GHz active probewith 1.8-cm ground leadal-purpose debugging and troubleshootingSignal loaded, now has 740 psec edgeSignal unaffected by probe, still hason most analog or digital circuitsProbe output contains resonance and630 psec edgeFor high-frequency applications ( greatermeasures 1. 4 nsec edgeProbe output matches signal andthan 600 MHz) that demand precisionmeasure 555 psecacross a broad frequency range, activeIn Figure 1-1 we see screen shots from aThe inductive and capacitive effects of theprobes are the way to go. They cost more600 MHz scope the Keysight Technologies, passive probe also cause overshoot andthan passive probe and their input voltageis limited, but because of their significantlyInc DSO 9064A)measuring a signal that ripping effects in the probe output ( purplehas a 500 psec rise time. On the left, atrace). Some designers are not concernedlower capacitive loading they give youmore accurate insight into fast signalsKeysight N2873A 500 MHz passive probe about this amount of measurement errorwas used to measure this signal On theFor others this amount of measurementright, a Keysight N2796A 2 GHz singleerror is unacceptableended active probe was used to measurethe same signal. the yellow trace showsWe can see that the signal is virtuallythe signal before it was probed and is the unaffected when we attach an active probesame in both cases. The green trace shows such as Keysight's N2796A 2 GHz activethe signal after it was probed, which is the probe to the dUt. The signal's character-same as the input to the probe. The purple istics after being probed (green trace)aretrace shows the measured signal, or thenearly identical to its un- probed characteroutput of the probeistics(N2796A 2 GHz trace). In addition, therise time of the signal is unaffected by theA passive probe loads the signal downprobe being maintained at 555 psec. Also,with its input resistance, inductance and the active probes output (green tracecapacitance(green trace). You probablymatches the probed signal (purple trace)expect that your oscilloscope probe will not and measures the expected 600 psec riseaffect your signals in your device under test time. Using the 1156A active probe's 2 GHz(DUT). However, in this case the passive bandwidth with superior signal fidelity andprobe does have an effect on the dut. the low probe loading makes this possibleprobed signals rise time becomes 4 nsinstead of the expected 600 psec, partlydue to the probes input impedance, butalso due to its limited 500-Mhz bandwidthin measuring a 583-MHz signal (0.35,600 psec =583 MHz)04 Keysight I Eight Hints for Better Scope Probing- Application NcteHint #2Key differences between passive and active probes are summarized below in Figure 1-2High Impedance Passive Probe Active ProbeProbe loading check withPower requirement NOYEStwo probesLoaderHeavy capacitive loading andBest overall combination oflow Resistive loadingresistive and capacitive loadingBandwidthup to 700 MHzup to 30 GHzApplicationsGeneral purpose mid-to-lowHigh-frequency applicationsfrequency measurementsRuggednessVery ruggedLess ruggedMax input voltage 300V~40VTypical PriS100S500SlkF gure 1-2 Compariscn of high-impedance passive and active probesBefore probing a circuit, connect yourTo check the probe loading ettect, first,00s2000/Autprobe tip to a point on your circuit and then connect one probe to the circuit under testconnect your second probe to the sameor a known step signal and the other end topoint Ideally, you should see no changethe scope s input Watch the trace on theSignal measuredon your signal. If you see a change, it isscope screen, save the trace and recall itwwc probescaused by the probe loadingon the screen so that the trace remains onthe screen for a comparison. Then, usingIn an ideal world, a scope probe would be a another probe of the same kind, connectnon-intrusive(having infinite input resisto the same point and see how the originaltance, zero capacitance and inductance) trace changes over the double probingWire attached to the circuit of interestFig 2-1. Probe loading check with twc probes and it would provide an exact replica of You may need to make adjustments to yourthe signal being measured. But in theprobing or consider using a probe withreal world, the probe becomes part of thelower loading to make a better measuremeasurement and it introduces loading toment. For instance, in this examplethe circuitshortening the ground lead did the trick. InFigure 2-2, the circuit ground is probed witha long 18 cm(7) ground leadFigure 2-2. Probe loading caused by a long ground lead5 Keysight Eight Hints for Better Scope Probing- Application NcteHint#3Compensate probe betore useMost probes are designed to match theinputs of specific oscilloscope modelsHowever, there are slight variations fromscope to scope and even between different o ede sinput channels in the same scope. Makesure you check the probe compensationFigure 2-3 Reduced probe loading with short grourd leadwhen you first connect a probe to anoscilloscope input because it may haveIn Figure 2-3, the same signal groundbeen adjusted previously to match ais probed with a short spring- loadeddifferent input. To deal with this. mostground lead. The ringing on the probedpassive probes have built-in compensation signal (purple trace)went away withRC divider networks. Probe compensationthe shorter ground leadis the process of adjusting the rc dividerso the probe maintains its attenuation ratioover the probes rated bandwidthIf your scope can automatically compensatefor the performance of probes, it makesigure 3-1. Use a small screw driver to adjustthe probe's variable capacitancesense to use that feature. otherwise, usemanual compensation to adjust the probesvariable capacitance. Most scopes have aProbe Tipsquare wave reference signal available onCableProbe bodyScopethe front panel to use for compensating theprobe. You can attach the probe tip to theprobe compensation terminal and connectComp9 MQthe probe to an input of the scope. viewingthe square wave reference signal, makethe proper adjustments on the probe usinga small screw driver so that the squarewaves on the scope screen look squareFigure 3-2. Compensation adjustment corresponds to the f at square waveThe diagram at the top of figure 3-2 shows low-frequency adjustment is not properlyhow to properly adjust the compensatingmade. This will result in high-frequencycapacitor in the termination box at theInaccuracies in your measurementsend of the probe. As you can see in theIt's very important to make surepicture, you can have either overshoot orthis compensation capacitor isundershoot on the square wave when thecorrectly adjusted06 Keysight Eight Hints for Better Scope Probing- Application NcteHint 4Unfortunately, this approach is notThe new N2820A Series high-sensitivityappropriate for measuring small currentscurrent probes from Keysight technologiesow current measurement tipsthat rapidly change between sub-miladdress the need for high-sensitivity currentamps and several amps because of themeasurements with a wide dynamic rangelimited dynamic range and sensitivity ofThese probes also offer the advantage ofAs modern battery-powered devices andintegrated circuits become more green andthe clamp-on type current probe, which is physically small connections to the deviceenergy efficient, there is a growing needlimited to a few milli amps. In the example under test (DUT) since today's applicationto make high sensitivity low level currentfor measuring the current consumption of a environments require an extremely smallmeasurements to ensure these devicesmobile phone, the idle state current is not form factor. The new N2820A/21A AC/DCcurrent consumption is within acceptablequite measurable because it is buried in the current probes offer the industry s highestlimits. The key applications calling forprobe noisesensitivity among oscilloscope currentprobes, going all the way down to 50 uAaccurate power consumption measurementare battery-powered applications such asAlso, for a more accurate measurementwith a maximum current range of 5 Awireless mobile devices and consumerone would occasionally degauss the probeelectronics. To maximize the battery liteto remove residual magnetism from theengineers need to minimize the power conprobe core and compensate for any dc offsumption over the lite of the product Powerset of the clamp-on current probe. T his extrais defined as p=v x l. the key enabler ofcalibration procedure makes the clamp-onreducing the power consumption of a devicecurrent probe cumbersome to useis to lower the average current consumptionfor a fixed supply voltage levelTransmit ]current pulsesa primary challenge in measuring thecurrent consumption of battery-poweredmobile devices, such as a cell phone ora tablet computer is that the dynamicReceive curent pulsesrange of the current signal is very wideThe mobile device typically switches backand forth between active states where it500 mA/divdraws very high and fast peak currents andIdle current2 ms/divan idle or standby current mode, where it0G060004060o10draws very small DC and Ac currentsFigure 4-1. The current drain measured on a GSM cell phone when making a calFigure 4-1 shows the current drain measured on a gSM cell phone when makingsa call. The active current peaks as high as2A. and at idle mode the current drain isextremely smallea simple way to measure a current withan oscilloscope is to use a clamp-on typecurrent probe such as Keysight's 1147Bor N2893a to directly monitor the currentgoing into the deviceFigure 4-2. A simple way to measure a current with an oscilloscope is to use a clamp-on typecurrent probe such as Keysight's 1147B or N2893A07 Keysight Eight Hints for Better Scope Probing- Application NcteKeysight's N2820A 2-channel high sensitivity current probe comes with two paralleldifferential amplifiers inside the probe withdifferent gain settings, where the low gainside allows you to see the entire waveformor the zoom out" view of the waveformand the high gain amplifier provides azoom in"view to observe extremely smallcurrent fluctuations, such as a mobilephones idle state. The N2820A/21Acur-rent probes are optimized for measuring thecurrent flow within the dut to characterizesub-circuits, allowing the user to see bothge signals and details on fast and widedynamic current waveformsFigure 4-3. The new N2820A/71A AC/DC current probes oter the industry s highestsensitivity among oscilloscope current probesThe probe offers an innovative methodof connecting the probe to your dUT. Thesupplied Make-Before-Break(MBB)connectors allow you to quickly probe multiplelocations on your DUT without having tosolder or unsolder the leads the mbbheader may be mounted on your targetboard or wired out of the dut. lt fits intostandard 0. 1"spacing thru- holes for 0.025square pins. Users should plan their PCBFigure 4-4. The supplied Make-Before-Breaklayouts accordingly. The MBBs are a great(MBB) connectors al ow you to quickly probemultiple locations on your DUT without havingway to easily connect and disconnectto solder or unsolder the leadsacross multiple locations on the targetboard without interrupting the circuitunder testr Cnrd -tp TrpThe innovation hasn t stopped there. WithQ自西current waveforms captured, you now wantB四to calculate the average current consumption of the system over time. Keysight'sInfiniium and Infinii Vision oscilloscopesprovide an area under the curve measurement(Charge), where you can easily calcu- Ulate the integrated current consumptions inAh(Ampere x Hour) over time. The Ah is aunit of measurement of a battery s electri-cal storage capacity One Ah is equal to a山山MLicurrent of one ampere flowing for one hourNow with the N2820A/21A current probesengineers in battery-powered product test-LEEEing are able to see the details and the bigpurrr-picture on dynamic current waveforms liken≌/never before with traditional clampon probesFigure 4-5. Keysight's Infiniium and Infinii Vision oscilloscopes provide an area under thecurve measurement Charge), where you can easily calculate the integrated current consumptions in Ah(Ampere x Hour)over time08 Keysight Eight Hints for Better Scope Probing- Application NcteHint #5Most digital oscilloscopes have a subtract Using a high-voltage differential probe suchmode where the two input channels can as Keysight's N2790A is a much betterMaking sate floatingbe electrically subtracted to give thesolution for making safe, accurate floatingdifference in a differential signal. Formeasurements with any oscilloscope. Withmeasurements with adecent results, each probe used should be a true differential amplifier in the probedifferential probematched and compensated before using it head, the n2790a is rated to measureIn this method, the common mode rejection differential voltage up to 1, 400 VDCScope users often need to make floatingratio is typically limited to less than -20db peak ac with Cmrr of -70 dB at 10 MHzmeasurements where neither point of the(10: 1 ) If the common mode signal on each Use a differential probe with sufficientmeasurement is at earth ground potentialprobe is very large and differential signal is dynamic range and bandwidth for yourFor example, suppose you measure amuch smaller, any gain difference between application to make sate and accuratevoltage drop across the input and output the two sides will significantly alter their floating measurementsof a linear power supply's series regulatordifferential" or A-B"result. a good sanityU1. Either the voltage in or out pin of thecheck here would be to double probe theregulator is not referenced to groundsame signal and see what the A-Bshows themA standard oscilloscope measurementwhere the probe is attached to a signapoint and the probe tip ground lead isattached to circuit ground is actually ameasurement of signal difference betweenBRIOUT+the test point and earth ground. MostAdscopes have their signal ground terminalsC1C4or outer shells of the bNc interface)connected to the protective earth groundsystem. This is done so that all signalsGroundapplied to the scope have a commonconnection point. Basically all scopeFigure 5-1. When measurement is not ground referenced, a differential measurement solutionmeasurements are with respect to earthIs necessary.ground. Connecting the ground connectorto any of the floating points essentially120V/2200V/0.0s200.0 Stop f1,22ypulls down the probed point to the earthground, which often causes spikes ormalfunctions on the circuit. How do you getaround this floating measurement problema popular yet undesirable solution to tineed for a floating measurement is theA-B technique using two single-endedprobes and a scope's math function2d/dt∫dtMenutFunctionOffset100mV/109mFigure 5-2. As a sanity check, double probe the same signal and see what the "A-B looks like09 Keysight Eight Hints for Better Scope Probing- Application NcteHint #6One of the most misunderstood issues with ground and onto the probe cable shieldprobing is that common mode rejection can Sources of common mode noise can beCheck the commonlimit the quality of a measurement Withinternal to the dut or external to it sucheither a single-ended or differential probe, it as power line noise, EMI or ESD currentsmode rejectionis always worthwhile to connect both probetips to the ground of the dut and see if any a long ground lead on a single-endedsignals appear on the screenprobe can make this problem verysignificant. A single-ended probeIf signals appear, they show the level ofdoes suffer from lack of commonsignal corruption that is due to lack ofmode rejection. Differential activecommon mode rejection Common modeprobes provide much higher commonnoise currents caused by sources othermode rejection ratios, typically as highthan the signal being measured can flow as 80 dB (10,000: 1)from ground in the dut through the probeFigure 6-1. Connect both probe tips to the groundand see if any signals appear on the screenTransmissionOutput多Q多几Figure 6-2. Differential active probe provides much higher common mode rejectionratic effectively elim nating common mode ncise currentHint #7The position of the ferrite core on thecable is important. For convenience, youCheck the probe couplingmay be tempted to place the core at thescope end. This would make the probehead lighter and easier to handle. HoweverWith your probe connected to a signal.the core 's effectiveness would be reducedmove the probe cable around and grab itwith your hands. If the waveform on thesubstantially by locating the core at theprobe interface end of the cablescreen varies significantly, energy is beingcoupled onto the probe shield, causingthis variation Using a ferrite core on theReducing the length of the ground leadon a single-ended probe will help someprobe cable may help improve probingFigure 7-. Using a ferrite core on the probeaccuracy by reducing the common modeSwitching to a differential probe willcable may help improve probing accuracynoise currents on the cable shield a ferritetypica lly help the most. Many usersdon t understand that the probe cablcore on the probe cable generates a seriesimpedance in parallel with a resistor in theenvironment can cause variations inconductor The addition of the ferrite coretheir measurements, especially at higherfrequencies, and this can lead to frustrationto the probe cable rarely affects the signalwith the repeatability and quality ofbecause the signal passes through themeasurementscore on the center conductor and returnsthrough the core on the shield, resultingin no net signal current flowing throughthe core10 Keysight Eight Hints for Better Scope Probing- Application NcteHint #8If you have to add wires to the tip of abe determined by first probing a knownprobe to make a measurement in a tight step signal through a fixture board like theDamp the resonanceenvironment, put a resistor at the tip toKeysight E2655C into a scope channeldamp the resonance of the added wireThen probe the signal with your proposedwire with a resistor at the tip When theThe performance of a probe is highlyFor a single-ended probe, put the resistance resistance value is right, you should see aaffected by the probe connection. asonly on the signal lead and try to keep the step shaped much like the test step, exceptthe speeds in your design increase, youground lead as short as possible For ait may be low-pass filtered If you seenay notice more overshoot, ringing, anddifferential probe, put resistors at the tip excessive ringing, increase theother perturbations when connecting anof both leads and keep the lead lengthsresistor valueoscilloscope probe Probes form a resonant the same. The value of the resistor cancircuit where they connect to the device. Ifthis resonance is within the bandwidth ofthe oscilloscope probe you alit willbe difficult to determine if the measuredperturbations are due to your circuit orthe pFigure 8-1. Put a resistor at the tip to damp theresonance of the added wireUndampedDampedZsr匙2 Inch leadInch leadVInFigure 8-2. With a properly damped probe input, the loading/input impedance will never dropbelow the value of the damping resistorZ5OMHz Clock, 100ps Rise TimeUndamped:jDampedVssourceHHH+++I++++++++H+H++++H+H+VinVout---:-1500ps/div--1:500ps/divFigure 8-3. As the speeds in your design increase, you may notice more overshoot, ringing andother perturbations Overcome the resonance formed by the connection of a probe by adding adamp resistor to your probe tip