How To Measure 5 Ns Rise/Fall Time On An RF Pulsed Power .
How to Measure 5 ns Rise/Fall Timeon an RF Pulsed Power AmplifierUsing the 8990B Peak Power AnalyzerApplication NoteIntroductionIn a pulsed radar system, one of thekey transmitter-side components isthe pulse power amplifier (see Figure1). Power amplifiers, whether solidstate or vacuum tube, used in pulseradar systems need to output up toseveral kilowatts of power. SinceWWII, engineers have been trying toperfect the design of pulsed poweramplifiers to support wider bandwidth, thus allowing more data to betransferred, and to increase efficiencyto enhance power consumptionefficiency.RF INWaveformGeneratorRF OUTPulse PowerAmplifierModulatorPower SupplyFigure 1. Transmitter block diagram
IntroductionOne of the key measurementsrequired for evaluating a widebandwidth (greater than 100 MHz)using a pulse power amplifier is pulserise time. Currently there are severalinstruments capable of measuringthe rise time of the RF pulsed poweramplifier, including a high samplingoscilloscope with a diode detector,an RF peak power meter, and an RFspectrum analyzer using zero-spanmode. This application note focuseson a new measurement instrument:the Agilent Technologies 8990B peakpower analyzer (PPA). It is the highest performance peak power meter inthe power meter test and measurement industry. The 8990B is easy-touse and provides highly accurate fastrise/fall times measurements.This application note illustrates howusing the 8990B peak power analyzerallows you to confidently measuresub-5 nanosecond rise time on radarpulses (Figure 2). The document alsoexplains how equivalent time sampling (ETS) is used to measure risetime. Some tips are provided to showyou how to ensure you obtain thecorrect rise/fall time measurementresults.Pulse rise and falltimemeasurementfunction andresultsFigure 2. Screen shot of the 8900B PPA measuring a fast RF pulse rise time2
Pulse Shaping (Profiling)When developing a wideband poweramplifier or transmitter device, signalprocessing and modeling techniquessuch as the pulse profiling or shaping have been used to make sureRF pulses transmitted out meet thebandwidth requirement (see Figure3). Filtering design also plays animportant role in determining the RFpulse profile.0400.e–3800.e–31.2For example, a sloping leading edge(rise time) affects the minimumrange as well as range accuracy orresolution. This means that the rangeresolution of a pulsed radar systemis fundamentally limited by the bandwidth of the transmitted pulse; thewider the bandwidth, the better therange resolution.1.6212VariousPulse TimeFigure 3. Pulse profiling illustration31.62
The Challenge in Measuring Fast Rise TimeWhen trying to measure a very fastpulse rise/fall time on an RF pulse,minimizing measurement uncertaintyis important. Consequently, obtainingan accurate rise time on the deviceunder test (DUT) requires the use of ameasurement device with a rise timethat is faster than the expected rise/fall time. To further reduce measurement uncertainty, it is desirable todirectly measure the RF pulse fromthe DUT, eliminating the use of adaptors or convertors.sampling (ETS). (ETS is explained inthe next section.) Using the 8990Bwith two wideband and fast powersensors (the N1923A and N1924A),the DUT’s RF pulse signals can bemeasured directly. This setup is ideal,particularly when measuring a fastrise/fall time, since measurementuncertainty is kept to minimumbecause the signal correction is doneinside the power meter.Figure 4 shows the rise time measurement error (in percentage) versusthe DUT’s actual rise time. The graphis constructed based on the followingsimple error equation:As mentioned earlier, the 8990BPPA has a sub-5 nanosecond risetime capability using equivalent timeRisetime Error 22RisetimeInstruments RisetimeDUT RisetimeDUTRisetimeDUT 100%60.0%Agilent 8990B PPA Rise Time Measurement Error (%)Measurement Error ser DUT rise time ( nsec )Figure 4. 8990B rise time measurement error vs. DUT rise time41617181920
Equivalent Time SamplingEquivalent time sampling (ETS) worksby constructing a picture or waveformof the input signal by accumulatingsamples over many waves of triggering cycles. As illustrated in Figure 5,when the ETS feature of the 8990BPPA is turned on, an accurate timeto-digital converter (DTC) subdivides10 more bins (points) over multipletriggers samples, meaning the DTCcan accurately locate 10 more smallertime samples in each trigger. Thesampling rate of the power meterchannels (channels 1 and 4) areeffectively boosted up from the 100MSa/s real-time sampling to 1 GSa/ssampling when ETS is used. This isa common sampling technique andis used in some of the conventionaldigital oscilloscopes to effectivelyincrease the sampling rate to capturefast and repetitive signals.Subsequent samples from multiple triggers andfilled into the 10 sub division binsTriggerlevel Bin 0Bin 1Bin 2Bin 3 Bin 9Reconstruction of therepetitive signals overmultiple trigger events,with accurate delta timeintervalFigure 5. 8990B equivalent time sampling illustration5
Equivalent Time SamplingIn the 8990B PPA, ETS is turned onautomatically only when the timebase setting is set below 500 ns/div. At 500 ns/div and above, ETSis turned off; in other words it is inreal time sampling of 100 MSa/s.To check whether ETS is turned onor off, simply click on the Statustab at the bottom of the 8990Bmeasurement panel (shown in Figure6 below). The panel also shows if theETS acquisition status is acquiring orcompleted.The limitation of this measurementmethod is that ETS works best onrepetitive signals. ETS cannot be usedfor single-shot or non-repetitive signals because there are not be enoughdata samples to reconstruct thesignal waveform. For these instances,there is an option to manually turn offthe ETS even if the time base is setto below 500 ns/div. This allows youto analyze non-repetitive RF pulses,provided the rise/fall time is greaterthan 50 ns.The other limitation of ETS is thetriggering time. The longer the waiting time for one trigger to be armedand completed, the longer it takes forETS to fill the 10 acquisition bins andsubsequently build back the RF pulsewaveform.Figure 6. 8990B ETS status bar6
Tips For Obtaining Correct Rise/Fall Time MeasurementThe flowchart shown in Figure 7is a guideline to optimize the rise/fall time measurement result in the8990B PPA. As it shows, there aretwo pre-determined factors that youneed to know in order to obtain thebest rise/all time measurement accuracy. The first is for you to know theexpected rise time. This data can befound in the DUT’s data specificationdocuments. The second is for you toknow whether the signal is repetitive.In most testing conditions, DUTpulses are repetitive since they canbe generated from external RF signalsources or when the DUT is operatingin service or self test mode.Rise/Fall timeExcepted ?YESLess than50 nsecRepetitiveSignals?NOYESSet time base lessthan 500 ns/div, makesure ETS is turned onManually turn off theETS. Rise/fall timemeasurementuncertainty will be highChange time base to getthe rising/falling edge of thepulse to occupy 50% thedisplayFigure 7. Flow Chart to Get Optimized Rise/Fall Time Measurement7NOChange time base to get therising/falling edge of the pulseto occupy 50% the display
ConclusionAccurately measuring RF pulse nanosecond range rise/fall time requiresthe use of a suitable measurementinstrument that is properly set up. Ina wide band pulse power amplifier,accuracy is especially importantsince the rise/fall time measurementbecomes a critical performancemeasurement parameter.The new Agilent 8990B peak poweranalyzer is an ideal test solution,ensuring measurement accuracywith its 5 nano-seconds rise timeperformance and easy-to-use graphicinterface.Reference1) Principles Of Modern Radar: Basic Principles, Mark A. Richards, James A.Scheer, William A. Holm, Scitech Publishing, ISBN: 978-1-891121-52-4, 20102) Agilent 8990B Peak Power Analyzer, User’s Guide, part number 08990-900053) Baseband Pulse Shaping for Improved Spectral Efficiency Technical Note,part number 5989-9492EN8
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Tips For Obtaining Correct Rise/Fall Time Measurement The flowchart shown in Figure 7 is a guideline to optimize the rise/ fall time measurement result in the 8990B PPA. As it shows, there are two pre-determined factors that you need to know in order to obtain the best rise/all time measurement accu-racy. The first is for you to know the
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