AnEmbeddedController fortheLSNAwithPulsed Measurement Capabilities

TheLargeSignal Network Analyzer (LSNA)digital controller hasbeenrevamped withasmallembedded computer instead oftheoriginal heavyandexpensive VXIrack.This computer takes careofallthemeasurement settings; itoffers several interfaces tocontrol thesystem (GPIB, RS232, LXI,front panelinterface). FIFObuffers managedwithinterruptions allow fullconcurrent operation ofeachinterface. Theaddition ofa boardwith timers andcounters provides thecapability tomeasure theRFtimedomain slopes inpulsed mode. Theso-called LSNA isdevoted tothemeasurement of voltages andcurrents ofperiodic signals atRFfrequencies. It samples 4RFwaves, usually theincident andreflected waves atthetwoterminals ofaDUT (device undertest). Thisisof great interest whentheDUT arenonlinear active devices (transistors orpoweramplifiers). Theresults ofLSNA measurements aretimedomainslopes ofvoltages and currents atthedevice terminals. Thisinformation, usually provided withoscilloscopes forlower frequencies, cannot be achieved effortlessly withoscilloscope atRF frequencies because ofmanycalibration issues. Thetimedomainslopes giveinvaluable information interms ofnonlinear behavior of highpowertransistors, onecannotice thatRF power amplifier designers within their CAD tools usethese slopes every day. Comparedwithclassical RF VNAs (vector network analyzers), theLSNAhasnatively thecapability torebuild thetimedomainslopes ofRFcurrents andvoltages. Thisisa consequence of itssampling strategy, withallthe fundamental frequency and harmonicfrequencies information acquired coherently inaone-shot ADC record (Fig. 1).Thankstoa special calibration algorithm withan absolute phasereference, theRF voltages andcurrents are plotted attheDUTplanes. TheLSNA hasbeenprototyped in1994attheVUB Laboratory (1)withthehelpoftheHewlett-Packard Company. ItisbasedontheMTA (Microwave Transition Analyzer, HP 71500A) hardware; basically theLSNA isa double MTA (ithas4 sampling headsinstead of2).The internal schemaoftheLSNA isgivenonFig.2, anda complete setup based onaLSNAisshownonFig.3. TheMTA digital controller wasnotabletohandle 4 samplers anddownconverters, soitwasrequired tobuild a newdigital controller. Ithasbeendecided atVUB totakea VXIrackandasetofVXIboards inaddition withanexternal PCtocontrol theLNSAsamplers. Thisdecision hasproven tobereliable, andHP/Agilent hassoldseveral complete LSNA systems withthis configuration. Butlater, itappears thathalfoftheprice ofthewholesystem wasduetothe expensive VXIcontrol rackandboards. Thispaperdescribes anewembedded digital controller of theLSNA.First, we willexplain ourneedsintermsof capabilities, andthenwewillpropose ourchoices. Finally, wewilldemonstrate thenewembedded features andpulsed capabilities ofourwork.