Case Study:Adding RF Test Capabilities for New Wireless DevicesScott TeigenBusiness Development ManagerNational Instruments

Making the Leap to WirelessProduct feature requested by customersWireless component cost drops over timeWireless components and antennas now small enough even for smallest portable devicesPotential large cost savings by adding wireless

Case Study: Wireless Meter

BackgroundUtility meter reading changing with government push for conservationCost prohibitive to pay meter reader in large residential areasReading sometimes inaccurate with manual readerCost and size of wireless components have shrunk dramatically in past decadeIndustry push for Smart Meter technology

ChallengesRF added to original system digital interfaceRF standard, frequency, power level and method needs to be selectedAdditional RF components need to provide minimal impact to test times to maintain product marginsTest system needs to be flexible to accommodate current standards and be able to grow with upcoming digital standardsIdeal to have test system perform RF, baseband and DC measurementsNeed familiarity with RF tests for selected device

Selecting the RF StandardUse the ISM Band to avoid costs and hassles of acquiring RF spectrumLow RF powerMinimize data collectors902—928 MHz2.400—2.500 GHz5.725— 5.875 GHz

Common ISM Band StandardsCustom Modulation FMUsed in cordless headphones and microphonesASKVery easy to implementUsed in remote controls, RFID, Tire Pressure Monitoring…FSK / PSKUsed in more advanced applications likeAudio & Video streaming applicationsIEEE StandardsIEEE 802.11 a/b/g/n WLANHigh data rate standard common in laptops and commercial devicesIEEE 802.15.4 and ZigBeeWireless mesh network and low power standard

Example: IEEE 802.15.4 (ZigBee)Works in ISM Band at 900 MHz and 2.4 GHzLow power operationInfrequency battery swappingOperates in mesh networkEnables peer to peer movement of data i.e. household to householdComplete PHY and MAC layer for security and ensured transmission

Defining test procedure for ZigBee interface with DUTInterfacing requirementsImportant tests for ZigBee: Power, Modulation Quality, SensitivityPHY layer only or verify with MAC layer?Other I/O for device control: digital, current (power management), bus communication (SPI, I2C, JTAG)

Interface example with DUTPower SupplyDigital I/ORF OutputDC SwitchingRF InputDMM

PXI Test Interface HardwareData Acquisition and ControlMultifunction I/OAnalog Input/OutputDigital I/OCounter/TimerFPGA/Reconfigurable I/OMachine VisionMotion ControlSignal ConditioningTemperatureStrain/Pressure/Force/LoadSynchro/ResolverLVDT/RVDTMany More. . .Modular InstrumentationDigital Waveform GeneratorDigital Waveform AnalyzerDigital MultimeterLCR MeterOscilloscope/DigitizerSource/Signal GeneratorSwitchingRF Signal GeneratorRF Signal AnalyzerRF Power MeterFrequency CounterProgrammable Power SupplyMany More. . .Bus InterfacesEthernet, USB, FireWireSATA, ATA/IDE, SCSIGPIBCAN, DeviceNetSerial RS-232, RS-485VXI/VMEBoundary Scan/JTAGMIL-STD-1553, ARINCPCMCIA/CardBusPMCProfibusLINMany More. . .OthersIRIG-B, GPSDirect-to-DiskReflective MemoryDSPOpticalResistance SimulatorFault InsertionPrototyping/BreadboardGraphicsAudioMany More. . .

Getting Familiar with RF Measurements for Wireless Meters:Common ZigBee TestsRF Power/Spectral MaskError Vector Magnitude (EVM)Sensitivity / Signal to Noise Ratio (SNR)

1. Measuring RF PowerCommon instruments:RF Power MeterSpectrum Analyzer / VSACombination of the aboveNI 5680 Features include:Dual sensor architectureUSB 2.0 connectivityFreq Range : 50 MHz to 6.0 GHzPower Range: -40 dBm to +23 dBmLinearity: +/- 0.13 dB

Relative versus Absolute Power?USB Absolute Power ~ 0.1 dBAnalyzer Absolute Power ~0.5 – 1.0 dBWhy use a VSA instead of a power meter?Power meter limited to average powerVSA test speed advantagePower Meter = 65 millisecondsVSA = 20 milliseconds

Power Spectral Density (PSD)Power is normalized to 1 Hz RBWSimilar to ORFS, ACP, other spectrum mask measurementsZigBee standard allows -30 dBc in adjacent channels

Sources of RF Power UncertaintySeveral sources of uncertaintySensor non-linearityNoiseVSWRCalibrationEach source contributes to the total uncertaintyAggregate uncertainty has a Gaussian distributionTypical specification is to within 2σ uncertainty

Several sources of uncertainty

Sensor non-linearity

Noise

VSWR

Calibration

Each source contributes to the total uncertainty

Aggregate uncertainty has a Gaussian distribution

Typical specification is to within 2σ uncertainty

2. Error Vector Magnitude (EVM)Measures quality of modulated signalsEncapsulates error from multiple sourcesSystem nonlinearityAdditive White Gaussian Noise (AWGN)Phase noiseQuadrature ImpairmentsUsed for quadrature modulation schemes BPSK, QPSK, 8-PSK, etc.QAM (4, 16, 64, 256, etc.)

Understanding the Constellation DiagramModulation produces changes in phase and amplitudeConstellation diagrams enable characterization these changesEach dot is a “symbol”Symbol spreading due to noise

Error Vector Magnitude (EVM) MeasurementsMeasured Symbol LocationQΔQIdeal Symbol LocationΔIVEVM typically reported as an RMS value over a specific number of symbolsӨ

EVM typically reported as an RMS value over a specific number of symbols

Effect of AWGN on EVM MeasurementsSNR = 44 dBSNR = 37 dBSNR = 30 dBEVM = 1.15%EVM = 29.54%EVM = 2.57%IQ SkewEVM = 6.5%IQ Gain ImbalanceEVM = 12.6%IQ DC OffsetEVM = 8.3%

3. Sensitivity MeasurementsCharacterizes receivers performance in low-signal strength environmentsMethod can be different for different receiver typesSpecified differently for various receiversCellular – Approximated by measuring BERGPS – Approximated by measuring C/N0ZigBee – Approximated by Packet Error Rate (or Bit Error Rate) with decreasing Tx powerIntimately related to receiver’s noise figure

ZigBee Receiver SensitivityZigBeeSensitivity approximated with a PER (packet error) and BER (bit error) testVSG generates low power signal starting around -80 dBm (or starting approximately at 0% error) and reduces power to compare PER until it reaches 100% errorGenerate ZigBee Packets(pseudo-random bistream)ZigBee ReceiverBasebandReceiverRead from ZigBee DUTSerial/UART/SPI

ZigBeeSensitivity approximated with a PER (packet error) and BER (bit error) test

VSG generates low power signal starting around -80 dBm (or starting approximately at 0% error) and reduces power to compare PER until it reaches 100% error

SeaSolve Software & National InstrumentsAlliance member Seasolve provides ZigBee test solutions:Transmitter testingReceiver TestingDesign validationCompliance TestingAutomated Manufacturing TestAll of this can be achieved on one PXI instrumentation platform at all ZigBee frequencies (865 MHz, 915 MHz & 2.4GHz).

Transmitter testing

Receiver Testing

Design validation

Compliance Testing

Automated Manufacturing Test

Demonstration: ZigBee Test with DUT

SummaryAdding wireless to your device makes sense with dropping costs and size of wireless componentsUtility meter case studyAdding wireless reduces costProcess to select ISM band deviceZigBee is a good standard for wireless meters because of power, cost, and mesh network capabilityPXI provides flexibility for total wireless meter testCommon RF Measurements for ZigBeeRF Power/Spectral DensityError Vector Magnitude (EVM)Sensitivity / Signal to Noise Ratio (SNR)