Agilent Measurement Journal - Issue 4 - 2008 - (Page 49)

Waveform metrology and choice of instruments A W-CDMA signal can be treated as a carrier signal with a complex modulation term, V(t) = Re(M(t) x exp(jwt)), where V(t) is the time-varying RF voltage at an angular frequency w and M(t) is the complex W-CDMA modulation term. The measured waveform will be imperfect, containing the modulation waveform and an error waveform Mmeas (t) = M(t) + Merror (t). Combining averaging with a knowledge of the measuring instrument characteristics reduces the instrument noise and makes it possible to recover the error waveform. Three types of general-purpose instruments seem to be logical choices for waveform metrology: sampling oscilloscopes, digital oscilloscopes and spectrum analyzers. Sampling oscilloscopes can demodulate repetitive RF signals and provide the closest link to the primary standards; however, the low frame-update rate of W-CDMA signals makes this approach impractical.5 Instruments such as the Agilent PSA E4440 Series performance spectrum analyzers can demodulate complex waveforms and are in widespread industrial use. NPL plans to use these instruments in later stages of the wireless project. Today, NPL recovers the W-CDMA modulation waveform mathematically from single-acquisition digital oscilloscope traces. These versatile instruments can be used to measure both repetitive and single-shot waveforms. Their weakness, though, when compared with sampling oscilloscopes or spectrum analyzers, is that the voltage-scale resolution is typically eight bits. Digital oscilloscopes also have complex noise and nonlinearity behavior, which restricts the choice of measurement frequency. To compensate for this shortcoming, digital oscilloscopes provide much higher levels of oversampling, typically > 500 points per chip. Digital-oscilloscope timebase accuracy is typically 1 x 10-6 or better. NPL uses an additional synthesizer, phase-locked to the W-CDMA signal, to provide a pilot tone that compensates for residual timing ﬂuctuation, improves the timebase performance by a factor of 10,000, and keeps additional EVM contributions below 0.1 percent at 2.5 GHz. To avoid the introduction of correlated errors into the result, the signal and pilot frequencies are chosen using a predictive model. Diagnosing errors with RF waveform metrology An NPL scientist worked with researchers at the Centre for Communications Research (CCR) at Bristol University, UK in January 2007 (Figure 3).6, 7 They used waveform metrology to locate a software fault that gave EVM values of eight to 10 percent when measured with commercial test equipment. The cause was a software error that shifted the symbol timing by 87.2 ns (0.335 chips) every 20 symbols (2560 chips; Figure 4). When calculated with the correct timing reference, the source EVM is less than 0.6 percent. Figure 3. NPL and CCR staff performing RF metrology waveform measurements at Bristol University Timing error 0.335 chips 19 20 1 2 3 19 20 1 2 3 1 Normalized value 0 -1 -5 0 Misaligned data 5 1- 15 2Aligned data 25 30 Symbol (128 chips) Figure 4. Constellation diagrams showing symbol-timing errors and alignment Agilent Measurement Journal 49

Table of Contents Feed for the Digital Edition of Agilent Measurement Journal - Issue 4 - 2008

Agilent Measurement Journal - Issue 4 - 2008 Delivering Confidence through Compliance with Standards Contents Emerging Innovations Trying Early Device Implementations at the IEEE 1588 PlugFest Achieving Greater Confidence in Measurement Accuracy through Consistency in Calibration Services Overcoming the Challenges of RFID Component Testing 3GPP LTE: Introducing Single- Carrier FDMA Ensuring Reliable Operation and Performance in Converged IP Networks Testing Storage Area Networks and Devices at 8.5-Gb/s Fibre Channel Rates Choosing an Appropriate Calibration Method for Vector Network Analysis Making Traceable EVM Measurements with Digital Oscilloscopes Exploring Terahertz Measurement, Imaging and Spectroscopy: The Electromagnetic Spectrum’s Final Frontier Interpreting Quoted Specifications when Selecting Digitizers

Agilent Measurement Journal - Issue 4 - 2008

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