Agilent Measurement Journal - Issue 4 - 2008 - (Page 16)

Step 1: Set center frequency and span The correct values depend on the signal of interest. For some signals, the process is simple: Use the carrier frequency as the center frequency and select a span wide enough to include any desired out-of-band signals. If the standard allows for frequency-hopped signals (e.g., EPCglobal Class1-Gen2), the measurements are more difﬁcult but a similar approach can be used, starting by setting the center frequency to the midpoint of the channel-hopping bands. The span should be set to include the entire frequency range of all active channels and allow for extra spectral content on the edges. Measuring hopped signals is comparatively easy if the signal analyzer has a gapless recording capability and the ability to change center frequency and span during post-capture analysis. By changing the analyzer’s center frequency and span to the values of the desired hop and selecting the appropriate portion of the recording for playback, any hop can be thoroughly analyzed with no need for prior knowledge of the hop timing or sequence. Step 2: Set input range The key is setting the input range as low as possible without allowing overloads. If the input range is set too high, noise will increase and cause greater errors. If the input range is set too low, it will overload the ADC. Selecting the optimal input range will provide the best possible data. Step 3: Set up triggering (if required) Rather than the typical magnitude increase, many RFID signals transition from a base magnitude to a lower amplitude. Solutions such as the 89600 VSA software can capture such signals with a “below level” mode that triggers when the amplitude falls below a user-deﬁned value. Step 4: Select a digital demodulation mode Many VSAs support various demodulation formats and standards. The 89600 VSA software has an RFID-speciﬁc demodulation mode that can be selected, along with the capability to select default settings for many of the current standards. Step 5: Specify the modulation format The type of forward (interrogator) modulation scheme depends on the applicable standard. With EPCglobal Class1-Gen2, for example, DSB-ASK, SSB-ASK, PR-ASK, FSK 2 or OOK may be used.2 Because the interrogator and tag may not have the same modulation scheme, the types of return modulation typically include DSB-ASK, FSK 2 and OOK. Similar to the forward and reverse modulation schemes, interrogators and tags have different line encodings (dictated by the relevant standard). Line coding helps guard against data corruption caused by noise and interference from other systems. Step 6: Specify the symbol rate In EPCglobal RFID demodulation, two important parameters must be set to match the analyzed signal: Tari and bit rate. (Note that other formats may use only bit rate.) Tari is deﬁned in the standards as the length of a data zero for pulse interval encoding (PIE). In the 89600 VSA software, the markers tool can be used to measure this value. Similarly, the bit rate can be measured by zooming in on a data burst and measuring the time between the start and end points of a high/low/high interval. Step 7: Select result length and points/symbol The acquisition length can be set to capture different time widths in the signal. With the “burst search” feature of the 89600 VSA software, the acquisition length and sync search length are the same. Because the software does not require symbol-clock timing, the clock can be manually adjusted to account for signal nonlinearities. Step 8: Select ﬁlter shape and alpha (measured and reference) In signal analysis using digital demodulation, adding a ﬁlter to the measurement can help reduce intersymbol interference or splatter into adjacent channels. Depending on the signal analyzer or software, several ﬁlter types can be applied, each offering tradeoffs between resolution in the time or frequency domain. In many cases, the user can specify the alpha or bandwidth/time (BT) product (for Gaussian ﬁlters) to deﬁne the ﬁlter’s shape and width, or create a user-deﬁned ﬁlter. Note that ﬁltering is not included in any of the RFID standards but may be a useful design and measurement tool, especially for reader/ interrogators, which are essentially fully functional transmitters. 2. DSB-ASK is double-sideband amplitude-shift keying; SSB-ASK is single-sideband ASK; PR-ASK is phase-reversal ASK; FSK 2 is two-level frequency-shift keying; OOK is on-and-off keying. 16 Agilent Measurement Journal

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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