Agilent Measurement Journal - Issue 3 - 2007 - (Page 43)

Designing the probe E EMI compliance is becoming more and more challenging as clocks and data transmission rates shift to ever-higher speeds. The increasing switching currents presented on integrated circuit (IC) power grids and input/output (I/O) ports are very often the ultimate cause of both signal integrity issues and EMI compliance failures. Measurement of those high-frequency currents is therefore important. Accurately measured currents could be used in many important ways: • Compare electromagnetic characteristics of ICs1 • Distinguish inductive or capacitive coupling mechanisms2 • Estimate power-plane ground bounce3 • Predict far-ﬁeld radiation4 • Optimize package pin assignment5 • Validate simulation results Unfortunately, it can be especially difﬁcult to make meaningful measurements of these high-frequency noise currents on an IC. Pinpointing the surface currents depends on ultra-ﬁne spatial resolution, excellent electrical ﬁeld rejection and the ability to extract stable relative phase at the harmonic frequencies. This article presents the development, characterization and application of a high-resolution thin-ﬁlm magnetic-ﬁeld probe. Probe diameter ranged from 5 µm to 100 µm. The 100-µm probe exhibits a 250-µm improvement in spatial resolution compared to a conventional loop probe, measured at a height of 250 µm over differential traces with a 118-µm spacing. Electric ﬁeld rejection was improved using shielding and a 180-degree hybrid junction to separate common-mode (electric ﬁeld) and differential-mode (primarily magnetic ﬁeld) coupling. A network analyzer with narrowband ﬁltering was used to detect the relatively weak signal from the probe and to allow detection of phase information. An application of the probe shows how it can be used to identify the magnitude and phase of magnetic ﬁelds produced by currents in very closely spaced IC package pins. The electromagnetic receiving characteristics of the probe may be realistically derived by assuming quasi-static ﬁeld conditions, since the probe is electrically small even to several gigahertz. Ideally, then, the probe’s received power is a linear function of only one ﬁeld component and the received power is an indication of that ﬁeld’s strength. For example, where Pprobe is the received power, Cy(f) is the frequencydependent coupling coefﬁcient between the y-directed magnetic ﬁeld and the probe and Hy is the magnetic ﬁeld in the y-direction. The coupling coefﬁcient can be obtained by either measurement or numerical calculations from a known probe structure. Numerical models can be used, but care is required since not all conditions seen in the test environment may be captured in the simulations. Measurement of Cy requires that Hy is well controlled and known while assuming the coupling from other ﬁeld components is minimal. In that case, Cy(f) can be calculated from the measured received power as In reality, however, negligible coupling from other ﬁeld components cannot be guaranteed. If the probe is not designed to minimize other ﬁeld components, the received power will be a function of multiple variables, for example, where Ez and Cz(f) are the z-directed electrical ﬁeld and the associated coupling coefﬁcient. The measured power can not be used to interpret any single ﬁeld component unless additional measurements are made. A good design should minimize coupling from all ﬁelds except the one of interest. Agilent Measurement Journal 43

Table of Contents Feed for the Digital Edition of Agilent Measurement Journal - Issue 3 - 2007

Applying Ingenuity to the Measurement of Emerging Technologies Contents Emerging Innovations Department Delivering Bigger Benefits by Optimizing Customer Workflows Measuring Material Properties at the Nanoscale Utilizing In Situ Atomic Force Microscopy in Life Science, Pharmaceutical and other Bio-Related Applications WiMAX™: Plotting a New Path to Global Mobility Addressing the Triple Complexity of Triple-Play Networks What Next for Mobile Telephony? Exploring the Inner Workings of Tire-Pressure Monitoring Systems Developing, Assessing and Applying a High-Resolution Thin-Film Magnetic Probe Making Accurate Settling-Time Measurements Using a Vector Network Analyzer Applying Metabolomics Methods to the Study of Bacterial Leaf Blight in Rice Plants Measuring Stream Dynamics with Fiber Optics survey

Agilent Measurement Journal - Issue 3 - 2007

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