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Improved Method for Measuring Impedance

IP.com Disclosure Number: IPCOM000033496D
Original Publication Date: 2004-Dec-13
Included in the Prior Art Database: 2004-Dec-13
Document File: 5 page(s) / 109K

Publishing Venue



In the past systems have been identified as having field (and laboratory) disk drive failure due to excessive noise from the power supply. Through functional testing, impedance measurements, and noise voltage measurements it was concluded that the addition of grounding clips on the disk drive rails reduced the impedance between the disk drive case (disk 'ground') and the chassis (chassis 'ground'). To prevent such failures in the future, a formal Chassis Impedance Measurement Procedure was developed and implemented. This procedure is a unique test process that is currently performed by the Environmental Stress Analysis Lab for each system.

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Improved Method for Measuring Impedance

     Our invention is a new technique for measuring the impedance between a sub-chassis (such as a hard drive case-and-rail assembly) and a computer chassis. Whereas a standard ohmmeter measurement would give us the answer for DC resistance only, our technique will tell us what the impedance is over a range of frequencies (up to hundreds of Megahertz). Using an impedance analyzer and a custom hand-held probe, we can make this measurement quickly, without special fixturing, and determine whether the connection's impedance is low enough at all frequencies of interest. The impedance between the main chassis and sub-chassis in computer systems should be maintained as a low level across a wide frequency range. Typically, the frequency range of interest extends to 100 MHz.

     A RF probe is attached to the end of the coax cable (included in the calibration). Note: It is important to keep the length of the other contact as short as possible, and less than 1" long to keep the probe accurate over the frequency range of interest.

     The probe is positioned so that one contact is on the sub-chassis, and the 'ground' contact is on the main chassis. The impedance is then measured directly. This measurement should be repeated for all sub-chassis-to-chassis connections, and if appropriate, and device-to-sub-chassis connections (e.g. disk drives to sub-chassis).

     The steps listed below are a detailed description of the test process that was briefly described in question #2: At this moment we do not believe any other vendor measuring at our is doing this test. A patent search should reveal such. This test or service could be a marketable service or sold as vendor based tool for product test.

Equipment Needed:

Agilent 8505 2D 35 mm Economy Calibration Kit

     Hewlett Packard 4396B Network/Spectrum/Impedance Analyzer Steps for Instrument Calibration: