Common-mode voltage probe for predicting EMI from unshielded...

Details Common-mode voltage probe for predicting EMI from unshielded... Common-mode voltage probe for predicting EMI from unshielded...

1998-12-03

2001-01-23

Metjahic, Safet (Department: 2858)

Electricity: measuring and testing

Impedance, admittance or other quantities representative of...

Parameter related to the reproduction or fidelity of a...

C324S072000

Reexamination Certificate

active

06177804

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention generally relates to devices for measuring the electromagnetic interference (EMI) from electrical and electronic devices. More specifically, the present invention relates to a device for predicting the EMI radiated from interconnect cables of electrical and electronic devices.
2. Description of the Related Art
Electrical and electronic devices typically generate electromagnetic emission sometimes called EMI. Significant contributors to the overall EMI of a particular device are caused by common-mode (CM) currents on the interconnect cables and cables within the device. CM currents at the input/output (I/O) cables are the result of CM voltage at the connectors of the device. Normally, in the functional design of a device only the differential mode currents are considered and these currents constitute the normal or functional signals generated by the device. However, CM current may sometimes be superimposed on the differential mode currents, thereby causing EMI. The EMI generated by these devices propagate as conducted or radiate missions and may degrade the performance of the nearby electronic devices. Various national and international standards such as FCC part 15 and EN55022 regulate the conducted and radiated emission to permissible levels.
Several conventional techniques exist for measurement of EMI and troubleshooting EMI problems. One such method is the radiated-emission measurement technique. A device under test (DUT) is placed in an open area test site (OATS) on a turntable and the EMI is measured with an antenna placed usually at 10 meters from the device.
FIG. 1
illustrates a CM model of a DUT with I/O cables showing the radiated-field measurement technique for determining the radiation characteristics of the DUT. A CM current I
CM
flows on an I/O cable causing radiation. The amplitude of the I
CM
depends on the CM voltage, the output impedance, and the impedance of the I/O cable. By rotating the DUT on the turntable, adjusting the height of the antenna, and finding the worst case I/O cable arrangement, the maximum radiation which varies in frequency can be found. The maximum measured field strength is therefore dependent on the position of the turntable, the antenna, and the cables. This procedure, however, is time consuming and typically takes a couple of hours to a day of testing.
In the case of shielded devices or small electrical devices with long interconnect cables attached, current clamps or a version called “absorption clamp”, which directly measures the CM current on the cables are sometimes used for determining the EMI. By finding the amplitude of the CM current, the EMI may be determined. The current clamp method reduces time and provides a simpler “bench method.” One disadvantage in using such a method, however, is that it is necessary to “maximize” readings by moving the clamp over at least 5-6 meters of the cable, and record the maximum at each frequency. Additionally, sometimes it is not feasible or practicable to measure a device with more than a couple of cables due to sizes of the cables involved with the current clamp. Further still, the ambient electromagnetic fields from nearby devices may also be present on the cables and the CM currents produced by such devices may also be measured with the current clamp, masking the CM noise under investigation.
Therefore, it is desirable to provide a device for measuring EMI generated from electrical and electronic devices that avoids the relatively complicated and time consuming procedures associated with the conventional methods of measuring EMI.
SUMMARY OF THE INVENTION
According to one embodiment, the present invention provides a voltage probe for measuring CM voltage of a DUT having differential input/output (I/O) signals. The probe includes a connector adapted to couple to the DUT, at least one output measurement port configured to connect to a measuring device for measuring the CM voltage, and at least one differential pair cable connected to said connector and to said at least one measurement port for coupling said differential I/O signals to said measurement port.
According to a second embodiment, the present invention provides a voltage probe for measuring CM voltage of a DUT having differential I/O signals while the DUT is simultaneously connected to an auxiliary equipment.
According to a third embodiment, the present invention provides a method for measuring EMI from a DUT. A CM voltage probe is interposed between the DUT and an EMI measurement device. The CM voltage is measured at an measurement port of the voltage probe. The measured CM voltage is compared to a limit for EMI.


REFERENCES:
patent: Re. 35644 (1997-10-01), Rogers
patent: 5130640 (1992-07-01), Gambill et al.
patent: 5345182 (1994-09-01), Wakamatsu
patent: 5495173 (1996-02-01), Bockelman et al.
patent: 5552715 (1996-09-01), Rogers
patent: 5701082 (1997-12-01), Rogers
patent: 5723975 (1998-03-01), Rogers
patent: 5949230 (1999-09-01), Kobayashi et al.

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