Electricity: measuring and testing – Impedance – admittance or other quantities representative of... – Parameter related to the reproduction or fidelity of a...
Reexamination Certificate
2002-04-04
2004-02-03
Le, N. (Department: 2858)
Electricity: measuring and testing
Impedance, admittance or other quantities representative of...
Parameter related to the reproduction or fidelity of a...
C324S637000, C324S557000, C324S641000
Reexamination Certificate
active
06686748
ABSTRACT:
The present application claims priority of EP application no. 01108668.9, filed Apr. 6, 2001, which is incorporated herein by reference.
FIELD OF THE INVENTION
The invention concerns a method and structures for the testing of the electromagnetic leakage of a junction between two adjacent, electrically conductive parts, especially the cover parts of a high frequency (HF)-shielded case for the electromagnetic alternating fields. However, electronic equipment itself often generates electromagnetic waves. To ensure the electromagnetic tolerance, (EMC), the cases for the installation of electronic equipment must therefore be shielded against irradiation and emission of electromagnetic waves.
The tightness of a case against high-frequency electromagnetic oscillation is based on the shielding effect of the metallic walls. Essentially, two physical effects are responsible for this. First, the incident waves induce high-frequency currents in the parts, which generate opposite arranged electromagnetic waves. Thereby the incident electromagnetic wave is reflected. On the other hand, the skin effect that occurs with high frequencies has the result that the currents caused by the high frequency only flow on the surface of the part facing the incoming wave. The penetration depth of the HF current decreases with increasing frequency and already remains clearly below the thickness of a common case wall at relatively low frequencies. This has the result that the electromagnetic waves hitting the surface cannot reach the inside of the case trough the metallic wall.
A problem regarding the HF-tightness of a case occurs in most of all the junctions between two adjacent parts made of metal or another, electrically conductive material, like between a door and door frame or between side and back wall for example. At such a junction the flow of the HF induced current is interrupted, and as a result, the junction becomes pervious to high frequency magnetic waves. For HF shielded cases the junctions between two adjacent parts are therefore sealed with the help of special, electrically highly conductive HF seals against the penetration or escape of HF radiation. Such HF seals are especially used between flexible cover parts, for example door and door frame of a case. The extent of the HF-tightness of such a case, sealed with HF seals that were installed later that seal the junctions, is essentially determined by the quality and the perfect fit of the HF seal between the two adjacent cover parts.
Two methods are common to test if a case assembled from several pieces possesses a sufficient HF tightness.
For the shield attenuation measurement, a HF transmitter is positioned outside the case and a receiving antenna inside, or vice versa. The attenuation, which the high frequency encounters through the closed case, is measured. A big disadvantage of this method that only the total attenuation of the complete case is measured at all times; individually leaking areas cannot be detected. An additional disadvantage is, that the measurement can only be performed on an empty case, and that the case must be irreparably damaged for the measurement. The reason is, that it is necessary to position either the transmission antenna or the receiving antenna in the (empty) inside of the case and to connect it with the other measurement equipment by means of at least one cable that runs through an opening of the case. That means, it is not possible to test a case already equipped with electronic parts for damaged shielding contacts or HF seals in the area of the doors, for example or if the case might not have been set up correctly, so that individual HF seals do not supply contact.
With the second method, the measuring of the HF tightness of a case by injection of current, a high frequency electromagnetic wave is generated between the case and an injection wire. The high frequency emission is received with an additional wire or a receiving antenna, which must be positioned inside the case. Here, too, only empty cases can be tested with this method. Another disadvantage is furthermore, that the cable shield must be galvanically connected with the case, which necessarily goes hand in hand with damage to the case surface, which is usually coated with an isolating varnish.
Both mentioned test procedures have the disadvantage in common that the measuring result is falsified by cavity resonances, leading to a bad reproducibility of the measurements. But extremely disturbing in praxis is that in each case the test piece is irreparably damaged and it is therefore not possible, to later examine the HF tightness of a case equipped with electronic devices, for example in case of a complaint. It is also not possible with the known methods to find individual HF pervious or permeable locations.
A test procedure is known from U.S. Pat. No. 5,477,157 that measures the current that decreases over the junction of a metal case. To this end, a signal source is connected with a final resistor via a coaxial conductor with inner and outer conductor. The outer conductor is sliced open and both open ends are connected to the case to be examined, so that the junction of the case is positioned between both ends and a current flows from the signal source over the junction of the case.
Besides this feed-in electrical circuit a second, separated measuring circuit is intended which consists of a measuring device and two connected electrodes. Each electrode is in contact with the case on each side on the junction. The voltage drop over the junction is measured with the measuring device.
This known testing device has several disadvantages. The inner conductors of the connected coaxial cable must be installed in a predetermined way due to design and geometric considerations. The testing device and the terminator must be electrically adapted to avoid reflections of the incoming signal, which falsify the test result. A miniaturization of the test device is possible only to a very small extent, due to the design.
The separation of the feed-in circuit and the measuring circuit results in the serious disadvantage that the measured values, established with the testing device show a positive result if a good, meaning low-impedance junction between the case parts is present, or if the electrodes are positioned wrongly, or if the equipment is defective. It is therefore not possible to distinguish between a low-impedance junction and a measuring error.
The invention at hand is based on the technical problem to test a junction between two adjacent parts reliably and without error, especially between two walls of a case, to test selectively for HF tightness, without any mechanical interference with the structure of the test piece.
The solution to this task is based on the thought that a measuring of the absolute value of the transmission and/or refection is not necessarily required to determine if a junction between two adjacent parts is pervious for high frequency electromagnetic oscillation or not, and that it is also not necessary to send electromagnetic waves through the parts, from the inside out, or in reverse, from the outside in.
SUMMARY
The problem is first of all solved with the procedure for testing the high frequency (HF) tightness of a junction between two adjacent, electrically conductive parts, especially cover part of a HF shielded case for the installation of electronic equipment, including:
Generating of a signal of high frequency electromagnetic oscillation;
Sending of the HF signal over the junction to be tested;
Measuring of the transmission and/or reflection of the HF signal on its way over the junction;
Comparing the measured transmission and/or reflection with a predetermined transmission and/or reflection characteristic for the junction.
In an embodiment of the present invention, the procedure of the present invention includes:
the HF signal consist of a series of electromagnetic oscillations with different frequencies; and
the course of the transmission and/or reflection is measured in correlation of the frequencies.
In an embodiment of the
Benko Robert
Reiser Peter
Schwenk Hans Martin
Le N.
Nguyen Vincent Q.
Schroff GmbH
Schwegman Lundberg Woessner & Kluth P.A.
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