Bifilar wound isolation network

Details Bifilar wound isolation network Bifilar wound isolation network

1998-11-16

2001-02-13

Metijahic, Safet (Department: 2858)

Electricity: measuring and testing

Impedance, admittance or other quantities representative of...

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

C324S627000

Reexamination Certificate

active

06188227

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Technical Field
This invention relates to electrical test systems, and more particularly to a test system incorporating an isolation network. The isolation network is adapted to be coupled between a device under test (DUT) and a DUT support system for generating an impedance which causes the DUT to emit its highest noise voltage levels during a test. This assists in developing worst case interference parameters which provide a guide for modifying the test item's components and connectors to minimize the effects of electromagnetic interference (EMI) generating sources and to immunize potential EMI receptors within the DUT.
2. Discussion
Various techniques have been employed to detect, measure and then suppress EMI in sensitive electrical equipment or test items. Interference or susceptibility detection and measuring should be conducted with the test item operating as close to service conditions as possible. Also, the test item normally is operated in its intended manner with anticipated inputs applied and its outputs normally loaded.
A problem exists with simulating service conditions in a normal manner of operation. To bring a test item out of its normal operating environment and to place it on a bench for a test, an actual duplication of operation of the test item seldom occurs. For example, if in a vehicle equipped with an engine control system that includes a microcomputer as a controller, assume it is desired to use a particular portable mobile two-way radio. Discovering that the radio works when the engine is shut off but doesn't work well while the engine is running, the engine control system is then removed from the vehicle and placed on a test bench for study. A simulator (support for the engine control system) is used to make the control system work as if it is in the vehicle. Also, assume the system responds as if it is operating in the vehicle controlling what it is supposed to be controlling. Assume also, the engine control system generates the same interference that it was putting out before it was placed on the bench, but now the interference reacts with the simulator. By virtue of changes in wiring, the coupling between harnesses, other components, and the impedance and length of wires, the engine control system generally radiates and conducts a different amount of interference.
Efforts have been made to standardize bench test setups in order to gain data that approaches actual circumstances. In prior bench tests under similar circumstances, line impedance stabilization networks (LISNs) have been recommended in a number of interference and susceptibility specifications for insertion in power leads to offer something approaching a standard impedance to the radio frequency (RF) current from test items. The LISN's, as required by some military specifications, introduce a standard 50 ohm power-source impedance for the test item so that conducted RF interference measurements can be compared to pass/fail limits without accounting for a source impedance variable. However, in several LISN designs a 5-microhenry coil is used so the device is suitable for use from 150 KHz to 25 MHz. Over this range, the source impedance varies from about 5 ohms at 150 KHz to 50 ohms at 25 MHz. It is not usable much above 25 MHz due to stray impedance. While it does furnish a standard impedance, it is not the impedance seen in the normal installation. It was never intended to be anything other than an A.C. power lines simulator.
Normally when trying to identify the potential of a device to be an interference source, it was thought that this determination depends upon how interference emanating from the source was measured. This implies that different test processes produced different results for the same interference source. Realizing the above conditions exist, efforts were made toward devising an interference measuring technique that didn't depend upon how the interference was measured. Systems which have been previously developed for this purpose are disclosed in U.S. Pat. Nos. 4,763,062 and 5,541,521, both assigned to the assignee of the present application, and hereby incorporated by reference into the present application. These test systems have performed in an exemplary fashion but nevertheless have been limited to devices under test which have one input referenced to ground.
With devices under test which do not have one signal line referenced to ground (i.e., balanced devices such as devices having differential inputs), previously developed EMI test systems have not been suitable for intercoupling between the device under test support equipment and the device under test itself. Accordingly, it is a principal object of the present invention to provide a test system forming an isolation network which is suitable to be intercoupled between a device under test and a device under test support system which is capable of providing a high isolation impedance to common mode signals present at input terminals of the isolation network, while still providing a relatively lossless path for differential signals present across the input terminals.
It is a further object of the present invention to provide an isolation network incorporating a bifilar winding which is relatively inexpensive to produce, which provides a broad bandwidth, and which enables balanced devices to be tested under worst case EMI operating conditions such that the device under test is forced to produce its highest noise voltage.
A test system as described above would therefore enable worst case interference parameters to be established and would also provide a repeatable scheme for determining worse case signals in any environment in which the device under test is employed. After establishing the worse case interference parameters, modification techniques and/or test item circuit components and connectors may be employed to minimize the effect of generating EMI sources and/or to immunize EMI susceptible receptors within the device under test.
SUMMARY OF THE INVENTION
The above and other objects are provided by a bifilar wound isolation network in accordance with a preferred embodiment of the present invention. The isolation network generally comprises a plurality of resonators which are serially interconnected by a single bifilar length of winding wound upon each one of a plurality of toroids. In a preferred embodiment of the present invention the isolation network comprises four independent inductors. The first inductor comprises approximately 45 turns (i.e., windings), the second inductor comprises about 30 turns, the third inductor comprises about 15 turns and the fourth inductor comprises about 10 turns. Each inductor is formed in part by a toroid which is supported upon a dielectric rod or dowel. The dielectric rod or dowel is in turn supported above a ground plane. An input end of the isolation network is coupled to an AC signal source and also to the device under test. An output end of the isolation network is coupled to a device under test support system. The bifilar winding allows the isolation network to be coupled to the differential inputs of the balanced device such that the device under test (DUT) essentially sees a high impedance to common mode signals, while the input terminals of the isolation network nevertheless provide a relative lossless path for the differential signals present across its inputs.
In the preferred embodiment of the present invention the first inductor has a resonant frequency of about 6.4 MHz, the second inductor a resonant frequency of about 15.4 MHz, the third inductor a resonant frequency of about 52.9 MHz and the fourth inductor a resonant frequency of about 87.5 MHz. The bifilar winding is comprised of two independent electrical conductors each being preferably a 26 gauge electrical conductor. The bifilar winding is wound upon each of the toroids such that the bifilar windings do not overlap one another (or have minimal overlap) when wound around each toroid, and further such that the bifilar winding extends from one inductor to th

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