GB2530793A

GB2530793A - Method and apparatus for measuring the screening effectiveness of passive equipment

Info

Publication number: GB2530793A
Application number: GB1417475.9A
Authority: GB
Inventors: Jack Chapman
Original assignee: Technetix BV
Current assignee: Technetix BV
Priority date: 2014-10-03
Filing date: 2014-10-03
Publication date: 2016-04-06
Anticipated expiration: 2034-10-03
Also published as: GB201417475D0; GB2530793B

Abstract

The screening effectiveness of a device under test (DUT) 16 is measured by connecting the device electrically in series between an input port 14 and an output port 12 of a network analyzer via a cable 18 and a test unit 20 comprising an impedance element or dummy load 22 and an attenuation device 24. A voltage drop over an input of the DUT to an output of the test unit is measured and this voltage is modified by an offset value based on a voltage drop across solely the test unit and couplings, to detect the screening effectiveness attenuation in a frequency range of 1 to 3000MHz. The dummy load 22 allows the voltage between the output of the DUT and ground to be measured and compared to the input voltage; this provides an insertion loss measurement corresponding to the screening effectiveness of the DUT.

Description

Title: Method and Apparatus for Measuring the Screening Effectiveness of Passive Equipment

Field of the invention

This invention relates to an apparatus and method for measuring the screening effectiveness of passive electromagnetic equipment and in particular for measuring components used in broadband communication systems.

Background to the invention

io The measurement of screening effectiveness of broadband electrical components has previously been carried out on circuits as used in a Cable TV (CATV) environment.

As the technical requirements have increased, the testing of EMC has been limited not by the components themselves but by the test circuits used. It is now required to measure screening effectiveness levels of -100 dB over a frequency range of 5 MHz is to 3 GHz and currently approved test circuits cannot cover this range.

Summary of the invention

In accordance with one aspect of the invention, there is provided a method of determining the screening effectiveness of a device comprising: connecting a device under test (DUT) in series with a test unit comprising impedance and aftenuation elements using coupling units and coaxial cable across the terminals of a wideband measuring system such that the DUT is correctly terminated in its characteristic impedances and the voltage across the output of the DUT to ground is measured relative to the voltage across the input of the DUT as an insertion loss measurement.

When a network analyser is used in this application, it may be calibrated with an attenuator equal to that in the test unit and coupling units in place of the DUT and test unit, The insertion loss then measured, is by definition, the screening effectiveness of the DIJT.

Thus there is provided a method of determining the screening effectiveness of a device comprising: connecting a device to be tested electrically in series with a test unit comprising impedance and attenuation elements using couplings; measuring a voltage drop over an input of the device to an output of the test unit; modifying the measured voltage drop with an offset value based on a voltage drop across solely the test unit and couplings so as to produce direct readings of screening effectiveness attenuation in the frequency range of 1 to 3000MHz This allows for the measurement of screening effectiveness to the official standard IEC 60728-2, Electromagnetic Compatibility for Equipment, Section 4.5, screening effectiveness for passive equipment to be measured directly and is suitable for measuring values in the range -60 to -110 dB.

io The test unit is configured to match the electrical characteristics of the device to be tested i.e. for IEC requirements the DUT has a characteristic impedance of 75 Ohm, and the impedance and internal attenuator within test unit are selected to match this to a 75 Ohm measuring system using 75 Ohm coupling units and cables. Test units for other DUT and network impedances may be configured to meet the screening is requirements of the Radio and Data Transfer Industry i.e. the DUT may have an impedance of 50 Ohm, or a 75 Ohm DUT may be tested with a 50 Ohm measuring system, coupling units, and cables.

Preferably the attenuation of the at least one attenuation element is between 5 to 20 dB. The attenuation is set to produce a high output return loss from the test unit and so ensure that there are no standing waves in the coaxial coupling cable but the value is low enough not to reduce the dynamic range of the measuring system.

In accordance with another aspect of the invention, there is provided apparatus for determining the screening effectiveness of a device, the apparatus comprising a network analyser with an output port and an input port, a test unit comprising at least one impedance element and at least one attenuation element, and at least one signal transmission cable, the at least one impedance element providing a dummy load over which the voltage to ground can be measured.

The test unit may have preconfigured electrical characteristics dependent on the electrical characteristics of a device to be tested.

Preferably the impedance and attenuation of the test unit is selected to match a device to be tested and that of the network analyser.

The test unit may be configured to match a device to be tested with an output impedance between 40 to 100 Ohm.

The invention will now be described, by way of example, with reference to the accompanying drawings in which: Figure 1 shows an electrical schematic of a passive screening attenuation unit in io accordance with the present invention; Figure 2 shows the screening attenuation measurement apparatus in use; Figure 3 shows the screening attenuation calibration /zero set-up process; and Figure 4 shows a test unit forming part of the apparatus.

is Description

In Figure 1, passive screening attenuation measurement apparatus comprises a Wide Band Network Analyser 10 having an output port U, and input port 14, which sweeps a range of electromagnetic frequencies. By creating an electrical path from output port t2, to input port 14, through a DUT 16, which requires the value of its screening attenuation to be measured, DUT 16 can be tested over its full frequency range, The network analyser is set-up at its maximum output level to ensure that an adequate signal to noise ratio is achieved for the measurement. By way of example, Figure 1 illustrates a galvanic isolator as a device 16 to be tested, which has a longitudinal impedance to ground of 0.75 mOhm, equivalent to a screening attenuation value of value of -100 dB.

Simply testing device 16 by connecting it between ports 12 and 14 using a test cable 18 connected directly to the dummy load will not produce suitable readings of screening effectiveness as test cable 8, typically a 75 ohm co-axial cable, will not be correctly terminated and may introduce standing waves into the system. Test unit 20 avoids this problem by providing an attenuation element 24 or matching pad of approximately 10 dB which in Figure 1 comprises a 100 Ohm resistor 26 and two Ohm resistors 28, 28'as shown, The values of the electrical components forming termination cap, or load, 22 and attenuation element 24 will vary with the impedance of device 16.

Load 22 is selected to match the impedance of device 16 and will generally be 75 Ohms although it can be other values such as 50 Ohms depending on the device to be tested. The resistance and attenuation values given are typical for a 75 Ohm system.

Attenuation element 24 is very closely coupled to the body of dummy load 22 with a low impedance coupling with a series inductance of 3 nI-I or less.

The series voltage drop across the attenuator 24, and all the coaxial units, may be zeroed out during the calibration of measuring system by using an identical attenuator 35 to item 24, in place of the test unit and DIJT during set up, see Figure 3.

When measuring screening attenuation values over a frequency range, see Figure 2, one end of device 16 is connected directly to output port 12 of measuring system 10 using an appropriate coupling unit 30, typically an N to F adaptor, with the other end of device 16 terminated by test unit 20, typically by using an F to N adaptor 32. Test cable 18 runs from the other end of test unit 20 to input port 14 and so completes an electrical path from output port 12, to the input port 14.

Test unit 20 provides a termination for the DTJT item 16, and is thus coupled in series with, and adjacent to, the device under test and is disposed between device 16 and test cable 18.

Attenuation element 24 is between device 16 and test cable 18 and is close coupled to the body of the termination cap item 22, and thus the body of DUT 16 itself By ensuring attenuator 24 provides a high return loss of the order of 20 dB, co-axial cable 18 is correctly matched over the full frequency range.

Test unit 20 is shown in Figure 3 and is approximately 90mm long over the terminals and 25mm in diameter with standard N' connectors.

Whilst capacitance to ground can be a limiting factor in screening effectiveness measurements, for attenuation va'ues in the range -75 dB to -100 dB the DUT has a very low impedance value to ground and stray capacitance values to ground of up to 1000 pF across device 16 have little or no effect on the measurement values, even at 3GHz, For example, if device 16 has a screening attenuation value of-85 dB (grade "A"), it has an impedance of only 4.2 mOhm and at -100 dB is 0.75 mOhm.

Series inductance within test unit 20 could be a problem. It is thus carefully constructed to maintain this at a very low leveL