GB2266380A - Electromagnetic compatibility testing - Google Patents

Abstract

In an electromagnetic compatibility test instrument with a hand-held field probe (10) and portable microprocessor controlled electronic circuits (42), a receiver antenna (38) senses electromagnetic fields radiated by equipment under test (34) connected to a power supply socket outlet (30) of the test instrument; a transmitter antenna (32) is used to direct electromagnetic fields onto the equipment under test (24); a coupling network (48) senses radiofrequency signals superimposed on the supply outlet (30) by the equipment under test; and an impulse amplifier (54) may be used to superimpose high voltage transients onto the supply (30) to the equipment under test. A polarisation sensitive receiver, e.g. an optical fibre coil (40), is used to sense transmitted fields (32), and a mixer (36) mixes incoming and outgoing fields. A display (26) displays all measured values <IMAGE>

Description

PORTABLE TEST INSTRUMENT FOR ELECTROMAGNETIC COMPATIBILITY ASSESSMENT OF ELECTRICAL AND ELECTRONIC APPARATUS BACKGROUND OF THE PRESENT INVENTION The present invention relates to a test instrument which is used to evaluate the radio frequency emissions and immunity characteristics of electrical and electronic equipment.

As is well known there has been a dramatic increase in the use of electronic control devices particularly those which incorporate static sensitive microprocessors both in military, industrial, scientific, medical and residential fields of application. Although this increase has resulted in substantial technological improvements in these fields, it has raised concern over potential hazards due to control and operational malfunctions caused by unintentional radio frequency interference and stray electromagnetic radiation. Such control malfunctions are of particular importance in safety or life support applications.

Recognition of this concern has promulgated regulations by various national authorities in many countries to define maximum permissable levels of electromagnetic emission and immunity. In addition it has long been recognised that different laws have caused technical barriers to trading in Europe and the drive to create a single European market has resulted in the adoption of harmonised standards and Community laws to ensure that manufacturers and producers of electrotechnical products comply with the electromagnetic compatibility (EMC) testing requirements. Compliance with the EMC Directive is essential if electrotechnical products are to be sold in the United Kingdom or anywhere else in the European Community.To date, most of the harmonised standards have specified the use of calibrated laboratory instruments and antennae operating in open-air, anechoic or screened environments. Because of the specific environmental, and expertise requirements and associated costs, many manufacturers would be unable to carry out their own tests and would engage the services of expert third party bodies with suitable testing facilities to undertake this work. It is widely recognised that such facilities in the U.K. and other European countries are in short supply and that delays attending submission for third party EMC testing would be inevitable especially since it is known in the art that a large proportion of EMC related measurements are undertaken during the development phase of new products.Thus a need in the art exists for a simple EMC testing device which is capable of measuring radiated and conducted emissions and quantifying electromagnetic immunity throughout the legislated frequency range which can be operated anywhere by individuals not expert in this field.

Although this need has been recognised to a limited extent in the prior art, the electromagnetic field testing devices to date can be categorised into three main groups; the first being sophisticated laboratory instruments fitted with near and far field probes; the second being portable survey equipment developed mainly for biological hazard assessment of non-ionising radiation and primarily designed to operate in the microwave and power frequency region; the third being simple detectors of electromagnetic fields for use as 'non-contact' voltage indicators and microwave oven leakage detectors.

The prior art sophisticated laboratory instruments are very expensive, require a high level of technical expertise and operational insight, and are calibrated for use in defined and controlled electromagnetic environments only. In addition numerous instruments would be required to cover the range of tests specifically addressed by the present invention. The prior art survey equipment and field detectors simply do not satisfy the measurement requirements.

SUMMARY OF THE PRESENT INVENTION The present invention specifically addresses and alleviates the above-referenced deficiencies associated in the art. More particularly, the present invention provides an improved electromagnetic field probe characterised by use of a transmitting and receiving miniature antenna pair, a mode mixing device, polarisation sensitive optical fibre, and microprocessor control electronics to provide a calibrated source of electromagnetic radiation and to provide a calibrated receiving system for quantifying radiated electromagnetic emissions.Due to the incorporation of the antennae, mode mixer, optical fibre field sensor and microprocessor controlled electronics, the present invention can be packaged in a compact and portable configuration having typical overall dimensions of less than 600mm by 300mm by 300mm.

The field probe of the present invention is designed to be hand held measuring less than 100mm diameter by 600mm long, and comprises an electrically screened cavity closed at both ends with a dielectric plate at one end to provide a localised controllable environment operating continuously over the frequencies of interest such that the closed dielectric end is swept across the major surfaces of the equipment under test, with the associated radio frequency amplifiers, detectors and control circuits, accomodated in an instrument case, which computes antenna factors, control funtions, stores measured field values, and reads back on demand the field levels monitored continuously with respect to frequency as well as providing error alarms and associated readings due to excessive or abnormal field conditions.

In the preferred embodiment, the field probe is basically an electrically screened coaxial arrangement of a receiving antenna centrally disposed between a transmitting antenna, a mode mixer, and a coil of polarisation sensitive optical fibre.

Various antenna, mixer andsensor geometries may be utilised which directly relate to the field probe performance parameters of bandwidth, efficiency, cavity resonance, polarisation, and gain. The field probe is connected via an umbilical into a control system that both generates and measures electromagnetic field levels, stores measured field values for control computations, and displays measured values of electromagnetic field strength in calibrated terms with respect to frequency.

Due to the field probe and microprocessor controlled electronic system, the present invention provides a relatively small, portable, electromagnetic field source and measurement device which possesses the high accuracy and repeatability necessary for meaningful EMC tests in ad-hoc electromagnetic environments. In addition, due to the field probe design of the present invention enabling the detection and generation of electromagnetic radiation over an effective area defined by the diameter of the cavity aperture at the dielectric end plate, the present invention is extremely suitable for the investigation of radio frequency 'hot spots' due to either electromagnetic emissions or susceptibility of the equipment under test.In addition, by use of microprocessor control techniques, the calibration of the field probe of the present invention may be maintained on a 'real-time' basis to cover a wide range of electromagnetic measurement environments enabling its widespread use during the development phase of new products.

DESCRIPTION OF THE DRAWINGS These as well as other features of the present invention will become more apparent upon reference to the drawings, wherein: Figure l is a perspective view of the present invention operated upon by an individual user and disposed in a typical EMC testing situation; Figure 2 is an enlarged 'cut-away' side view of the improved field probe of the present invention; and Figure 3 is an electrical block diagram of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT A preferred embodiment of the present invention will now be described with reference to Figures 1 through 3.

The field probe device 10 of the present invention is formed having an electrically conductive tubular housing, defined by a front dielectric plate surface 14 and rear surface 16, and having approximate dimensions of 100mm diameter and 600mm length so as to be capable of being held in the hand of a user 18 using the handle 20. By way of example and not limitation, the instrument case 22 is preferably formed having a rectangular front surface 24. As best shown in figure 1, the front surface 24 is provided with visual display 26, manual control functions 28 and filtered mains output 13 amp socket 30.The working components of the field probe 10 basically comprise four parts as best shown in Figure 2: the transmitting antenna 32, adapted to radiate electromagnetic energy onto the equipment under test 34; the mode mixing device 36 adapted to reflect and mix electromagnetic radiation emitted from the transmitting antenna 32 and thereby prevent resonance peaks from occuring within the tubular housing and also to reflect and mix electromagnetic radiation entering the tubular housing via the dielectric plate surface 14 emitted or reflected from the equipment under test 34; the receiving antenna 38 adapted to collect electromagnetic radiation emitted from the equipment under test 34 during normal radiated emissions measurement operations and to collect electromagnetic radiation emitted from the transmitting antenna 32 during normal calibration operations of the receiving antenna 38; the polarisation sensitive optical fibre 40 adapted to collect electromagnetic radiation emitted from the transmitting antenna 32 during normal radiated susceptibility testing operations to provide feedback to the microprocessor controlled electronics 42 (shown in Figure 3) to enable accurate and precise control of the level of electromagnetic field radiated onto the equipment under test 34.

More particularly, the working components of the field probe 10 may be alternately configured depending upon overall design and performance requirements. In the preferred embodiment, the particular transmitting antenna 32 is a conducting dipole printed onto a dielectric substrate and the receiving antenna 38 is a conducting loop printed onto a dielectric substrate, both adapted for use from 20 MHz to 1000 MHz; the particular polarisation sensitive optical fibre is adapted for use from 1 V/m to 10 V/m. Of course, other antenna in the form of spiral, annular slot, annular strip or crosseddipole configurations, printed onto a dielectric substrate or free-standing could be used for higher frequencies of 1 GHz to 400 GHz.

It is an important feature of the present invention that the particular receiving antenna 38 is capable of detecting electromagnetic radiation over the full frequency range of interest and that, in particular, the present invention by calibration against a known level of electromagnetic field emitted by the transmitting antenna 32 is adapted to provide measured values of electromagnetic field strength received from the equipment under test 34 such as that associated with calibrated laboratory instruments operating in defined and controlled electromagnetic environments.

It is likewise an important feature of- the present invention that the particular transmitting antenna 32 is capable of producing electromagnetic radiation over the full frequency range of interest and that, in particular, the present invention using feedback from the polarisation sensitive optical fibre 40 is adapted to provide measured values of electromagnetic field strength radiated onto the equipment under test 34 to enable accurate and precise control of the level of this field such as that associated with calibrated laboratory instruments operating in defined and controlled electromagnetic environments.

In addition, it is an important feature of the present invention that additional testing facilities are provided to determine levels of conducted susceptibility and levels of conducted emissions of the equipment under test 34. In particular, the present invention is adapted to quantify conducted electrical noise produced by the equipment under test 34 which in the preferred embodiment is in the frequency range from 150 kHz to 30 MHz, and to generate voltage transients which in the preferred embodiment are in the range up to 2000 V, which are superimposed onto the mains supply 44 of the equipment under test 34.

Referring to Figure 3, by way of example and not limitation, the instrument case 22 contains microprocessor controlled electronic circuits 42 which are a combination of active and passive elements using components of standard manufacture recognised by those skilled in the art.

More particularly, the working sections of the microprocessor controlled electronic circuits 42 basically comprise four parts: the conducted emissions measurement section utilising filter network 44 to isolate mains-borne interference from the measurement of conducted 'terminal voltage' emissions, is connected in series with coupling network 48 which is used to pick-off the radio frequency signals superimposed on the mains supply voltage output 30 produced by the equipment under test 34. A radio frequency amplifier and detector 50 provides an amplified voltage for conversion to a digital signal by the microprocessor 52 which enables measured values of conducted emissions to be observed on demand at the visual display 26; the conducted susceptibility test section utilising filter network 44 to block and prevent high voltage transients produced by impulse amplifier 54 from interfering with other mains operated equipment not under test, is connected in series with coupling network 46 which is used to superimpose the transients onto the mains voltage supply output 30. The microprocessor 52 provides short timing pulses of approximately 5 ns rise time for amplification by the impulse amplifier 54.A software controlled switch may additionally be provided to de-activate or attenuate the radio frequency signal amplifier 50 during this particular testing operation; the radiated emissions measurement section utilising a radio frequency amplifier and detector 56 connected to a receiving antenna 38 is used to detect radio frequency electromagnetic emissions passing through the dielectric plate surface 14. In the receive calibration mode, a defined electromagnetic field is emitted from the transmitting antenna 32 and sensed by the receiving antenna 38 to provide an amplified voltage which is converted to a digital signal by the microprocessor 52.The difference between this signal and a stored response with respect to frequency is calculated by the microprocessor 52 to produce a control parameter which adjusts the voltage gain with respect to frequency of the radio frequency amplifier and detector 56. In the receive emissions mode, the defined electromagnetic field is switched off and the electromagnetic energy emitted by the equipment under test 34 is detected by the receiving antenna 38 at a distance d2 within the field probe 10. A field source substitution principle is thus used to derive the gain characteristics of the receiving system with the field produced by the transmitting antenna 32 acting as a transfer standard.The amplified voltage of the radio frequency amplifier and detector 56 is converted to a digital signal by the microprocessor 52 to provide measured values of radiated emissions to be observed on demand at the visual display 26; the radiated susceptibility testing section, in the preferred embodiment, consists of a transmitting antenna 32 connected to a radio frequency power amplifier 58 which is driven by a synthesised frequency generator programmed by the microprocessor 52 to produce a quasi-continuous frequency swept output.

Electromagnetic energy emitted by the transmitting antenna 32 is received within the field probe 10 by the polarisation sensitive optical fibre 40 which feeds back a visible light signal to the optical polarisation detector 60 which produces a voltage proportional to the electromagnetic field strength emitted by the field probe 10. This voltage is converted by the microprocessor 52 to provide a digital signal which when compared with an amplitude setting fixed by a manual control function 28 (indicated also in Figure 1) is used to adjust the power gain of the radio frequency power amplifier 58 and to provide measured values of electromagnetic radiation on the visual display 26.

Thus, in summary, those skilled in the art will recognise that the present invention comprises a significant improvement in the art by providing a calibrated measuring instrument which may be easily carried by the individual 18 and be used to produce swift and repeatable measurements for qualititive EMC assessment purposes. In addition the system maintains calibration in response to the testing environment and may also be used as a trouble shooting device for locating sources of radio frequency interference and points of radio frequency sensitivity.

Claims (15)

1 A testing device for assessing electromagnetic compatibility comprising: a field probe housing adapted to be held and used in the hand of an individual; a microprocessor electronics housing adapted to be carried in the hand of an individual; receiving antenna means carried by said field probe housing for detecting electromagnetic fields incident upon a defined surface of said field probe housing; means responsive to said receiving antenna means for displaying the level of electromagnetic fields incident upon said defined surface of said field probe housing within said microprocessor electronics housing to the individual; transmitting antenna means carried by said field probe housing for emitting electromagnetic fields from said defined surface;; means responsive to said transmitting antenna means for displaying the level of electromagnetic fields emitted from said defined surface of said field probe housing within said microprocessor electronics housing to the individual; coupling means carried by said microprocessor electronics housing for displaying the level of radio frequency noise on electrical supply of equipment under test to the individual; transformer means carried by said microprocessor electronics housing for coupling high voltage transients of specific levels and repetition rate onto said electrical supply of said equipment under test;

2 The device of Claim 1 wherein said receiving antenna means includes means for detecting electromagnetic fields on a quasi-continuous frequency range basis.

3 The device of Claim 2 wherein said transmitting antenna means includes means for producing electromagnetic fields on a quasi-continuous frequency range basis.

4 The device of Claim 3 wherein said coupling means includes means for detecting radio frequency noise on a quasi-continuous frequency range basis.

5 The device of Claim 4 wherein said transformer means includes means for generating said high voltage transients for each of said specific levels at said repetition rates.

6 The device of Claim 5 comprising means for connecting said equipment under test to microprocessor electronics housing.

7 The device of Claim 2 and Claim 3 further comprising means for calibrating said receiving antenna.

8 The device of Claim 2 and Claim 4 wherein said detecting means further comprises switch means for setting amplitude threshold levels for said quasicontinuous frequency range.

9 The device of Claim 8 further comprising means responsive to said amplitude threshold levels to alert the user of increasing or decreasing levels.

10 The device of Claim 3 further providing switch means for manual frequency adjustment over said quasi-continuous frequency range.

11 The device of Claim 10 wherein said means for producing electromagnetic fields provides means for mode mixing electromagnetic radiation to prevent resonant peaks occuring within said field probe housing.

12 The device of Claim 11 further comprising field strength detection means responsive to said mode mixing.

13 The device of Claim 12 wherein said field strength detection means comprises a polarisation sensitive optical fibre.

14 The device of Claim 13 comprising polarisation detection means for amplitude control of said electromagnetic field.

15 The device of Claim 14 wherein said display means comprises a digital display