METHOD AND APPARATUS FOR MONITORING ELECTROMAGNETIC EMISSION LEVELS

Abstract: Electromagnetic emission from a printed circuit board (39) is monitored by energizing the board while it is located adjacent an array of electromagnetic emission measuring probes (42). The probes are successively addressed and currents induced in the probes are measured at a receiver. A memory map of the electromagnetic emission as a function of board position is generated and is displayed together with the circuit board layout so that regions of high emission level can be identified in the circuit. Each probe of the array is a series connected pair of wire loops (24, 26), the planes of the loops being perpendicular to each other and to the plane of the array.

Description: This invention relates to a method and apparatus for locating sources of high levels of electromagnetic emission in printed circuit boards and similar circuit packages. Communications regulatory authorities in many countries have introduced regulation requiring that electromagnetic emissions from computing equipment and other digital equipment be limited in order to prevent electromagnetic interference to licensed radio communications. For example, in the U.S.A., the Federal Communications Commission has introduced Part 15, Subpart J of the FCC Rules which specifies test methods and limits. One way of limiting emission is by mounting the equipment in a shielded enclosure which acts as a Faraday cage. This can prove expensive because of the numerous measures to ensure that the shield is not compromised by electromagnetic leakage through enclosure openings or external electrical connections. Thus special conductive gaskets to electrically join panels, and radio-frequency filtering of all wiring leaving the equipment may be required. A cheaper method in the long term is to ensure that the various electrical and electronic components within the equipment do not in fact generate high levels of electromagnetic emission. An important tool in circuit board design is therefore a method for easily and accurately locating and measuring the electromagnetic emission from an operating circuit board. Using a known system, a prototype circuit board package is placed in an environment such as an anechoic chamber devoid of any electromagnetic interference. A monitoring device is then set up in spaced relationship to the circuit board or equipment to ascertain the gross electromagnetic emission. This does not really assist in determining exactly where on the circuit board the electromagnetic emission is high. For this a more localized monitor is required. U.S. Patent 2,509,045 discloses an apparatus for determining the pattern of ultra-high frequency or micro-wave type radiation from a radiator, e.g. an antenna, at a location spaced from the body by a comparatively large distance, e.g. a distance measured in metres. Such apparatus measures electrically induced currents indirectly by sensing heat that such currents generate, using thermocouples. Furthermore, such apparatus determines overall field radiation patterns, not emissions from specific, localised parts of the radiator. According to one aspect of the invention there is provided a method of testing a body for electromagnetic emission therefrom, the method comprises positioning of an array of electromagnetic emission monitoring probes occupying predetermined positions within the array immediately adjacent to the body, successively addressing the probes to measure detected magnetically induced current in the probes by electromagnetic emission from respective immediately adjacent regions of the body, and analysing electrical outputs from the probes to obtain a position-dependent measure of electromagnetic emission levels immediately adjacent to the body. Particularly for testing a circuit board for electromagnetic emission therefrom, the method can further comprise energizing a circuit supported by the circuit board, addressing an electromagnetic emission monitoring probe located immediately adjacent the board at successive locations in a grid thereof extending parallel to the board, measuring the current induced in each of the probes during an address period using a receiver tuned to a predetermined frequency, directing circuit layout design data to a controller, operating the controller to illustrate the circuit layout at a visual display unit, directing said peak current data to the controller, and operating the controller to illustrate a corresponding electromagnetic emission map at the visual display unit. The method can further comprise generating a contour map having contour lines joining identical electromagnetic emission levels, and then displaying both the contour map and said circuit layout design at the display means whereby emission levels can be related to specific elements of the circuit. Alternatively, the method can comprise generating a colour coded map, each of the displayed colours denoting a predetermined level of electromagnetic emission and similarly displaying the colour map and the circuit layout design. According to another aspect of the invention there is provided apparatus for testing the body for electromagnetic emission therefrom, the apparatus comprises an array of electromagnetic emission monitoring probes; addressing means for successively addressing the probes of the array; a receiver connected to said addressing means for measuring magnetically induced current in the monitoring probes by electromagnetic emission from the body when located immediately adjacent to the array; controller means for developing a memory map of electromagnetic emission from the body; and display means for displaying said map whereby the level of electromagnetic emission is related to locations on the body. Particularly for testing a circuit board for electromagnetic emission therefrom, the apparatus can further include a memory at which circuit layout design data is stored, and means for inputting said circuit layout design data to the controller means, said display means operable simultaneously to display said memory map of electromagnetic emission from the circuit board and said layout of the circuit whereby the level of electromagnetic emission is related to elements of the circuit. Preferably the probes are located in an XY lattice, the XY lattice having a plurality of column conductors and a plurality of row conductors with the probes located at intersections between the row and column conductors. Said means for addressing the probes can comprise decoder and driver circuits associated with each of the column and row conductors. The apparatus can further include a signal processor for digitizing an output from the receiver. Preferably the apparatus includes a microprocessor controller for developing the memory map and for producing an output signal to the visual display unit for simultaneously displaying said map and circuit layout. The probes can each comprise a pair of wire loops, the loops of each pair electrically connected in series with one another and with a switch whereby a particular probe is only addressed when both the column and row conductors intersecting at that probe are energized. The switches can comprise switching or PIN diodes. The probes can be received in recessed portions of a supporting board for supporting the probe array. The board can have a thin dielectric layer overlying the probe array for physical protection and a number of layers of connecting metallization. In order to conduct receiver signal to the receiver, the probes can be interconnected by strip line interconnect. An embodiment of the invention will now be described by way of example with reference to the accompanying drawings in which:- Figure 1 is a block schematic drawing of electromagnetic emission monitoring apparatus according to the invention; and Figure 2 is a schematic perspective view, not to scale, showing one probe of a scanning array thereof forming part of the apparatus. Referring to the drawings in detail, Figure 1 shows a block diagram of an electromagnetic emission scanning system. The system has a probe array 10 driven by decoder and drive circuits 12, a measuring receiver 14, a signal processor 16, a controller 18, and a visual display unit 20. The probe array has a series of probes 22 packed in an XY grid array. As shown in Figure 2, each probe consists of two serially connected wire loops 24, 26, the wire loops being mounted with their planes perpendicular to the plane of the array (horizontal as shown) and mutually perpendicular to one another. A current is induced in each loop which is proportional to electromagnetic flux in a direction perpendicular to the plane of the loop. The area of each loop is a compromise between a large area required for probe sensitivity and a small area enabling more probes to be packed into the array so as to increase the resolving capability of the scanner. A suitable compromise area is 25mm² (approximately 0.04 square inches). The crossed probes are used in order to provide an omnidirectional characteristic. Although not described in this embodiment, a further loop mounted perpendicular to the other loops, can be used to measure electromagnetic emission perpendicular to the plane of the array. A single loop may be used for measuring in a single direction. A circuit board package 39 to be tested is positioned close to the scanning array so that changing magnetic fields generated by currents within board conductors 41 induce corresponding currents in the probes 22. These induced currents are measured by the receiver 14. The array of probes is scanned electronically using the decoder and driver circuits 12, each probe 22 in turn being conducted to the receiver. Normally the electromagnetic emission scan is performed for a range of frequencies. Priority is given to those signals on the board having the fastest repetition rates. Only those signal harmonic frequencies which fall within the range of the specific application need be considered (e.g. 30 MHz to 1000 MHz for FCC Part 15J requirements). Alternatively, the total radio-frequency radiated emissions of the circuit board may be measured by conventional means using a radio receiver and an antenna at a distance, and the frequencies of emissions having the highest amplitude selected for board scans. The receiver 14 is first tuned to a particular frequency at which electromagnetic emission is to be measured. Current induced in an addressed probe 22 develops a voltage across a terminating input impedance of the receiver. The receiver has variable bandwidth, e.g. 10 KHz and 120 KHz, and detection levels (peak or average) selectable for measuring electromagnetic emission at different frequencies. The output of the receiver is passed to the signal processor 16 where it is digitized into a form suitable for input to the controller 18. The controller 18 carries out necessary housekeeping tasks such as addressing the probes 22 of the row column array in sequence. It also sets up the parameters of the receiver such as frequency, bandwidth and detection mode (amplitude or frequency modulation). In addition, it instructs the signal processor 16 in its digitization of the signal from the receiver. Lastly, the controller 18 formats data for input to the visual display unit 20 according to whether the displayed map is required in contour or colour form. Selection of each probe 22 is achieved by means of a diode matrix arrangement. A switching of PIN diode 29 such as D16 (Figure 1) is serially connected to the loops of each probe. A particular probe at a row/column intersection is selected by applying a positive voltage to one of the columns 32 and a negative voltage to the appropriate row 30 to turn on the associated diode 29. Current induced in a selected probe, for example D11, flows to the left along the row 30 through D22 and C19 to a collector bus 34 and to the receiver 14. The path followed from the selected probe 22 to the measuring receiver 14 is of controlled impedance, typically 50 ohms. At high frequency, above 20 MHz, the length of each row 30 becomes an appreciable part of a wavelength. To avoid fluctuations in the received signal from signal reflections, the rows 30 are terminated by an impedance such as R11 equal to the characteristic impedance of the transmission line, typically 50 ohms. The signal is carried from the probes 22 by means of strip line 36, the strip lines having flanking ground tracks 37 bonded to upper and lower ground planes 38 by vias 41 at frequent intervals, typically every 7.5mm (approximately 0.3 inches). One end of each probe is AC grounded by means of a capacitor such as C9. This is a surface mount type of capacitor which provides a high self-resonant frequency. Control signals for selection of the probes are carried on lower layers 40, 42 of the multilayer printed circuit board. In use, the printed circuit pack 39 is placed track side down (component side up) on top of the probe array but separated from the probes by a thin insulating layer 44 to prevent electrical shorts. The printed circuit pack under test is subjected to signal and supply voltages to be expected in normal operation. The array is then scanned, one probe at a time, and the magnitude of the output from each probe is measured at the receiver 14. The output of the receiver is then processed and passed to the controller 18. A memory map is generated at the controller and is displayed coincidentally with computer aided design (CAD) data for the particular board. The data is formatted so as to show the various levels of electromagnetic emission as different colours or different contour lines. Consequently the circuit elements responsible for high emission levels can be identified. The invention has particular application at the circuit board design stage but can be used also in production control to avoid the fabrication of boards having unusually high levels of electromagnetic emission. The scanning apparatus described has applications additional to measuring EMI emissions from printed circuit boards. The apparatus can, for example, be used in the fault diagnosis of printed circuit boards. Each printed circuit board (PCB) when energized and exercised, has its own characteristic electromagnetic signature (or contour map) as measured by the electromagnetic scanning system. For a given type of PCB, any fault in the circuit conductors or components can cause this signature to be altered. This happens because signal currents are either interrupted (e.g. by open circuits), diverted (by short circuits), or otherwise changed in amplitude or waveshape by malfunctioning components. By comparing the signature from a PCB under test with a reference signature obtained from a PCB known to be good, the location and nature of the fault may be deduced. If the fault exists on parts of the circuitry operating at DC or at very low frequencies where the EM scanner is not sensitive, use is made of a tracer signal of appropriate frequency injected into the circuit in question. The scanner is operated to map the path followed by this signal to aid in fault diagnosis. An example of this technique is to inject a tracer signal current between a power supply and ground terminals of a PCB to help trace the location of a possible short circuit between these terminals. The apparatus can also be used in electromagnetic shielding evaluation. Good electromagnetic shielding requires that the shielding material (particularly thin, conductive coatings applied to plastic equipment housings) be free of flaws such as cracks and thin areas. Also, where joints occur between shielding panels, the electrical bonding must be continuous. If such a shield is energized with a radio-frequency current from an external generator, areas of RF leakage at these flaws are prominently displayed and localized using the scanner display. Similarly, flaws in metal coatings or sheets can be detected by the scanner if the samples are suitably energized with RF energy. This is accomplished by injecting RF currents through the samples under test or by illuminating the samples with external RF radiation and then observing the resulting pattern of RF current flow on the sample surface using the scanner. Flaws such as cracks or badly bonded metal panels show up as anomalies on the visual display. In another application, the scanner is operated in reverse. The scanner port which is normally connected to the receiver is instead connected to a source of RF energy such as a signal generator. When the scanning circuitry is operated, the applied signal causes RF current to sequentially flow in each individual RF probe on the scanner head. In effect, the scanner operates in reverse, causing localized RF magnetic fields to be generated at probe positions. The scanner head is placed in close contact with a PCB known to be sensitive or susceptible to RF, and the precise time of occurrence of any PCB disturbance or malfunction is monitored. This is related to the particular probe of the scanner which is being energized at that time. This technique is used to generate a map of PCB sensitivity to RF. Depending on the particular application, appropriate adjustment can be made of the RF receiver sensitivity and bandwidth selection. The receiver can be designed to respond to amplitude or frequency modulation. Alternatively, a peak, average, RMS or logarithmic response signal detector can be used. As previously indicated, the display can show different emission levels as different colours, colour levels, as a contour map, colour intensity modulated map, or a simulated three-dimensional graph. Known image processing techniques can be used to increase the apparent resolution of the displayed image or to enhance its contrast or information-to-noise ratio. The display intensity or colour corresponding to each probe location can be binary to indicate a pass/fail situation or it can consist of many quantized steps or levels corresponding to a continuous measurement of some parameter of the circuit under test. Finally, in the scanner itself, the probe size, aspect ratio, and number of turns of the loops can be selected according to the particular application. Optionally, the probe wires can have magnetic cores to increase probe sensitivity to low frequency electromagnetic fields or an electric field shielding layer to exclude the interfering effects of unwanted electric fields. Although the scanner described has a planar rectangular probe array, the scanner is not restricted to this shape and, depending on its application, can be cylindrical, spherical, or disc-shaped.

Claims: A method of testing a body (39) for electromagnetic emission therefrom, said method comprising positioning of an array (10) of electromagnetic emission monitoring probes (22) occupying predetermined positions within the array immediately adjacent to the body, successively addressing the probes to measure detected magnetically induced current in the probes by electromagnetic emission from respective immediately adjacent regions (41) of the body, and analyzing electrical outputs from the probes to obtain a position-dependent measure of electromagnetic emission levels immediately adjacent to the body. A method as claimed in claim 1 wherein the body comprises a circuit board (39), the method being further characterised in that it comprises energizing a circuit (41) supported by the circuit board while addressing electromagnetic emission monitoring probe means (10) at the predetermined positions located immediately adjacent the board, measuring current magnetically induced in the probe means at each predetermined position and displaying the respective measures of electromagnetic emission levels arranged in a spatial arrangement corresponding to the spatial arrangement of the predetermined positions. A method as claimed in claim 2, wherein the step of displaying a map of electromagnetic emission level measures comprises generating a contour map having contour lines joining identical electromagnetic emission levels and displaying said contour map. A method as claimed in claim 3, wherein the step of displaying a map of electromagnetic emission level measures comprises generating a colour coded map, each of the displayed colours denoting a predetermined range of electromagnetic emission levels and displaying said colour coded map. A method as defined in claim 4 further comprising displaying a map of the circuit board under test with the map of the circuit board and the map of the electromagnetic emission level measures. Apparatus for testing a body (39) for electromagnetic emission thereform, the apparatus comprising an array (10) of electromagnetic emission monitoring probes (22); addressing means (12) for successively addressing the probes of the array; a receiver (14) connected to said addressing means for measuring magnetically induced current in the monitoring probes by electromagnetic emission from the body when located immediately adjacent to the array; controller means (18) for developing a memory map of electromagnetic emission from the body; and display means (20) for displaying said map whereby the level of electromagnetic emission is related to locations on the body. Apparatus according to claim 6 wherein the body comprises a circuit board (39), characterised in that the apparatus further includes means for energising a circuit (41) supported by the circuit board, means for addressing (12) the probe means (10) at the predetermined positions, means for measuring (14) current magnetically induced in the probe means and means for displaying (20) measures of electromagnetic emission levels arranged in a spatial arrangement corresponding to the spatial arrangement of the predetermined positions. Apparatus according to claim 6 or 7 characterised in that each probe means (22) comprises at least one wire loop (24,26) with each wire loop being perpendicular to the plane of the array. Apparatus according to claim 8 characterised in that each probe (22) comprises a pair of such loops (24,26) electrically connected in series and wherein one of the two loops is perpendicular to the other of the two loops, both loops being perpendicular to the plane of the array. Apparatus according to claim 8 characterised in that the apparatus further comprises a plurality of row conductors (30) and a plurality of column conductors (32), each row conductor connected to each column conductor by a respective switch (D1-D13) in series with one of the probes. Apparatus according to claim 8 characterised in that the probes (22) are received in recessed portions of a supporting body for supporting the probe array. Apparatus according to claim 8 or 9 characterised in that there is provided a thin insulation layer (44) to separate the probe means from the body under test. Apparatus according to claim 10, characterised in that each conductor of one of said pluralities of conductors comprises a conductor strip (36) flanked by ground tracks (37), disposed between underlying and overlying ground planes (38), terminated at one end in an impedance matched to a characteristic impedance of the resulting transmission line and coupled to the receiver. Apparatus as claimed in claim 8, characterised in that probe wire has a magnetic core to increase probe sensitivity to low frequency magnetic fields. Apparatus as claimed in claim 8, characterised in that the probe wire has electric field shielding outer layer.

Inventors: GOULETTE, RICHARD RALPH, GREENFIELD, RAYMOND LESLIE, XAVIER, STANILUS KISITO
Applicants: NORTHERN TELECOM LIMITED
Application year: 1987
Main classification: G01R31
IPC codes: G01N27, G01R29, G01R31