METHOD AND APPARATUS FOR HIGH-SPEED SCANNING OF ELECTROMAGNETIC EMISSION LEVELS

Abstract: A method and apparatus for high-speed scanning of electromagnetic emission levels includes a synchronizer for synchronizing scanning of a device under test with digitizing an output video signal from a spectrum analyzer. The synchronizer can be responsive to a synchronization signal from the device under test or can generate a synchronization signal to the device under test. Methods for calibrating the level and frequency of the video output signal are provided. Once calibrated the high-speed scanning can be used to develop single probe multifrequency/time scans yielding amplitude versus frequency and time plots, and single frequency multiple probe/time scans yielding amplitude versus space and time plots.

Description: Method and Apparatus For Hiαh-Soeed Scanning of Electromagnetic Emission Levels FIELD OF THE INVENTION The present invention relates to a method of and apparatus for high-speed, scanning of printed electromagnetic emission levels and is particularly concerned with testing circuit packages and like devices. BACKGROUND OF THE INVENTION An important tool in circuit board design is the ability to easily and accurately locate and measure electromagnetic emissions from an operating circuit board. The prior art electronic emission monitors are represented by Canadian Letters Patent 1 ,286,724 assigned to Northern Telecom Limited, granted on July 23, 1991 ; United States Letters Patent 4,829,238 granted on May 9, 1989; and, United States Letters Patent 5,006,788 granted on April 9, 1991. These patents describe a method and apparatus for monitoring electromagnetic emission levels from operating printed circuit boards. The prior art enabled the user to carry out two types of scans: spectral, that is, signal strength relative to the frequency of the scanner, and spatial, that is, signal strength relative to the position of the signal on the printed circuit board. The spectral scan retains only the highest value observed at each frequency point, regardless of the probe, and the display is, therefore, merely a profile of peak electromagnetic amplitudes plotted against frequency. All other readings are discarded. A spatial scan takes a reading of the signal level at the designated frequency, for each probe within the selected scan area. The scan results are retained in a display file which provides a colour coded map of the current flows (signal strength) in the scan area at the defined frequency. The amount of time required to complete the scan is in the range of 95 seconds. The disadvantages and limitations associated with the prior art apparatus relate to the fact that it cannot scan the board under test fast enough for use in production line testing. To be effective in such an application requires an method and apparatus for gathering spectral information from all locations of the printed circuit board under test in much less than one second. SUMMARY OF THE INVENTION An object of the present invention is to provide an improved method and apparatus for monitoring electromagnetic emission levels from printed circuit packages. In accordance with an aspect of the present invention there is provided an apparatus for testing a body for electromagnetic emission therefrom, the apparatus comprising: probe means positionable in a plurality of predetermined positions immediately adjacent to the body, said predetermined positions having a known spatial arrangement; addressing means for successively addressing the probe means at each predetermined position; a receiver connected to said addressing means for measuring current induced in the probe means by electromagnetic emission from respective immediate adjacent regions of the body; signal processing means for analyzing electrical outputs from the probe means to obtain a measure of electromagnetic emission levels immediately adjacent the body associated with each predetermined position; spectrum analyzer means to gather and analyze spectral information; digital computing means to convert the analog output of the spectrum analyzer means to a digital output; calibration means to calibrate the digital output. In a further embodiment of the present invention, the spectrum analyzer output is connected to an analog to digital converter via the "video out" output socket. In yet another embodiment of the present invention, the initial analog to digital conversion is triggered to start the sweep of the printed circuit package under test by the electromagnetic scanning apparatus thereby synchronizing the operation of the printed circuit package under test with the scanning apparatus allowing the collection of scanning data with respect to time. In an embodiment of the present invention, a spectral scan of the printed circuit package under test is obtained resulting in a display of the profile of peak amplitudes plotted against frequency. Several peaks are chosen by the computer system and the frequency of each peak is recorded. From the frequencies of these peaks, a piece wise linear frequency calibration curve is produced. In another embodiment of the present invention a method is provided whereby measured level of electromagnetic radiation from the printed circuit board is calibrated. The "Cal Out" output of the spectrum analyzer is serially connected via an attenuating means of 40dB to the spectrum analyzer input. Particularly for testing a printed circuit board package 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 controller means being operable to generate a circuit layout map from the circuit layout design data and said display being means operable simultaneously to display said map of electromagnetic emission level measures and said circuit layout with said maps superimposed. In accordance with the present invention, a spectral analyzer is placed in series with the electromagnetic probe array. In accordance with another embodiment of the invention, the "video out" terminal of the spectral analyzer is connected to a high speed analog to digital (A/D) converter. By using a spectral-spatial scan, the present invention seeks to overcome the disadvantages found in the prior art. The use of a spectral- spatial scan makes it possible to measure electromagnetic emissions from printed circuit boards orders of magnitude faster than prior art scanner apparatus. The increased scanning speeds enable the user to build large data structures of scan data. In addition, the systems provide powerful manipulative, comparative and analytical tools which allows multiple perspectives of the scan results, thus establishing a new level of visualization capabilities. A further objective of the present invention is to provide a device of the character herewithin described which permits the production of improved products; significantly reduces pre-compliance times with a consequent reduction in the time to market of the printed circuit package; reduced re-engineering or modification costs; cost effective means for ensuring on-going electromagnetic emission compliance with preset standards; and, minimized life-cycle costs for the product. The spectral- spatial scan system provide a low cost, automated, non-contacting, testing of printed circuit packages quality and a testing of their performance on the manufacturing line. BRIEF DESCRIPTION OF DRAWINGS The present invention will be further understood from the following description with references to the drawings in which: Fig. 1 illustrates known art scanning apparatus; Fig. 2 illustrates, in a block diagram, a high-speed scanning apparatus in accordance with an embodiment of the present invention; Fig. 3 illustrates in a flow chart a calibration method in accordance with an embodiment of the present invention; Fig. 4 graphically illustrates the amplitude verses frequency spectrum collected by the apparatus of Fig. 2 with a window superimposed upon a peak in the spectrum in accordance with the method of Fig. 3; Fig. 5 graphically illustrates evaluation of a peak for suitability as a calibration point in accordance with the method of Fig. 3; Fig. 6 graphically illustrates narrow span sweep of the peak in accordance with the method of Fig. 3; Fig. 7 illustrates a sample piece-wise linearization curve used by the method of Fig. 3 to calibrate the frequency of the collected spectrum data; Fig. 8 illustrates in a flow chart a method of spectrum amplitude calibration in accordance with an embodiment of the present invention; Fig. 9 illustrates an apparatus set-up used for the method of Fig. 8; Fig. 10 graphically illustrates an exemplary amplitude calibration curve obtained by the method of Fig. 8; and Fig. 11 graphically illustrates an exemplary time-frequency-level plot obtained by the method of Chart B. DETAILED DESCRIPTION In the known apparatus, as shown in Fig. 1 , the method for testing a body for electromagnetic emissions therefrom, the method comprising positioning 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 current induced in the probes by electromagnetic emission from respective immediately adjacent regions of the body, and analyzing electrical outputs from the probes to obtain a position-dependent measure of electromagnetic emissions immediately adjacent to the body. For example, the co-pending device described in Canadian Patent Application 2, 161 ,292 has a electromagnetic sweep range of plus one gigahertz ( + 1 GHz). Using a 100 KHz bandwidth, a minimum of 20,000 readings would be necessary to cover all of the information contained in the electromagnetic spectrum under analysis. The internal digitizer of the spectrum analyzer on the prior art apparatus would require 34 separate sweeps with each sweep taking 100 msec to complete. This speed is not adequate for high speed production applications. One method to increase the speed of the spectral scanner is to connect an external high-speed digitizer. However, the only available output is the "video out" output of the spectrum analyzer. This output is not calibrated and therefore would yield inconsistent scanning results. Referring to Fig. 2, there is illustrated in a block diagram, a high¬ speed scanning apparatus in accordance with an embodiment of the present invention. The high-speed scanning apparatus includes an electromagnetic scanning (EMSCAN) probe array 10, a spectrum analyzer 12, a computer 14 having an analog-to-digital (A/D) converter 16, a synchronization (SYNC) block 18 for testing the electromagnetic radiation emitted by the device under test (DUT) 20. The EMSCAN probe array 10 is connected to the spectrum analyzer 12 and the computer 14 via an IEEE 488 bus 22. The spectrum analyzer 12 is connected to an A/D connecter via a video out line 24 and a blanking out line 26. The computer 14 is connected to the SYNC block 18 via a line 28 to provide a ready or arm signal thereto. The SYNC block 18 has an input/output port connected to the DUT 20, for receiving from or sending to the DUT 20, a sync signal via a sync signal line 30. The SYNC block 18 has an output coupled to the spectrum analyzer 12 via a first trigger line 32 and to the A/D converter 16 via a second trigger line 34. In operation, a high-speed scan is accomplished by externally digitizing the video out signal, provided on the video out line 24, in the A/D converter 16. The digitizing process is initiated in synchronization with the spectrum analyzer. Optionally, the synchronization signal can be applied to the device under test (DUT) 20 or alternatively, can be received from the DUT 20. The synchronization is accomplished by the SYNC block 18. The SYNC block 18 first receives a ready signal form the computer 14, then in one mode of operation sends a sync signal to the DUT 20 and trigger signals to the spectrum analyzer 12 and A/D converter 16. Thus, activity in the DUT 20 and both the spectrum analyzer and the A/D converter synchronous. In another mode of operation, the armed sync block 18 receives a sync signal, generated by the DUT 20, and in response thereto, sends trigger signals to the spectrum analyzer 12 and the A/D converter 16. The A/D converter 16 uses the video blanking signal from the spectrum analyzer 12 to stop its digitization process. The spectrum analyzer, for example, may be a Hewlett-Packard 8594E. In order to consistently map a predetermined number data points for each sweep of the spectrum analyzer, data interpolation or decimation is performed upon the digitized data output from the A/D converter. For example, at a sampling rate of 100 kHz and a sweep time of one second, each sweep of the spectrum analyzer would provide 100,000 data points. However, the blanking signal used by the A/D converter 16 to stop digitizing data likely does not have sufficient accuracy to ensure that precisely 100,000 data points are generated. In this event, the data are interpolated or decimated such that exactly the predetermined number of data points or samples result from each sweep. The step of the high-speed measurement process are summarized in the following chart: CHART A HIGH-SPEED MEASUREMENT PROCESS Step Process 1. Setup the spectrum analyzer and read back the sweep time. (Fig. 2) 2. Select the first Emscan probe. 3. Set the "Ready Line" and arm the Synchronizer. 4. The synchronizer waits until the DUT is ready or runs immediately for unsγnchronized scans. The spectrum analyzer's sweep and the Analog to Digital Converter (A/D) are then triggered. 5. The A/D digitizes the signal at the spectrum analyzer's "Video Out" and "Blanking Out" for a duration of the sweep time plus a percentage. 6. Determine the end of sweep from the digitized blanking data. 7. Linearly rescale the 'Video Out' data to correct to the number of digitized points desired. 8. Apply the Level Calibration formula to the data. Note: this formula is only determined from time to time (once per month). 9. Step to the next Emscan probe and repeat step 3-8 until the last probe is completed. The spectral data for each probe is stored in an array. 10. Run the Frequency calibration algorithm. 1 1. Display the data. Referring to Fig. 3, there is illustrated in a flow chart, a calibration method in accordance with an embodiment of the present invention. Once the data has been collected in accordance with steps 1 -9 of Chart A, at step 10, the frequency calibration method of Fig. 3 is applied as follows. The first step as represented by a block 100 is to find the highest peak in each of several windows, evenly spaced across the frequency range as illustrated in Fig. 4. Then, as represented by a block 102, based upon a minimum level and the area under the peak evaluate the peaks in each window for acceptability for use as a calibration point as illustrated in Fig. 5. Then as represented by a block 104, find the uncalibrated frequency of the first acceptable peak. Then, as represented by a block 106, set the spectrum analyzer center frequency to that frequency. Then, as represented by a block 108, set the spectrum analyzer span to a small percentage of the original span. Then, as represented by a block 110, take a sweep with the spectrum analyzer. As represented by a block 112, move the spectrum analyzer's marker to the peak, as illustrated in Fig. 6. In a block 114 determine the frequency of that marker (peak). At a block 116 repeat from steps 3-8 for each acceptable peak. Then, as represented by block 118, generate a piece- wise linearization curve from the measured values, as illustrated in Fig. 7. Finally, as represented by a block 120, use this curve to adjust the displayed spectrum to the calibrated frequency. Referring to Fig. 8, there is illustrated, an a flow chart, a method of spectrum amplitude calibration in accordance with an embodiment of the present invention. For the apparatus of Fig. 2, at the spectrum analyzer 12 attach "Cal Out" to "In" through a 40dB attenuator, as shown in Fig. 6, as represented by a block 200. Set the spectrum analyzer Reference Level and Attenuation Level so that the trace falls on the bottom graticule, as represented by a block 202. Measure the voltage on the "Video Out", as represented by a block 204. Increase the spectrum analyzer Reference Level and Attenuation Level so that the trace falls on the next graticule, as represented by a block 206. Repeat steps 3 and 4 until the top graticule has been measured, as represented by a block 208. Calculate a calibration formula from the data, as represented by a block 210. The formula is a least square fit to a straight line, i.e. Y = aX + b , as illustrated in Fig. 7. Once the set-up of Fig. 2 has been calibrated, thereby allowing high-speed scanning, collection of time related data becomes practical. There are two methods for collecting time related data: single probe multiple frequency (spectral); and single frequency multiple probes (spatial). To gather single frequency multiple probe data, the spectrum analyzer Center Frequency is set to the desired frequency at zero span and the output is digitized for each probe for the duration under consideration. The start of the digitization is synchronized to the activity on the DUT. Detailed steps are given hereinbelow in Chart B. CHART B SINGLE FREQUENCY TIME SCANNING Step Process 1 . Set up the spectrum analyzer for the desired Center Frequency, Bandwidths, etc. Set it to zero span and turn the sweep off. 2. Select the desired probe. 3. Wait for a trigger signal for the DUT. 4. Digitize the 'Video Out' of the spectrum analyzer for the duration desired and save the data. 5. Select another probe and repeat steps 3-4. 6. Display the data. To gather single probe spectral data, the spectrum analyzer is setup for the desired spectrum sweep and a single probe is selected. The spectrum analyzer sweep is triggered at time 0 of the activity on the DUT and the data collected. The spectrum analyzer is then triggered at time 0 + 1 n sample duration(s) and process is repeated until data has been collected for the desired time intervals. Note: multiple probes could be scanned in this fashion to produce a data set of position verses time and frequency. That is a combination of the two types of scans. Detailed steps are given hereinbelow in Chart C. CHART C SINGLE PROBE TIME SCANNING Step Process 1. Setup the spectrum analyzer and read back the sweep time. (Fig. 2) sϋ«8sτrrυτES r €ET(Rϋi£2β) 2. Select an Emscan probe. 3. Set the 'Ready Line' and arm the Synchronizer. 4. The synchronizer waits until the DUT is ready. The spectrum analyzer's sweep and the Analog to Digital Converter (A/D) are then triggered. 5. The A/D digitizes the signal at the spectrum analyzer's 'Video Out' And 'Blanking Out' for a duration of the sweep time plus a percentage. 6. Determine the end of sweep from the digitized blanking data. 7. Linearly rescale the 'Video Out' data to correct to the number of digitized points desired. 8. Apply the Level Calibration formula to the data. Note: this formula is only determined from time to time (once per month). 9. Repeat step 3-8 waiting one sample time more each time until the desired duration has passed. (Fig. 11 ) The spectral data for sample time is stored in an array. 10. Run the Frequency calibration algorithm. Note: for a combination time scan, select another probe and repeat steps 2-10 until all the desired probes are scanned. 1 1. Display the data. Numerous modifications, variations, and adaptations may be made to the particular embodiments of the invention described above without departing from the scope of the invention, which is defined in the claims.

Claims: WHAT IS CLAIMED IS: 1 . An apparatus for testing a device for electromagnetic emission therefrom, comprising: probe means positionable in a plurality of predetermined positions immediately adjacent to the device, said predetermined positions having a known spatial arrangement; addressing means for successively addressing the probe means at each predetermined position; a spectrum analyzer for analyzing electrical outputs from the probe means to obtain a measure of electromagnetic emission levels immediately adjacent the device associated with each predetermined position and having a video output; a digitizer connected to the video output for digitizing a signal received therefrom; a synchronizer connected to the spectrum analyzer, the digitizer and the device; and a means for frequency calibrating the digitized output signal. 2. An apparatus as claimed in claim 1 , wherein the digitizer and means for calibrating comprise a computer. 3. An apparatus as claimed in claim 2 wherein the digitizer is connected to a video blanking output of the spectrum analyzer. 4. An apparatus as claimed in claim 3 wherein the synchronizer is connected to the computer for sending or receiving a ready signal. 5. An apparatus as claimed in claim 4 wherein the synchronizer is connected to the device for receiving a synchronization signal therefrom. 6. An apparatus as claimed in claim 4 wherein the synchronizer is connected to the device for sending a synchronization signal to the device. 7. A method of high-speed scanning of electromagnetic emission levels comprising the steps of: synchronizing a spectrum analysis sweep and digitizing a video output signal generated thereby; calibrating the video output signal level; for each of a plurality of probes repeating the synchronizing and calibrating steps; calibrating the video output signal frequency; and displaying the calibrated data. 8. In an apparatus for testing a device for electromagnetic emission therefrom, comprising probe means positionable in a plurality of predetermined positions immediately adjacent to the device, said predetermined positions having a known spatial arrangement; addressing means for successively addressing the probe means at each predetermined position; a spectrum analyzer for analyzing electrical outputs from the probe means to obtain a measure of electromagnetic emission levels immediately adjacent the device associated with each predetermined position and having a video output; a digitizer connected to the video output for digitizing a signal received therefrom; a synchronizer connected to the spectrum analyzer, the digitizer and the device; and means for frequency calibrating the digitized output signal, a method of high-speed scanning of electromagnetic emission levels comprising the steps of: synchronizing a spectrum analysis sweep and digitizing a video output signal generated thereby; calibrating the video output signal level; for each of a plurality of probes repeating the synchronizing and calibrating steps; calibrating the video output signal frequency; and displaying the calibrated data. 9. A method as claimed in claim 8 further comprising the steps of: synchronizing the device with the spectrum analysis sweep and the video signal digitizing. 10. A method as claimed in claim 8 wherein the steps of calibrating the video output signal level includes the steps of: applying an attenuated calibrated output signal to the input of the spectrum analyzer; adjusting reference and attenuation levels so that a trace of the spectrum analyzer falls on a bottom graticule; changing reference and attenuation levels so that the trace falls on a next higher graticule; repeating the step of changing until an upper graticule is reacted by the trace; calculating a calibration formula based upon the reference and attenuation levels. 1 1 . A method as claimed in claim 10 wherein the formula is a least square fit to a straight line. 12. A method as claimed in claim 8 wherein the step of calibrating the video output signal frequency comprises the steps of: for an initial spectrum analysis sweep, for each of several windows spaced across the frequency range, finding several high peaks; finding an uncalibrated frequency of the peaks; sweeping a frequency band narrower than the initial sweep including each of the peaks; marking the frequency of the peak within the frequency band; generating a calibration curve using the marked frequency of each highest peak. 1 3. A method as claimed in claim 12 further comprising the step of adjusting. the video output signal in accordance with the calibration curve. 14. A method as claimed in claim 12 wherein the calibration curve is a piece-wise linearization curve. 1 5. In an apparatus for testing a device for electromagnetic emission therefrom, comprising probe means positionable in a plurality of predetermined positions immediately adjacent to the device, said predetermined positions having a known spatial arrangement; addressing means for successively addressing the probe means at each predetermined position; a spectrum analyzer for analyzing electrical outputs from the probe means to obtain a measure of electromagnetic emission levels immediately adjacent the device associated with each predetermined position and having a video output; a digitizer connected to the video output for digitizing a signal received therefrom; a synchronizer connected to the spectrum analyzer, the digitizer and the device; and means for frequency calibrating the digitized output signal, a method of high-speed scanning of electromagnetic emission levels comprising the steps of: synchronizing a spectrum analysis sweep and digitizing a video output signal generated thereby; calibrating the video output signal level; repeating the synchronization and calibrating after waiting one sample time more each sweep, until a desired duration has elapsed; calibrating the video output signal frequency; and displaying the calibrated data. 1 6. In an apparatus for testing a device for electromagnetic emission therefrom, comprising probe means positionable in a plurality of predetermined positions immediately adjacent to the device, said predetermined positions having a known spatial arrangement; addressing means for successively addressing the probe means at each predetermined position; a spectrum analyzer for analyzing electrical outputs from the probe means to obtain a measure of electromagnetic emission levels immediately adjacent the device associated with each predetermined position and having a video output; a digitizer connected to the video output for digitizing a signal received therefrom; a synchronizer connected to the spectrum analyzer, the digitizer and the device; and means for frequency calibrating the digitized output signal, a method of high-speed scanning of electromagnetic emission levels comprising the steps of: synchronizing digitizing a video output signal generated thereby with activity of the device; calibrating the video output signal level; for each of a plurality of probes repeating the synchronizing and calibrating steps; calibrating the video output signal frequency; and displaying the calibrated data.

Inventors: GUNTHORPE GARY, JAMES DON, GUNTHORPE, GARY, JAMES, DON
Applicants: EMSCAN CORP, GUNTHORPE GARY, JAMES DON, EMSCAN CORPORATION, GUNTHORPE, GARY, JAMES, DON
Application year: 1996
Main classification: G01R31
IPC codes: G01R29, G01R31