AN-04: EMI Considerations Designing DDX Amplifiers
Contents
1. At the Test Site Look for the problems you know about be ready with more than one method of suppressing an emission if possible When you fail to suppress an emission get as much information at the test site as possible Apogee Technology Inc 10 Document 07010001 01 00 Appendix D Schematic Examples of Suppression Components This is an example of EMI filtering for DC Power and Speaker lines for a DDX amplifier Generally all components will not be required in most cases but is illustrative of typical methods of suppressing unwanted signals from conducting off a board The combination of ferrites and capacitors produce simple RC filters Note that different values of capacitance and types of ferrites will be required depending on the frequency to be filtered Care must be taken to not run noisy signals under or near the input for the DC Power or the output for the Speaker lines of these filters or noise can be coupled to wires leading off of the PCB nullifying any benefit of the filter In some instances such as prototyping it is advantageous to have a separate interface ground to bypass to This allows the Interface Ground to be connected to either circuit ground or chassis ground in any manner AC with a capacitor or DC with a zero ohm jumper 0JMP It also keeps normal circuit currents away from the I O areas of the PCB Power to DDX 2060 DC Power in CM Ferrite OJMP or Cap Interface Ground From the DDX 20602. A second source at the same frequency or other outside factor is involved Apogee Technology Inc 12 Document 07010001 01 00 Do not leave the verification site without gathering as much information as possible about a failure it will be invaluable back in the lab Regulatory requirements have been reviewed and methods of identifying and suppressing emissions relevant to DDX amplifiers have been offered that will be effective in meeting those requirements A specific example of configuring a DDX amplifier is given in Appendix A Using the techniques and methods presented in this application note the Apogee EB 2060S stereo evaluation board a Class A laboratory device was modified to meet Class B limits for a typical application without benefit of a shielding chassis Appendix B shows scanned images of the Certificate of Compliance report highlights and photographs of the EUT Appendix C is a checklist of do s and don ts for quick reference when designing your system Appendix D shows schematic circuit examples of typical suppression components applied at PCB interfaces Additional information on EMI suppression is available from the ferrite and suppression component manufacturers listed in the References section Apogee Technology Inc 13 Document 07010001 01 00 Appendices Appendix A Modifications to EB 2060S to be Class B compliant The EB 2060S evaluation board was tested on 5 17 01 as an unenclosed board and passed radiated emiss
3. Ground Plane Trace A Void Ted Trace B gt A Source Load gt lt Ground Plane Figure 4 Apogee Technology Inc 8 Document 07010001 01 00 All digital signals contain high frequency components due to their fast rise and fall times As good practice do not allow digital signals or power traces to digital devices to cross a void in a ground plane Sometimes it is not possible to avoid this or it gets overlooked during layout In those cases it may be necessary to increase the circuit impedance at the noise frequency to significantly reduce the noise current Figure 5 shows a representation of this The inductor can be a real physical inductor or can represent a ferrite bead Ferrite beads increase circuit impedance by dissipating energy as heat and so act primarily as resistive components in the circuit They are generally only effective under low bias current conditions for differential mode noise as their impedance can be significantly reduced with increasing bias current Typically the use of ferrite beads is limited to signal levels for reducing differential mode noise and physical inductors or chokes are applied to power bus signals The capacitor shown in Figure 5 is generally a real physical component for power circuits and is a stray capacitance when ferrite beads are applied In either case the principle of reducing the loop radiating area applies Signal gt Source LOAD Noise Source Figure 5 Differential noi
4. Radiated emission limits for the US and Europe Frequency FCC Class B EN55022 Class B Quasi Peak Quasi Peak dBuV m dBuV m MHz 3meter 10meter 3meter 10meter 30 to 88 40 30 40 30 88 to 216 44 33 40 30 216 to 230 46 36 40 30 230 to 960 46 36 47 37 960 to 1000 54 44 47 37 Note that as DDX amplifiers use less than a 108MHz clock the maximum frequency subject to examination is IGHz Apogee Technology Inc 5 Document 07010001 01 00 Again as for the conducted emissions a design specification for the strictest requirements can be generated Table 5 Design Specification for DDX amplifier radiated emissions Frequency Class B Quasi Peak dBuV m MHz 3meters 10meters 30 to 88 40 30 88 to 216 40 30 216 to 230 40 30 230 to 960 46 36 960 to 1000 47 37 As with the conducted emissions the stricter limit applies at a transition frequency Note also that a 2 dBu V m margin should be allowed to account for measurement repeatability Clearly it is much more difficult to replicate the radiated emissions test set up primarily due to space constraints and the need to be away from interfering background signals For development purposes both the inductive current clamp mentioned for conducted emissions and a near field probe kit are used to determine the frequencies that are emitted and again in a relative manner the effectiveness of the suppression techniq
5. can have the earth itself coupled through stray capacitance as the return path A good example to avoid in DDX amplifiers is power supply traces to the DDX power output devices should not run under or be made into a plane layer under the audio output filter components This will cause noise to be coupled to inductors or traces that then connect to the outside world after the PWM signal filter resulting in a radiated emission from common mode noise on the speaker lines Fortunately common mode filtering is effective and easy to apply Figure 7 shows the use of a common mode choke essentially a ferrite core transformer wound to induce opposing currents in the opposite winding to cancel the common mode currents Ferrite beads are also extremely effective in this application as the net field due to the differential signal current flow is zero they do not suffer from impedance decrease due to a bias current as a bead around a single wire would This allows ferrite clamps to be used on multi conductor cables as well The transformer principle applies to beads as a few turns can be made on a bead core to increase its loss impedance Generally frequency determines the choice of a common mode bead or common mode choke as the inter winding capacitance of multiple turns limits the loss at higher frequencies and they are sometimes both used to fully cover a broad frequency emission range Signal Source Noise Source Common mode Choke or Bea
6. conducted by wires inside or outside the product and determining in a relative manner the effectiveness of the applied suppression techniques This can be of either the passive or active variety but should be for 50 ohm termination to be consistent with input impedance of most spectrum analyzers Radiated Emissions and Limits Radiated emissions are measured as electric field strength expressed as dBu V m by an antenna at a test distance of 10 meters from the EUT A 3 meter distance is sometimes used to distinguish the emitted signal if a background signal is very strong note that 3 meter testing is not accepted by all standards This measurement is done with the EUT mounted on a non conductive table at a height of 0 8 meter the antenna is mounted such that it can be raised or lowered from 1 to 4 meters and rotated so that it can be set to measure the maximum signal emitted from the EUT Both the EUT and receiving antenna are mounted over the same ground plane of metal sheeting or screening to permit additional EUT signals to be reflected to the antenna The antenna and receiving system must be calibrated and correction factors applied to the raw measurement data to compensate for signal loss in the antenna and interconnect cable At a test facility this test can typically run six hours or more as both the EUT and antenna must be rotated and moved in height to ensure that the maximum signal is being received in each scanned frequency range Table 4
7. have the proper equipment on hand for evaluating EMI and understand how to use it Plan for pre screening your product and make allowances for a second iteration During the design and prototyping Treat the product as a whole the chassis design and board layouts must compliment each other to minimize emissions Understand how and where current flows in your design and identify critical areas to layout first to minimize emissions Make allowances for suppression components in critical areas especially at I O interfaces it is easier to remove or leave out a component than to add one Pay particular attention to power supply routing and bypassing and to the routing of the PWM outputs and the filter layout Current flowing out needs to return back on the same path Use a ground plane unbroken as possible and keep connections short Do not let digital signals cross a void in the ground plane allow space for using a ferrite bead if you can t avoid it Do not make unconnected stub traces on digital signal lines Be aware and check for common mode coupling do not route noisy signals were they would couple to traces or wires leading outside the chassis Review the entire design before committing to hardware A fresh set of eyes may catch something you have missed even better if they have EMI experience Pre scan your PCB and product with near field probes know what emits and how before the verification test
8. CI AS NZS 3548 Class B Approved By Remarks Testing ts performed using calibrated equipment traceable to the National Institute of Standards and Technology NIST This certificate is valid for products tested as described in the accompanying test report Specific modifications necessary to meet the above requirement recommended by Integrity Design amp Test Services Inc are described therein Integrity Design amp Test Services Inc ts accredited by the National Voluntary Laboratory Accreditation Program NVLAP for Electromagnetic Emissions Testing Apogee Technology Inc 3 Document 07010001 01 00 Report 56920 e1 2 Test Environment 21 EUT Description Mode number EB 20608 S N SNOO0 Model name DDX Stereo Evaluation Board Description The EUT is for evaluating the Apogee Technology Inc DDX Digital Audio Amplifier Chip Sei the DDX2000 processing IC and the DDX2060 power IC It converts standard PCM audio data to a Pulse Width Modulated signal completely by digital means to reconstruct an audio signet with sufficient power to drive loudspeakers Its intended environment is for laboratory cvaluation use by manufacturers of consumer audio products who would then incorporate the DDX chip set into their products This testing is intended to demonstrate that product Ibat meets regulatory agency requirements can be produced with the DX chip set 2 1 1 System Operation The system was configured in a typical operat
9. EMI Considerations Designing DDX Amplifiers For Applications Assistance Contact Mike Danielson Apogee Technology Inc 129 Morgan Drive Norwood MA 02062 USA mdanielson apogeeddx com Apogee Technology Inc Document 07010001 01 00 Table Of Contents CPPS UREA ee Sot ba tuu utut tb pet utes 2 Introduction tee ROME SONG MO RR EO OR RR ARES aad eR 3 luec lonnen eee ee ree beaan S 3 EMI Regulations and Measurements eeeeeeeeeenee 3 Conducted Emissions and Limits ee tee su Dena ERR NORRIS ME LATIS 3 Radiated Emissions and Limits 3 Sources of Emissions Noise and General Suppression Methods 6 Differential Mode Noise 6 tetro ederet ona Re ER ne Ya e XR ad Reds 7 Common Mode INGISE a ied tos atv ed as eee esa e is NENNT SR IRA RAS SE ER 9 Finding the Noise Sources d io e edili eiiis 11 Conclusion 22 ounce bae ea aaoi oae e feed 12 PAPPCMCICeS ee o 1 Appendiz PE os ee sca acc id cc ut a ds Ca Se ag gee De ECOL en oed 1 Modifications to EB 2060S to be Class B compliant 1 PRP PSURs ceed Sota tue X 3 Verification Test Report 5 eL E RUNE ESSEN DRAN EE ENS 3 JNDDOBODOC Soie dice casa Caleta es te Late Rss can Ghat i eid CUL ed De 10 Checklist of Do s and Don ts eeseeeeeeenene 10 Append D SS ota te Son ose i er ie use oet ma 11 Schematic Examples of Suppression Components 11 Ee Crenu
10. ZS 3548 Class B Kadiated Emissions Radlated Emissions See Note 1 Passed 3 5 dB at 372 4 MHz Passed 2 dB at 214 4 MHz Line Voliage 120 VAC 60 Hz Line Voltage 120 VAC 60 Hz See Table A See Table Al Azimuth Angle see diagram below Azimuth Angle sce diagram below oy 270 Antenna Height 2 Meters Antenna Height 1 Meters Polarity Horizontal Polarity Vertical Notes n Final scan No modifications installed Azimuth Angle Diagram The ubove results pertain only fo the specific item submitted for testing identified by the product s model and serial numbers 37 7 Ayer Road Littteion MA 01460 Page 13 of 30 Tel 978 486 0432 Fax 978 486 0592 Email iniegrity idts com Apogee Technology Inc 6 Document 07010001 01 00 Repon 6920 eL 3 25 Results The EB 2060S met the FCC and EN55022 Class B conducted emissions requirements when tested as described below See Appendix A for a complete listing of data points Worst case emissions measured EN55022 Conducted Emissions Conducted Emissions Condueted Emissions See Note 1 Passed 6 6dB at Passed 8 2dB at Passed R 2dB at 0 7MEHz 0 15MBHz 0 15MHz Linc Voltage Line Voltage Line Voltage 120V AC 50H2 120V AC 60Hz 120VAC 60Hz Seo Table A2 See Table 43 See Tablo A4 Notes 1 Added missing 0 tuf rectifier suppression capacitor The above resulis pertain only to the specific ltem submitted far testing identified by rhe product s mod
11. al mode noise is conducted on a pair of signal lines or on a power line and ground in opposite directions to each other This is represented as normal current flow in a system and is shown in Figure 2 below Signal Source Noise Source Figure 2 Differential mode noise The Noise Source represents the high frequency current generated by the switching of digital circuits i e the emission generator and the Signal Source can represent either a power source or information signal source i e the desired signal The load can represent a digital IC load or other active device on a power supply or a circuit load like a loudspeaker depending on the function of the Signal Source Differential noise can be considered a conducted emission in that the unwanted signal is conducted between the Signal Source and the Load or between the Load and Signal Source depending on which is generating the noise It only becomes a conducted emission from a regulatory sense when the Signal Source is the AC Mains and the Load is the EUT with the Load generating the noise However it is possible for this conducted emission generated by a products internal circuitry to become a real in the regulatory sense radiated emission Remember that any changing current generates a magnetic field H field around the wire carrying it this H field can radiate directly or induce currents in nearby circuits that will carry the noise to other places in the product Layin
12. d Cstray Cstray Reference ground surface Figure 7 Common mode suppression with ferrites Apogee Technology Inc 10 Document 07010001 01 00 Another effective method of suppressing common mode noise particularly where cables enter or exit a product is to use capacitors to couple the common mode noise to the product chassis This is represented in Figure 8 The noise is then able to couple back to the source and remain inside the chassis A separate interface ground is typically used on a PCB to allow a single point connection to chassis or circuit ground of multiple bypass capacitors at an I O interface area It is also possible to combine choke and capacitor techniques for EMI filtering Signal Source Noise Source Cstray Cbypass Metallic chassis Cbypass Reference ground surface Figure 8 Common mode suppression with capacitors Finding the Noise Sources The time to look for noise sources is at the start of a design Identify those signals that can potentially cause problems with conducted or radiated emissions For a typical DDX amplifier these signals will be power supply distribution clocks the PWM output and drive circuits and to a lesser extent general logic signals Pay close attention to the power supply bypassing and the PCB and chassis layout keeping in mind how differential and common mode noise becomes a radiated or conducted emission Missing this step can result in t
13. el and xerial numbers 37 7 Ayer Road Littleian MA 01460 Page 16o0f30 Tei 978 486 0432 Fax 978 456 0592 Email integrity aidty com Apogee Technology Inc 7 Document 07010001 01 00 Report 66920 e1 Configuration Photograph Apogee Technology Inc EB 2060S Worst Case Radiated Emissions Test Configuration 37 7 Ayer Road Littleton MA 01460 Page 28 of 30 Tel 978 486 0432 Fax 978 486 0592 Email integrity idts com Apogee Technology Inc 8 Document 07010001 01 00 Report 66920 e1 Configuration Photograph Apogee Technology Inc EB 2060S fj j f iii HN Uil Ill lil Worst Case Conducted Emissions Test Configuration 37 7 Ayer Road Littleton MA 01460 Page 30 of 30 Tel 978 486 0432 Fax 978 486 0592 Email integrity idts com Apogee Technology Inc 9 Document 07010001 01 00 Appendix C Checklist of Do s and Don ts Before beginning the design Understand the requirements of the countries your product will be sold in Be familiar with the operation of DDX amplifiers and the spectrum they generate the PWM frequency and harmonics dominate conducted emissions and the master clock harmonics dominate radiated emissions Be familiar with other digital components in your product particularly switch mode power supplies Understand the concepts of Differential and Common Mode Noise Understand the techniques and components used to suppress emissions Make sure you will
14. es ee eR ud ee REE orn aL DER ene ER PER Tene Lut E UN tem c tead i Apogee Technology Inc 1 Document 07010001 01 00 Abstract Contrary to popular belief meeting EMI regulations with digital PWM switching amplifiers is not black art Switch mode power supplies have been doing it for years What is required is an understanding of the sources of noise and the hidden circuit paths that allow it to escape to the environment In most cases it is not the actual PWM output that is the culprit but rather things that are already familiar to engineers designing consumer digital audio equipment power supply currents and the clock signals used in all digital circuits This application note discusses signals that create emissions from a DDX amplifier circuit and effective methods of identifying and suppressing these emissions Using the guidelines presented in this application note an EB 2060S evaluation board was modified and demonstrated to pass Class B emissions requirements Apogee Technology Inc 2 Document 07010001 01 00 Introduction When designing products utilizing digital signal processing or digital signals in general audio or otherwise attention must be paid to meeting Electromagnetic Interference EMI regulatory requirements for different types of equipment The importance of meeting these standards is obvious in countries where EMI regulations are in force using or selling a non compliant product is not permitted In this applicatio
15. g out circuit boards such that the physical conductors carrying the noise current in opposite directions are close together can minimize this Each conductor will generate an equal but opposite H field resulting in a net H field of zero This especially applies to the power bus structure supplying the circuit s active devices and to the switching output of the amplifier power stage where the noise currents are actually required for proper operation of the circuit These traces should be kept as short as practical and their high frequency impedance minimized as any differential voltage will also generate an electric field E field between points in the circuit Again the E field can either radiate directly or couple to other circuits Design Considerations Consider the case of the power supply to the amplifier power output stage The power output stage draws large peak current during the switch transitions resulting in a very high change in current during a very short time This current if allowed to circulate as shown in Figure 2 will result in a large radiated signal from both E and H fields fortunately good layout and circuit bypassing are effective at reducing the emissions from this necessary Apogee Technology Inc 7 Document 07010001 01 00 current flow High frequency bypass capacitors such as ceramic or tantalum dielectric should be placed as close to the power device as practical this will constrain the majority of high frequency c
16. he need for excessive suppression components and methods or in the worst case having to re spin the design The method of finding the residual noise sources is straightforward after examining the design and layout there should be some expectation of what to look for and where The product must be configured as the final consumer will use it that is as generally hooked up and operating in a typical fashion Before going for verification test a pre scan should be performed to identify emissions and steps taken to determine where and how to suppress them most effectively Some of the steps can be implemented directly or planned to be implemented at the test site if required which to do becomes a matter of experience In any event you should arrive at the test site with a plan based on information from the pre scan The scan needs to be performed with sufficient resolution to determine a signal from a group of signals for a DDX amplifier a detection bandwidth of 120 KHz is generally adequate for radiated pre scans When doing a pre scan outside of a screen room the background level being picked up for each measurement should be determined with the EUT switched off Apogee Technology Inc LE Document 07010001 01 00 Using the spectrum analyzer and the inductive current clamp determine the signal components that are being conducted out of the EUT on its cables both for common mode and differential signals record frequency and amplitude for each sig
17. ion During testing the EUT was connected to a portable CD by optical cable to SPDIF input Test CD supplies 14 dBFS digitally generated pink noise to both channels of BUT The linear power supply provides 28 VDC under load to EUT 30V with no load The EUT provides 12dB of gain to thc signal to simulate music played with full power Crutpuls are loaded each with 8 37 7 Ayer Road Littietan MA 01460 Page Tof30 Tel 078 486 0432 Fax 978 486 0592 Email integrity idts com Apogee Technology Inc 4 Document 07010001 01 00 Report 2 66920 21 2412 Support Equipment Leod optica SPDIF cable Linear Power Supply Apogee Nol ER Not open frame Technology 28V DC 811 50W Resistive loads Not Labeled Not Labeled Description Cabk Unahielded Shielded Shield Length Loopback pen Type Termination Meters Ended Connecied Bralded Foll 160 ibrale Optical SPDIF Unshielded LE A Bi BE Power IGAWG Unshielded NA os Bi 2 Speaker 16A WG Unshielded WUA X 37 7 Ayer Road Littleton MA 01460 Page Bof30 Ted 978 486 0432 Fux 978 486 0592 Email integrity idts com Apogee Technology Inc 5 Document 07010001 01 00 Report 66920 el 3 1 5 Results The EB 2060S met the FCC and EN55022 Class B radiated emissions requirements when tested as described below See Appendix A for a complete listing of data points Worst case emissions measured EN55022 VCCT Modifications FCC Chss B AS N
18. ions at 10 meters A simple open frame linear power supply was constructed and used and the 5 volt switching regulator supplied on the stock EB 2060S evaluation boards was replaced with a linear regulator This was to insure that switch mode regulator and power supply emissions did not obscure EMI issues related to DDX amplifier components The optical input was used also to not obscure EMI issues The input signal was 14 dBFS pink noise with 12 dB of gain applied to replicate a typical full power music output at approx 9Wrms Ch No modifications were made during the radiated test but pre verification testing the board was modified based on the pre scan activity as follows Filtering components added to inputs or outputs Speaker leads Modified to include common mode beads Steward P N CM3322X630R 00 between the filter output and speaker 8 ohm loads These were mounted on the PCB and filter a common mode current from the speaker leads Power leads from EB 2060S to open frame linear supply A Fair Rite 2673002402 shield bead with 6 turns of 24 wire wound common mode on the power input wires this was to address low frequency common mode conducted emissions that radiated off the DC power lines to the power supply it is unlikely this bead would be required if the power supply and amplifier share the same PCB A Fair Rite 2643002402 shield bead with 3 turns of 24 wire wound common mode was installed between the 2200uf bypass ca
19. n note major regulatory requirements are reviewed as are the types of noise that cause problems Methods of identifying and suppressing emissions will be discussed and a modified DDX evaluation board is shown to pass Class B requirements without needing to be enclosed in a chassis Background EMI Regulations and Measurements DDX amplifiers are for the purpose of EMI regulations considered Information Technology Equipment ITE as they receive process and use digital data The major standards are FCC Part 15 subpart B in the US EN55022 in Europe and VCCI in Japan At the time this application note was written meeting the most stringent requirements of these standards covered the requirements of most countries however the manufacturer is responsible to meet the requirements of any country that your product will be sold or used in at the time your product is manufactured Always check for the most current standards as regulations are subject to change This application note does not cover the methods or requirements of setting up a test facility to guarantee measurements can be made to the accuracy required for product approval but is intended to show methods of discovering the areas that cause emissions and address typical solutions when developing products with DDX technology For product approval measurement of the emissions should be made at a facility that is certified to be in compliance with the criteria established by the relevant
20. ne its relative effectiveness If no change is apparent then the source was not properly identified If the emission is eliminated or significantly reduced then it was the only or main source In addition to the frequency information from the spectrum analyzer a time domain waveform of the radiated signal should be viewed either by demodulating the signal with the analyzer or by hooking the probe to an oscilloscope to gain more insight into the offending waveform More extensive information on determining the sources and types of emissions can generally be found in the near field probe manufacturers users manual Conclusion In summary during the product design phase minimize the problems you know about Pre scan in the lab to understand what remains and determine methods to suppress it Go to the verification test site prepared to implement several techniques as schedule and budget requirements generally have a role in determining the best solution Remember that the test conditions of pre scan and verification are usually not the same frequencies that had not been identified as problems can appear and vice versa Be ready to identify and suppress new emission sources at the verification site If the attempt to reduce an emission fails it is due to one of four reasons 1 The diagnosis of the source is wrong 2 The suppression technique is not correct for the identified source 3 The suppression technique is not properly applied 4
21. nificant signal Use the current clamp to determine the common mode signal on the AC power cord and the LISN to determine what is conducted on each wire Suppression of line conducted emissions will likely use a line choke in combination with a capacitive bypass to chassis filtering due to the lower frequencies provisions for this should be made at the design stage Use knowledge of the system design to determine if the frequencies are fundamentals of the system or harmonics Do not be surprised to see very high order harmonics the small geometries on PCB s do not make efficient antennas at low frequencies Next take the most sensitive near field loop probe H field and bring it near enough to the EUT to start seeing a signal as an emission can be radiated in any direction the probe should scan a spherical radius about the EUT Identify these far field signals first as they are more likely to cause compliance issues The ones closer to the EUT are likely related and will be suppressed along with them Find the strongest signal and note the distance and direction from the EUT as well as the frequency and amplitude Switch to progressively less sensitive probes and follow the line of maximum emission to identify where it is coming from on the EUT Trace the emission to its source inside the unit using the smallest loop or small E field probe and determine which method of suppression can be applied Apply the suppression and repeat the measurement to determi
22. o 46 dBuV 0 5 to 5 MHz 56 dBuV 46 dBuV 5 to 30 MHz 60 dBuV 50 dBuV Note that the stricter limit applies at the frequency transition points Table 2 FCC Class B Conducted Emission Limit Frequency Quasi Peak Limit 0 45 to 30 MHz 48 dBuV A comparison shows that the FCC limits for conducted emissions are stricter than EN55022 limits between 450KHz and 30MHz but that EN55022 extends down to 150KHz where the FCC has no specification When designing for conducted emission the goal should be to meet both resulting in the following Table 3 Design Specification for Conducted Emissions Limit Frequency Quasi Peak Limit Average Limit 0 15 to 0 45 MHz 66 dBLV to 48 dBuV 56 dB V to 38 dBuV 0 45 to 30MHz 48 dBuV 38 dBuV Apogee Technology Inc Document 07010001 01 00 Average measurement is the typical mean value of a waveform whereas quasi peak better represents the envelope of a waveform and detects short duration higher energy signals better than the average measurement The spectrum analyzer chosen for development work ideally should be capable of both but in most cases an average responding instrument will be sufficient Bandwidth up to 1GHz is desirable as it will cover the radiated emissions range for DDX amplifiers as well A third item an inductive current clamp is required for development work It is useful for identifying the frequencies of currents being
23. output filter CM Ferrite C7 10nF OJMP or Cap d Circuit Ground Interface Ground Apogee Technology Inc LE Document 07010001 01 00 Typically if the application has a chassis and the power source is in the chassis the DC Power input filter is not likely required Its purpose is to prevent coupled noise from radiating off a connecting cable to the power supply It that case a similar filter will likely be used on the AC power line for conducted and radiated emissions The frequency range of most common mode ferrites starts to become significant in the range of 10MHz up below this frequency wound choke coils tend to be more effective Apogee Technology Inc 12 Document 07010001 01 00 References EMC Test Systems 1999 User s Manual Near Field Probe Set Model 7405 http www fair rite com online product catalog Choosing Ferrite Materials http www murata com online product catalog http www steward com Application notes Use of Ferrites and EMI Testing Apogee Technology Inc i Document 07010001 01 00
24. pacitor and the power input connector for the high frequency emissions Note that these beads appear in series and each covers a different frequency band Using type 31 material from Fair rite would accomplish the function of both with a single bead but was not available at the time Also note that the Steward P N CM3322X630R 00 is effectively the same as a Fair Rite 2643002402 shield bead with 3 turns of 424 wire but is easier to use as it is a surface mount component More information on these beads and ferrite material characteristics can be found at the respective manufacturers websites www steward com and www fair rite com On the evaluation board connectors J6 and J7 are expansion headers to allow use of other IIS sources for development purposes in a real application they would not be present All traces from these headers were cut at the signal source on the PCB and grounded to prevent them from acting as antennas for the 256 fs clock recovered from the S PDIF receiver Apogee Technology Inc 1 Document 07010001 01 00 A ferrite bead Fair rite 2508051217Z0 was added to the 256 fs MCLK output of the S PDIF receiver to damp a differential mode emission that coupled common mode to the power supply and speaker leads The emission is due to the MCLK line crossing several ground plane voids During the conducted test a 0 luf capacitor was added across the rectifier bridge transformer windings to tame the diode commutation spikes This is a
25. regulations and agencies Certification is generally an indication of traceable calibration and measurement repeatability to recognized standards Repeatability of measurements is key to demonstrating a product meets standards What exactly is measured Two types of emissions are of concern Conducted Emissions is the radio frequency energy that a product transmits along its power cord to the AC Mains also DC Mains in some countries and Radiated Emissions is the radio frequency energy transmitted by the product into the air Conducted Emissions and Limits Conducted emissions are measured as a voltage expressed in dBUV using a line impedance stabilization network LISN and a measurement receiver typically a spectrum analyzer connected to the LISN s output port A typical LISN schematic is shown in Figure 1 Its function is to supply a stable 50 ohm load impedance for the noise signals on the equipment under test EUT power cord A commercially available LISN and an AC line conditioner are recommended for development work Apogee Technology Inc 3 Document 07010001 01 00 AC Power In EUT AC Power Cord To Receiver Ground i AC Power In Neutral AC LISN Figure 1 Conducted emission limits for the US and Europe are given in the tables below Table 1 EN55022 Class B Conducted Emissions Limit Frequency Quasi Peak Limit Average Limit 0 15 to 0 5 MHz 66 dBuV to 56 dBuV 56 dBuV t
26. se suppression Common Mode Noise Common mode noise is conducted generally on a pair or group of signal lines or on power lines in the same direction This is represented in Figure 6 Signal Source Noise Source Cstray Cstray lt y Reference ground surface Figure 6 Common mode noise Apogee Technology Inc 9 Document 07010001 01 00 As shown in Figure 6 common mode noise is similar to differential mode noise in that current flows in a loop This loop is again the emissions radiator What is different is that the return conductor the reference ground surface is not always as easy to clearly identify For most electronic products the metal chassis becomes one fortunately it is also a very good shield for most internally generated emissions An example of this would be a digital clock line that goes to an expansion header but is unconnected to other circuitry The return path for this clock signal is now through stray capacitance to the chassis back to its source Signals such as these can radiate internally and appear at multiple places in the product thus the importance of controlling EMI at its source one suppressor is better than many Design Considerations More common mode emission problems tend to appear at the interfaces of products where cabling enters or leaves the chassis Generally coupling is due to pickup of internal emissions or from board conductors to signal paths that lead outside the chassis These signals
27. typical linear power supply that one would use in a stereo powered speaker application The transformer winding was rated at 53VA and supplied 28VDC to the evaluation board during operation Full bridge rectification was used and filtered at the power supply with a 4700uf cap The transformer core and mounting were connected to earth ground by the third prong of the power cord The circuit ground was connected to the earth ground by a 2200pf coupling capacitor During the pre scan the conducted emissions were found to be due to primarily common mode conduction of the output PWM frequency and required the installation of a common mode choke The power line filter use was a Murata PLY10A2821R2R03BI combination differential common mode line choke Specifications are available at www murata com Apogee Technology Inc 2 Document 07010001 01 00 Appendix B Verification Test Report Excerpts from the EB 2060S evaluation board verification test report by permission of Integrity Design and Test Services Inc SARGENT m An Entela Inc Company Certificate of Compliance The following product was found to comply with the requirement stated below when tested in accordance with the test procedures described in the accompanying test measurement report Reference report number 66920 e1 Manufacturer Apogee Technology Inc 129 Morgan Drive Norwood MA 02062 Product Name DDX Stereo Evaluation Board Requirement FCC Part 15 EN55022 1998 VC
28. ue Near field probe kits are available commercially and are offered as both active and passive devices again 50 ohm termination will be required to match the spectrum analyzer input impedance Near field probes are usually constructed to detect either magnetic fields or electric fields while rejecting the other Magnetic probes are essentially loop antennas made from a single turn of semi rigid coax cable and electric field probes are made from a stub of semi rigid coax cable It is possible to construct near field probes but the commercial variety typically are more durable and come with sensitivity curves similar to antenna correction factors These probes are connected to the spectrum analyzer and brought into close proximity to the circuit in order to determine both the frequency content and type of emission being radiated Sources of Emissions Noise and General Suppression Methods As previously discussed there are two types of emissions that are of concern conducted and radiated What is most important in suppressing these emissions is to understand how they are generated and to identify the source Suppression of EMI is most effective when applied at the source A good board layout is the first step to minimizing emissions Noise can be classified into two types according to the mode of conduction Differential Mode Noise and Common Mode Noise Apogee Technology Inc 6 Document 07010001 01 00 Differential Mode Noise Differenti
29. urrent flow to the immediate area of the bypass capacitors and the power output stage as shown in Figure 3 note that these are in addition to the large bypass capacitors required for the lower audio frequencies LOAD Power IC Signal Source Power Supply Noise Source Figure 3 Load Bypassing Power supply lines should be laid out so that supply and return lines run together as previously mentioned These two items properly applied to all active devices will significantly reduce the area this noise signal travels around and subsequently the associated emissions A ground plane is strongly recommended for DDX amplifier applications for both EMI and Thermal considerations When using a ground plane in high frequency applications care must be taken to understand the impedance and current flow in the actual circuit path As an example Figure 4 shows a ground plane with two signal trace routings Trace A is the most direct path to the load from the perspective of the source but passes over a void in the ground plane This forces the return current to flow around the void to follow the path of least resistance back to the source At DC this would not be an issue however at high frequencies a radiating loop antenna is formed and circuit inductance is increased Trace B offers the lowest impedance at high frequency as the conductors run together minimizing circuit inductance It also produces the desired field cancellation
Download Pdf Manuals
Related Search