EMI/EMC COMPLIANCE TESTING
J. Bellott, MSEE
EMI/EMC compliance certification frequency ranges and power magnitudes are designed to ensure that two products can operate properly without affecting one another.
Design reviews are the least expensive moment to catch non-conformance to DFSS goals. Screening tests/mitigation are the second rung for catching EMI/EMC issues. Failures after delivery can be difficult or expensive to correct. The most stringent agency rules on compliance are related to your product’s effect on another company’s products. It is advantageous to prevent the reverse also.
Areas to consider during product design and design reviews:
Printed Circuit Boards and Backplanes
· Length of conductors – keep signals short to avoid radiation or susceptibility issues.
· Signal transition rates – edge dv/dt rates or di/dt rates. Control transition rates and terminate properly. Applies to voltage or current mode signals.
· Proper analog/digital isolation to prevent digital noise on quiet analog signals. One method is use of moats in layout. Moats can be used to separate analog and digital power/ground back to point near backplane ground and backplane power/on-board voltage regulators. Proper bypass filtering in analog and digital circuit areas.
· Bypass solution design for signal rates on circuit boards – Adequate well caps (typically electrolytic), and small caps near ground pins. Small cap impedance vs. frequency plots are available in data sheets. For high speed CMOS IC’s, often two caps are used (small and smallest in parallel) to provide bypass in appropriate frequency range.
· Type of signals implemented on PCB (internal between ground planes, outer surface microstrips. Microstrip impedance can be controlled to greater tolerance by board manufacturers and are used by designers for high speed transmission lines (single wire or differential mode). Fastest signals are often implemented with microstrip impedance as low as 50 ohms (a practical limit in PCB manufacture). Typical slow signals often have impedances as high as 75 ohms. (See manufacturer’s association websites, Design Resources Page, or contact your PCB manufacturer for details on specifying impedances and tolerances available from their factory.
· Placement of routing on proper layers, spacing between high speed nets. Use of automated cross talk analysis recommended to layout engineers.
· Avoidance of ground loops (In general, signal carrying conductors in free space that do not have a ground return or shield). See rules on “Design Resource Links” page.
· Earth ground connection, return to ground at single point. (In general: PCB ground is backplane ground; backplane ground and internal power supply transformer ground output is tied to equipment box and earth ground. AC ground input goes to transformer in this scenario and is not connected to third wire earth ground.)
· Ventilation coverings (hole size and depth can be selected to minimize RF in bands of interest).
· Use of balanced signals, differential signals for noise tolerance when appropriate
· Cable shielding around signal and return (one rail signals or differential pairs) connected to earth ground at equipment chassis for maximum external cable shielding.
· Mitigation including study of signals that have lengths impacted by compliance failures at specific frequencies.
· Follow rules regarding use of production releases of software during final certification testing.
· Maximize signal activity with software and firmware modes of operation during testing
Product EMI/EMC Test Plan - Example Outline by J. Bellott, MSEE
Table of Contents
3. Applicable Documents and References
4. Product Specifications and Test Criteria (Agencies, standards, frequencies, pass criteria charts/tables.)
5. Certification Application Status
6. Product Hardware, Firmware, and Software Versions Under Test
7. Test Configurations
8. Test Equipment to be Used
9. Test Labs - Info
a. Lab test equipment for each test (Cells, current probes, power line filters, ESD guns, etc.)
b. Lab’s Measurement Equipment Description for Agency/EMC Standards tests
c. Schedule (Setup, cell times, staff, removal of DUT and engineering dept. test equipment)
10. Product Compliance Test Procedures Summary
· Use representative modes of operation with test configuration representative of field use.
Use a substantial representation of operating modes to attempt to catch problems so they can be avoided in the field. This typically involves maximizing hardware operation and I/O plus representative and common modes of software operation while this is accomplished. Any software modes of operation that involve special amounts of traffic or occur at unusual rates might be tried as additional good test cases.
· Up to three phases of testing:
a. Screening Test Period
b. Official Tests - Certification (Production version)
c. Field testing (if appropriate)
11. Product Radiated Emissions Testing
Test procedures (determine/use appropriate orientations, use appropriate equipment modes of operation, use proper antennas and distances for cell/frequency range (lab equipment and procedures), save final run charts.
12. Product Radiated Susceptibility Testing
Test procedures (equipment orientations and modes of operation, means of identifying level when induced failure occurs e.g. timestamps, single step frequencies/ramp levels). One goal of testing is to verify that induced failures only occur when the input power levels are above the “pass” criteria. Knowing the input frequency and power levels when failures occur helps verify “pass” status or points to areas that may need design mitigation steps (e.g. shielding, long wires that act as antennas, etc.). Mitigation helps get the product through certification.
13. Product Conducted Emissions Tests
Test procedures (Install measurement probes on cables, DUT modes of operation, measure coupled emissions as a function of frequency.) Mitigation involves techniques including band limiting of output signals with chokes, corrections to internal grounding scheme, use of additional shielding in cables. Mitigation helps get product through certification.
14. Product Conducted Susceptibility Tests - Each Product Signal I/O Jack
Test Procedures (Install, calibrate injected signal level, run equipment test modes of operation, identify induced failure info/product tolerance to EMI through I/O wires.)
15. Product Conducted Susceptibility Tests - Power and Ground Wires
16. ESD Tests
16. Product EMI/EMC Test Result Logs
Here is a description of some important areas of consideration for EMI/EMC Agency Compliance Testing work to obtain certification:
Official certification testing is only valid with production versions of products. Exercise products in the correct modes and spatial orientations using production hardware, software, firmware, and equipment housings as required by the certification agency for the approvals sought.
When testing for susceptibility, test fails when a fault occurs and normal operation ceases. This needs to be visible outside of the test chamber. Consider using software with timestamps inserted during the frequency sweep so that the time of failure can be noted to correlate to what the RF generator in the test cell was doing at that time in terms of frequency and energy output at the moment of failure. Knowing the failure frequency and by calculating the wavelength of that frequency, one can then use PCB layout tools to locate all traces that would be excited as receiving antennas.
Digital products (except for their switching power supplies and analog I/O) that may have high frequency signals that may radiate or be susceptible to interference. Often radiated signal failures occur at the fundamental clock frequency of one of the digital boards in a system. This can be fixed with re-layout, improved grounding, or increased shielding.
The test measurement magnitudes and frequency bands are specified by the FCC (or international standards agency or product buyer).
A good reference for FCC compliance is the FCC Title 47 rules and regulations on the FCC website.
The letters "CE" are the abbreviation of French phrase "Conformité Européene" which literally means "European Conformity". The term initially used was "EC Mark" and it was officially replaced by "CE Marking" in 1993. "CE Marking" is now used in all EU official documents.
Obtaining CE Certification