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EMC Testing in Medical Device Development

Wed, 11/20/2013 - 12:11pm
Bjarne Hansen, Electrical Engineer, StarFish Medical

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Medical devices containing electronics require Electromagnetic Compatibility (EMC) testing to ensure they can be used in the intended environment without failing or causing other equipment to fail.

My blog Electromagnetic Compatibility (EMC) challenges in medical device design lists roadblocks that delay medical product launches and strategies for avoiding these delays. This presentation focuses on product development pre-testing using inexpensive pre-compliance screening gear to improve and simplify EMC compliance.

EMC testing addresses two categories of interference:

  • Emissions testing measures Electromagnetic (EM) interference radiated or conducted out of the device. Emissions from your device can cause malfunctions in nearby equipment.
  • Susceptibility testing measures the device’s immunity to external EM interference conducted or radiated into the device. An example of external interference is Electrostatic Discharge (ESD).

Emissions testing confirms that the device is unlikely to interfere with other devices, while Susceptibility testing confirms that the device will keep operating despite outside interference.

EMC test methods for medical devices are essentially identical to those used in consumer electronics. Performance requirements however may be more stringent, reflecting the more serious consequences of product failure. The FDA has several good discussions about concerns particular to medical devices; Medical Devices and EMI: The FDA Perspective and Radio Frequency Wireless Technology in Medical Devices - Guidance for Industry and Food and Drug Administration Staff. The latter says:

“…When considering commercial off-the-shelf RF wireless components or systems that conform to industry standards (such as IEEE 802.11 standards), medical device manufacturers should take into account that some equipment might not have been adequately tested or qualified to address the needs and risks for use in medical devices. This is because such equipment may conform to standards that are not written specifically for medical devices...”

Determining the level of EMC requirements that apply usually occurs while preparing the risk management file for the device. Consider the types of components that make up the device, the type of environment and jurisdictions the device will be used in, and the risks associated with its operation. Including input from an EMC test lab (which may not have a lot of medical-specific experience), as well as a product designer familiar with medical devices, will ensure that the risk assessment and usage environments are properly accounted for and that the planned tests are complete and efficient.

Full EMC testing to IEC 61000 and CISPR 11 standards is complex and expensive; expect to spend several days and at least $5000 for an uncomplicated device – more for a complex one. These fees pay for access to calibrated equipment and an accredited or certified lab. Many lab personnel also provide helpful advice when technical difficulties arise during testing.

Due to the specialized nature of EMC testing, you may not find a local facility that performs it, and test sessions may have to be booked weeks in advance. It is also common for a new product to fail the first time through testing; retesting will incur additional costs and delays. It is in your best interest to be fairly certain the device will pass the first time submitted.

How can you maximize the likelihood of passing? Knowledge and past experience with similar devices help a lot. Prior in-house testing and pre-screening at one’s own facilities help even more. In a few hours the likelihood of achieving EMC compliance can be assessed, and device modifications (which are common during initial development phases) can be quickly evaluated.

Some form of EMC testing gear is needed for in-house prescreening, though it need not be as expensive as that used by a certified lab. For example, Hameg’s EMC-PCS3 instrument kit provides basic conducted and radiated emissions testing ability for about $7000. An ESD gun for checking susceptibility to static discharge is useful and costs $1500 and up. After saving one or two visits to the EMC lab, this gear will have paid for itself.

Here are some examples of simple and inexpensive tools an engineer can build for quick EMC prescreening:

Magnetic Field Probe
A near-field magnetic probe acts like a transformer’s secondary winding, picking up signals from alternating currents flowing in nearby conductors. Moving a probe about the surface of the PCB and experimenting with different loop orientations can locate where interfering signals are emanating. Magnetic probes can be constructed with different sizes of loops and numbers of turns to get the desired sensitivity, frequency response, and spatial resolution.

Douglas C. Smith’s paper Signal and Noise Measurement Techniques Using Magnetic Field Probes (PDF) discusses loop construction in detail, as well as how to use them to assess crosstalk, current flow in planes, and noise sources.

Electric Field Probe
This type of probe is more sensitive to electric fields (rather than the magnetic fields of the previous probe) and will sniff out common mode voltages in a circuit, cable or chassis. K. Armstrong and T. Williams provide construction details in an excellent paper EMC testing Part 1 – Radiated emissions.

Conducted Emissions Probes
Using readily-available ferrites, one can quickly make probes that clamp on to a cable being tested, and will give you the ability to assess conducted emissions.

Used in conjunction, these three types of probe will help you localize problematic emissions. Hooked up to a signal generator, they can also be used to inject signals into a circuit to test immunity.

Measuring and Display System
Any of these probes can be connected directly to an oscilloscope to view the signals they are picking up. If the scope supports it, a spectral view (as opposed to time-domain view) of the signal is often quite useful. One can make changes to a circuit and quickly assess the effect on amplitudes and frequencies of emissions.

Unless one has a higher-end scope, the spectral view capability may be quite limited in update rate, bandwidth or resolution. An alternative to an oscilloscope is to use a radio frequency receiver together with software running on a PC. One interesting system is built around the RTL2832 tuners commonly used for DVB-T digital television, coupled with software like GNURadio or SDR# (SDR Sharp).

For under $50, one can build a shielded radio receiver that works with the probes described above, and connects to a PC via USB.

Open-source or free software can be used to control the frequency of the receiver and display the resulting signals. Sensitivity and resolution is sufficient to detect potentially problematic frequencies and perform comparisons between equipment configurations. After gaining some experience testing devices with known EMC performance (from formal testing at a lab), one can judge whether a new device is likely to pass its EMC testing.

I hope that I have piqued your interest in EMC testing, by showing you a few ways in which relatively inexpensive equipment can be assembled to help with EMC prescreening. If you have any thoughts or comments to add, or want more details on how these tools might be used, I’d love to hear from you.

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