Eliminating broadband emissions interference

Electromagnetic Interference (EMI) appears in many different forms. Most prominent are the intentional radiators, such as cell phones and other communication devices. Another form of interference is conducted EMI, which is a subject for another time. What is not widely talked about, or widely understood, is EMI radiated from incidental sources: devices and/or systems that are not designed to intentionally transmit.

This form of interference, also known as broadband emissions or “noise,” can be generated from any electronic or electrical device and/or system. It manifests itself through a very rapid, very steep change in voltage, pulse, or burst. This could be from components that normally operate in this manner, or from defective components.

Broadband noise can radiate from heating, ventilation, and air conditioning (HVAC) control units; lighting ballasts; elevator motors; time-clock components; fans; dimmer switches; shredders; hair dryers; automatic door openers; portable equipment, such as floor buffers; and any number of mobile devices brought in by patients. Obviously, the list is far too long and ominous to believe that every one of these suspected devices can be completely managed or controlled. However, making an effort to do so will help eliminate the majority of potential incidental sources and should be part of an overall electromagnetic compatibility (EMC)-management program in a facility.

Interference issues must be dealt with from all angles. Manufacturers must strive to fully understand the end-user’s environment and design products that are compatible within that environment. Standards and regulatory bodies must research, develop, and improve test standards to encompass the ever-growing wireless-technology race. And end-users must manage the use of their devices in their own environment by understanding, tracking, and managing the frequencies being used. Not all interference issues can be predicted or prevented. Only by managing this problem from all angles can interference incidents be reduced to tolerable levels.

Telemetry and Broadband Emmissions
Consider how broadband emissions affect a telemetry system. When an interfering signal exceeds the amplitude of a transmitted signal from the patient, even if only momentary, it can cause a loss of transmitted data. Telemetry antenna networks are usually combined in a final network, preceding the receivers, and there are too many unknowns to predict how far any given incidental emitter will radiate. However, if broadband emissions are received by just one telemetry antenna, the entire system is vulnerable.

When signal to noise (SNR) is compromised, the signal loses integrity. In other words, if a signal is maintained 16 dB (manufacturing specification) above the noise floor, it retains integrity and transmits the correct data. If it drops below that specification, the transmitted data from the patient can become corrupted. In addition, if a signal maintains this SNR, and interference occurs at a level that exceeds it, the channel may drop below the specified SNR and the transmitted data can become corrupted.

The fading factor of a telemetry signal (how much the signal is allowed to fluctuate) must not encroach on the 16 dB SNR. If the fading factor is 30 dB, the minimum peak must not drop below 46 dB above the noise floor. Since telemetry signals are transmitted from the patient, whose body contributes to the transmitting antenna, patient movement affects the signal stability being transmitted. RF reflectors in the environment also contribute to signal stability. Consistent and correct electrode application, routine battery replacement, and allowances for unusual radio-frequency (RF) reflectors in the environment all contribute to the integrity of telemetry signals. Failure to maintain signal strength by ignoring these factors will compromise performance.

Often, telemetry-system integrity is compromised by maintenance issues, such as cable or component damage by workmen installing new systems or performing maintenance. When an entire string of antennas is inadver-tently disconnected, the transmitters will be received by the nearest working antenna. If low-level noise is introduced into the environment, interference may result with the low signals transmitted near these disconnected antennas. Biomedical personnel responsible for the operation of telemetry systems should be notified when maintenance is to be performed around system components, and integrity should be reverified afterward.

Hospitals pay millions for a telemetry system but fail to protect it in the same way a smaller, less expensive device is protected from damage. Allowing a loss of transmit or receive integrity is a sure opening for broadband noise interference. Protecting the integrity of your wireless systems will ensure that a manufacturer’s design performs as intended.

Consider the case of a hospital that installed new control components for the HVAC system on a floor adjacent to a critical care unit. One of these components generated broadband noise, which radiated into the telemetry system and interfered with it intermittently for more than a year (SA2). The manufacturer of the controller, it was discovered, knew about the emissions and advised an adjustment to correct the problem. The long months of interference, frustration, and high tempers could have been avoided by two simple steps: asking the manufacturer about emissions from any purchased device or system (medical or nonmedical); and communicating the plan to purchase and install the equipment, to adjacent departments in most cases, and to the biomedical group without fail. Often, just the recorded date of a new installation will coincide with the time line that another system began malfunctioning.

Another case involved occasional interference in only one patient wing. While the broadband noise found in the environment was not very severe, it had an effect on almost all transmitters in one hallway, which appeared very weak. Upon investigation, that entire string of antennas was not functional in that hallway. Sometime previously—perhaps months before—during ceiling work, a cable had been pulled too hard and ripped completely from its connector. It went unnoticed because all the transmitters in that hallway were being received (though weakly) by the closest working antenna string. Only when a broadband-noise source was introduced did those transmitters begin having long-term intermittent problems. The damage was eventually discovered by the manufacturer’s service engineer. This is a common occurrence. Though the noise source was discovered and eliminated, the telemetry system could easily withstand the level of noise had it not been damaged.

Manage the RF Spectrum
Along with protecting the integrity of your wireless systems, the entire RF spectrum should be managed by documenting each and every frequency in use, where it is used, and who is responsible for it. The purchase and installation of any new or upgraded equipment, even if it is not medical equipment, should be communicated to all departments.

Using a spectrum analyzer capable of high sensitivity, a fast sweep speed, and resolution bandwidth, scan your environment for an initial benchmark of the entire spectrum. Then, repeat the process in selected frequency bands whenever something changes or new equipment is installed. Document the vulnerable frequencies of your critical devices and/or systems, and prevent the occupation of interfering sources within those vulnerable bands. Always scan for broadband noise as well as intentional transmitters.

Spectrum analyzers come in all shapes and sizes. A handheld analyzer provides mobile capability. Remote access allows it to be monitored from a desktop computer at your workbench. Most analyzer manufacturers provide classes regarding the use of their products. Analyzers do not have to be exceedingly expensive to cover the capabilities needed to detect broadband noise, but do not waste money on one that will not do what you need. The sensitivity needed to capture broadband noise in an analyzer is technology driven. You will not find a suitable analyzer for RF surveillance on the bargain rack, but the cost of an adequate one will be a fraction of what an EMC consultant will charge for just one interference investigation or hospital-wide survey.

Broadband noise can quickly spray through the spectrum. Some occurrences cannot be detected with a slow sweep speed, further creating the illusion that the spectrum is clean. Broadband noise can appear as tiny spikes of varying amplitude, or it may contain repetitive characteristics indicating a switching mechanism. However, no one occurrence is exactly like another. It may spray across the spectrum at high speed and disappear, or it may rise up for minutes at a time. It may be there all the time at low amplitudes, and grow stronger when a triggered event occurs, or it may occur only at strange times in the middle of the night.

When scanning for broadband noise, a narrow frequency span (that views fewer frequencies at a time) should be used to increase the analyzer’s sweep speed, 200-kHz frequency span or less, somewhere in the frequency band of interest.

Remote access allows another area in the hospital to be monitored while viewing the data on your desktop computer or making adjustments to the analyzer’s measurement setup based on that data. Taking care of routine preventive maintenance (PM) while monitoring for interference is efficient and cost-efficient.

When troubleshooting your environment for broadband noise, consider everything electrical or electronic near the point of entry into the system being affected. Turn devices and systems off, interrupt power, and systematically eliminate components until you know what it is. Do not rely on anecdotal information.

Survey the RF Environment
An RF environmental survey is an excellent tool for managing the RF environment. Whether performed by a consultant or by hospital personnel, every hospital should consider doing a full survey as a benchmark, and subsequent surveys before a new installation is installed or a major change occurs in the environment.

The cost of an interference investigation, after experiencing months of problems with your wireless system, can cripple a tight budget. Managing the environment yourself keeps your systems in prime condition, resulting in fewer complaints. It also negates the cost of hiring an EMC consultant to track down the interference for you.

Manage the maintenance of your equipment, medical or nonmedical. If you suspect that the equipment is generating RF noise—especially if you have spectrum analysis showing the emissions—ask the service personnel to investigate it. Ensure the integrity of networks by supervising the work done around them. Communicate between all departments. Remember, all equipment—not just medical—is subject to scrutiny. Assign the job of spectrum manager to an individual who will manage this information and know what frequencies are in use in the environment. Scan your environment regularly for possible broadband emissions, especially in areas where wireless systems are in use.

Cell phones can be removed from the environment or banned, even at the slight possibility of interference. How will you deal with a permanent HVAC system that interferes with one of your wireless systems? Or the elevator that emits noise every time it is used but cannot be shut down?

Be proactive about your RF environment, and prevent problems before they occur. 24x7

Dara McLain is the EMC Engineer for the hardware quality engineering group at Philips Medical Systems. She has 25 years of experience working with environmental electromagnetic interference (EMI).