Hospitals are known for being safe and orderly, but this isn't always the case when it comes to their radio frequency (RF) systems. Unfortunately, stray signals can cause problems for many devices such as MRIs, telemetry devices, and ultrasound instruments.

A good understanding of the environment in which a medical wireless network works helps those installing the network as well as technicians responsible for its operation. And an understanding of common interference sources helps engineers design better wireless components. Certain patient telemetry systems show how easily problems can arise if such designs are not well thought-out.

For example, equipment in the 608 to 614 MHz WMTS band and in the 2.4 GHz band in Europe typically has a distributed antenna system installed along halls of areas where patients are treated. Generally, many antennas feed a single monitoring receiver. This configuration lets patients be monitored anywhere within the area. The system works with a single receiver because a patient's telemetry signal can be picked up by any of the antennas. However, a single interference source anywhere in the monitored area can obstruct all telemetry signals, even when the patient is nowhere near the antenna receiving the interference.

Because these systems operate in the same frequency range as microwave ovens and many cordless phones, WiFi interference is a real problem. In fact, cheap microwave ovens have been known to emit broadband interference in the medical telemetry band (608 to 614 MHz). Imagine the problems that could arise as a result of a microwave oven being placed in a break room just inches from a telemetry antenna on the other side of the wall. In this case, it is the equipment that interferes with a wireless network.

A wireless network can also interfere with equipment. Interference frequently comes from power supplies with switching frequencies in the same range that ultrasound equipment uses for imaging. Ultrasound and MRI systems subject to these stray signals usually have artifacts on their displays that make image interpretation difficult.

Receiver intermodulation

It is important to note that any signal exceeding a telemetry receiver's input power specification, regardless of frequency, can cause intermodulation or signal distortion in the receivers and generate unwanted signals on output. Exceedingly large signals can even cause a receiver to desensitize. This might happen when, for instance, a telemetry receiver has a maximum input-power specification of -20 dBm, which is the same power level as the leakage from a microwave oven.

Receiver specifications differ, but all receivers have a limit on input power. Therefore, it's important to survey the entire frequency spectrum for signals that might exceed this specification for troubleshooting and pre-installation site surveys. Biomed technicians can easily get confused when not finding interference signals in a particular frequency band of interest when, in reality, the receiver itself is generating interference because of a high-powered signal elsewhere in the frequency spectrum.

A common belief is that cell phones are a major source of interference in hospitals. In general, this is not true. Frequencies used by cell phones are well separated from those used by patient-monitoring systems. The only exception might come from the rare occasion when an active cell phone is within a few inches of a medical device and the output power of the phone causes the device to overload.

Causes of RF interference

Most RF interference is caused by unintended emitters — devices that shouldn't be radiating signals at all. Common among these are fax machines, elevator motors, floor buffers, treadmills, and light dimmers. When broadband intermittent interference affects a wireless network, equipment with motors that operate occasionally is a likely suspect. That's because worn brushes in motors or cheap motors can spark and thereby generate noise. Interference can come from the control circuitry of an elevator that employs a brushless motor as well. This interference might happen only during times of high usage.

Telemetry interferences also come from HVAC systems. Noise can result from the arcing of old variable-control switches, such as those that control individual HVAC fans in patient rooms. Sometimes these switches are not rated for the voltage they control. Over time, the switches corrode, which increases the noise amplitude. This noise can be difficult to locate because it is dependent on which switch was arcing and which fan was being adjusted at the time.

Tracking interference sources

Finding interference sources is more art than science. A good procedure is to first determine general attributes of the interferer. These might include time of day and whether the signal is broadband noise or discrete. Analyzing signals is easiest when done with a handheld spectrum analyzer.

Analyzers that automatically save a sweep are helpful in the tracking of intermittent signals. Usually either the technician or the device itself can create a limit line that defines the maximum value of acceptable signals. The line can be straight, or complex to avoid known signals.

Because any antenna in a telemetry network can pick up interference, the next step is to find out which one is receiving the unwanted signals. A spectrum analyzer with save-on-event capability makes this effort simple. Basically, technicians tap into the antenna network with a power splitter and assess half the antenna system. The instrument records any interference. If this area has no interference, the technician moves the tap to the other half of the antenna system and tries again. It doesn't take many half-splits to find the culprit antenna. A spectrum analyzer equipped with an omni-directional antenna can easily find such signals.

Pre-installation checks

Newer ITS telemetry systems operating in the 1.4 GHz frequency band cannot be evaluated with this procedure because they do not use a distributed RF network. But spectrum analysis is still the most accurate way to identify stray signals. The most efficient way to prevent problems here is to survey the environment before installation. This is typically done with handheld spectrum analyzers such as the Anritsu MS2721B Spectrum Master.

Some handheld spectrum analyzers can even be used on the production floor. The use of the same test equipment on the floor and in the healthcare environment allows for more repeatable measurement and fosters a greater confidence in network performance.