anritsu-radiofrequencyinterferencehospitalsan.pdf

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Conducted Interference

Ultrasound equipment tends to be susceptible to interference conducted into the equipment over the power line in

the 2 MHz to 18 MHz range. Harmonics of the oscillators in many switching regulator power supplies can be in the

frequency range used by ultrasound equipment. You can recognize such interference by repetitive lines on the

instruments display. Most small power supplies employ switching regulators so if such interference is noted, a

hunt for such power supplies is in order.

Cell Phones

Cell phones operate on many different frequency bands. The ITU-R approved the following bands for cell phone

operation: 806 MHz to 960 MHz, 1710 MHz to 2025 MHz, 2110 MHz to 2200 MHz and 2500 to 2690 MHz.Not all countries follow this exact plan. In the United States, for example, these bands are in use.

Experienced interference hunters have noted that very seldom are cell phones the cause of interference to

telemetry systems in hospitals and usually when there is interference it is because a doctors cell phone went

off inches from the patients telemetry equipment.

Current / Planned Technologies Band Frequency

SMR iDEN 800 MHz 806 MHz to 824 MHz and 851 MHz to 869 MHz

GSM, IS-95 (CDMA), IS-136 (D-AMPS), 3G Cellular 824 MHz to 849 MHz and 869 MHz to 894 MHz

GSM, IS-95 (CDMA), IS-136 (D-AMPS), 3G PCS 1850 MHz to 1910 MHz and 1930 MHz to 1990 MHz

3G, 4G, DVB-H 700 MHz 698 MHz to 806 MHz

3G, 4G AWS 1710 MHz to 1755 MHz and 2110 MHz to 2155 MHz

4G BRS/EBS 2496 MHz to 2690 MHz

Wi-Fi and other sources in the 2.4 GHz band

The 2.4 GHz IMS band is unlicensed, and hence difficult, if not impossible, to manage. Telemetry equipment that

operates in this band generally has sophisticated interference avoidance algorithms, allowing them to change

frequencies to move away from occupied channels. If the use of other 2.4 GHz equipment is carefully managed in

or near a hospital, such deployment can be done successfully. Bear in mind that microwave ovens operate in the

2.4 GHz band and, while reasonably well shielded, they do radiate enough energy to disrupt telemetry operation if

the oven is too near a telemetry antenna.

Outside Signals

There is beginning to be smart grid deployment in the 1392 MHz to 1395 MHz and 1432 MHz to 1435 MHz bands inthe United States. It is worth noting in that these assigned frequencies are adjacent to the 1395 MHz to 1400 MHz,

and 1427 MHz to 1432 MHz WMTS bands. Since it appears that smart grid deployment will eventually be ubiquitous

in the United States, being aware of the potential for interference is worthwhile. In other places in the world 1.4 GHz

is used for cell phones.

The 608 MHz to 614 MHz band is in the middle of the UHF TV band - between channels 36 and 38. If you have

those channels or other strong TV channels in your area, they can readily deliver a very strong signal inside a

building and potentially cause harmful interference. The transition of analog to digital TV in the United States

caused many channels that previously operated in the VHF portion of the TV band to move to available UHF

channels, raising the potential for interference that hadnt previously existed.

On the roofs of many hospitals is a forest of antennas for various radio systems, including paging transmitters, land

mobile of various sorts, microwave links, cellular base stations, etc. Most tall buildings lease space on their roofs for

such uses. It would be a good idea to get on the roof so you can discover and measure what is there. That way you

will be knowledgeable about the potential for interference. A good survey would cover as wide a frequency range as

your equipment can do. Keep records of the frequencies being used and take screen shots of the various signals so

if they show up as interference you will already know what they are. Ambulance companies usually have UHF radios

in their vehicles and walkie-talkies for the paramedics. Usual frequency assignments for such services are in the

470 MHz area; while not particularly close in frequency to the IMS and WMTS bands, a walky-talky be used close to

a telemetry antenna can cause fundamental overload of the telemetry receiver. When that happens, the telemetry

receivers sensitivity is severely regarded and may not be able to receive telemetry signals.


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Once you have a good sample of the interference then build a limit line to let the instrument capture future

occurrence of interference while not saving lots of traces in which there is no interference. To do this, create a limit

line that is higher than the expected signals but lower than the interfering signal. Using Anritsu handheld spectrum

analyzers you can create a complex limit line that has 40 segments. See Figure 2a for an example.

Figure 2a: complex limit line to capture an intermittent interferer Figure 2b: Menu of save on event.

This limit line was built automatically by using limit envelope. Once you have the limit line, use it to capture enough

occurrences of the interference to get an idea of the time of day and the rate at which the interference occurs.

You do this by using the save-on-event capability to capture only traces in which a signal exceeds the limit line. It may

take a couple of tries with the limit line to capture the interfering signal when it occurs, but not too many other traces.

After you have captured a group of useful traces, copy them into your PC in a separate directory. You will be

creating a spectrogram from the traces by using the folder spectrogram creation tool in Master Software Tools,

software shipped with every Anritsu handheld spectrum analyzer.

In addition to creating a spectrogram, the folder spectrogram can show you the total power versus time, average

power versus time, peak frequency versus time, a time chart that shows the number of sweeps captured versus

time. All of these are useful when searching for an intermittent interferer.

SpectrogramThis is a great tool for noticing time and frequency patterns of intermittent signals. A spectrogram provides the

frequency, time and power level of all signals in the measured bandwidth.

Figure 3: Folder Spectrogram


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Average Power versus Time

If you have interference that becomes worse at certain times of day, this is a great view to see what time of day

the interference is happening. If you spot a pattern, you will be able to be on-site to look for interference at the

times when interference has occurred.

Figure 4: Average Power Vs. Time

Peak Power versus Time

This display gives more clues that may be able to help you determine when you need to be in the hall with your

spectrum analyzer to find an interferer. Time is on the horizontal axis. The times of the first and last saved files is

shown in the legend.

Figure 5: Peak Power vs. Time


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Total Power versus Time

This display can help you to discover relationships between bouts of interference and the total power in the

bandwidth being swept.

Figure 6: total Power vs. Time

Peak Frequency versus Time

This display shows you the frequency at which the highest power level in the sweep occurred. This display can

help you to discover what frequency range is actually causing harmful interference by correlating occurrences of

interference to the frequency at the highest power level at the time that interference occurred.

Figure 7: Peak Frequency vs. Time


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Time Chart

For intermittent interferers, this is a powerful tool for discovering patterns in the timing of the interferer. If, for

example, interference peaks at the start and end of work shifts, think about activities that peak at those times,

such as heavy elevator use. It is possible that the elevator or elevator controller is generating interference.

Figure 8: Time Chart

Depending on the nature of the interference, you may not get useful information out of all the displays available to

you in folder spectrogram. However, one of them may give you just the insight you need to solve your interference

problem. In this case the spectrogram and the peak frequency vs. time chart give useful information. The Time

Chart, which shows the number of measurements saved per minute, shows that the vast majority of the signals

occurred in the last afternoon and morning and basically went to zero at night.

Keep notes of the measurements you make, where interfering signals are strong and you can generally find theproblem and devise a fix.


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11410-00619, Rev. A Printed in United States 2011-072011 Anritsu Company. All Rights Reserved.

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