FYI: The phantoms at 411 KiloHertz

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Old 01-29-15, 07:16 AM
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FYI: The phantoms at 411 KiloHertz

Patient Wandering systems are used in nursing homes to prevent patients from leaving certain areas, or leaving the facility. The patient wears a small "badge", or transmitter, and receivers near the doors lock a maglock when a badge enters the area. Staff can override by entering a code into keypads.

This particular system was installed 15 or more years ago. It operates at 433kHz -- a frequency that wasn't crowded when the system was installed. (Today's systems operate in the 500 MHz range, and they handshake with door controllers and servers to verify their presence.)

At 2pm last Wednesday all 20 of the doors in the facility locked as if they had detected badges in the field. The company that installed the systems bailed, saying it was not their problem.

We were called in to solve the problem. We checked the area and walked the halls with an RF spectrum analyzer, which showed nothing until we plugged in a laptop's power supply. The power supply was not the cause, however it was amplifying the RF spike at 411kHz so we could read it on the meter. The RF was riding on the building's power system.

The only option was to turn off every electronic device, one at a time, in this 450-bed nursing home. On the second day of searching one of the techs found an old CRT television with a bad oscillator. It was in a patient's room, and the patient liked to have it on 24/7. The TV's oscillator was producing a strong 50kHz-wide upper harmonic centered at 411kHz which was blasting into the building's power infrastructure.

Ya never know.
 
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Old 01-29-15, 07:57 AM
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The 26th harmonic of the CRT horiz sweep puts it at 411kHz. It is the single highest powered frequency source in the old analog TVs, by far. Good find on that one!
 
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Old 01-29-15, 09:34 AM
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That poor ol TV. I hope you gave it a good home. Definitely a good find.
 
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Old 01-29-15, 10:53 AM
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Did you try plugging the TV into a power line noise suppressor? This should attenuate most of the wire borne 411 KHz coming out of the TV set in the same manner as electrical noise from the line is prevented from going into the TV set.
 
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Old 01-29-15, 11:12 AM
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Whose TV - patient's or nursing home's? If the latter, bet there are a ton more waiting to behave similarly. Also sounds like they need to update their wandering prevention.
 
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Old 01-29-15, 12:14 PM
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But the fix might be almost as much as the solution; namely a new, low cost flat screen TV.
 
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Old 01-29-15, 09:24 PM
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I think in most care centers, at least all the ones I've worked in recently, have replaced all the tube sets with flatscreens. This was probably one old relic just hanging around waiting to wreak havoc.
 
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Old 01-30-15, 05:38 AM
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The home's CATV system is analog so they only swapped out the old CRTs as they failed. I'll bet they all get swapped now.

Originally Posted by telecom guy
The 26th harmonic of the CRT horiz sweep puts it at 411kHz.
Yep. Funny thing is, the first thing I did after they found it was divide 411k by 15,750. I would never have figured that out in advance, though, to tell them what to look for. It put me in mind of the old security video systems in the malls. The oscillator frequency was so loud I couldn't stand to walk into a store.

In the process we recommended that an electrical contractor be brought in to look for anomalies. They found the power phases to be way out of balance, with one leg at 91 degrees and another at 141. So this place is getting some much-needed TLC.
 
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