Close Calling LED Signs

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tanger32au
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Close Calling LED Signs

Post by tanger32au » Mon Dec 19, 2016 10:01 am

Around six weeks ago, I was driving along Wellington Street in Launceston when I stopped at the traffic lights near the Westbury Road turn off. I had my Uniden UBC126AT scanner with me, running in close call mode.

I got a close call hit on 162.500MHz which is the repeater output frequency for Boral on Mt Arthur. This was "open carrier". I thought this was a little strange as I was over 22km from where this repeater is located (see the map below)

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I thought it might have been a driver using this in simplex mode, but I have never heard them do this before. A couple of weeks later I was again driving in this area and again had a close call hit on this same frequency, 162.500MHz. This I found very strange.

Last week I was driving past Queechy High School when I again got a close call hit on 162.500MHz, this really puzzled me. I parked my car and got out, by following the signal strength I was able to locate the source of the signal to this LED sign.

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On my drive home I drove along Wellington Street and got another close call hit on this frequency, looking around I spotted this sign.

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It seems that these LED signs radiate a open carrier signal on 162.500MHz. Range is up to 25 metres.

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Blake
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Re: Close Calling LED Signs

Post by Blake » Mon Dec 19, 2016 10:13 am

Well, that's a bit dodgy.
I've never heard of that before.

Most LED signs use PWM, which I suppose could coincide?
But that's generally just a clock signal, unless the pulses turning the array on & off, are dumping voltage, which is forming a rudimentary transmitter???

Quite strange, would be interesting to see if that level of interference is legal.
I'd be speaking to someone about that, as it appears it might be interfering with a repeater's operation?



EDIT:
After a bit of research, this is not an isolated problem. :o
From what I've found, the gist of the problem is this:

LED driver chips have two sets of signals, both PWM (meaning they are square waves with a fixed frequency, but a variable duty cycle).

One of the PWM drivers sets the peak current of the LEDs. These usually run between 500kHz and 1MHz.
The other PWM signal is used to dim the LEDs. These are usually 300HZ to 400Hz.

This in it's self, should not interfere.
However, when the PWM signals' harmonics are added, they contribute to the RFI emissions at specific frequencies.
Coincidentally, the PCB traces start to become very effective antennas after about 100MHz.

This all results in a large amount of RFI between 30 and 300 MHz.
I would say, that this may be the cause.
I'm no expert though, take this with a grain of salt :?
VESARS


"cognitio vincit tenebras"

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