118.775 VHF AWOS bleeding into sound system. - Page 1
Forum section: electronics / projects

author: WakeUpWolfgang
author_rank: Contributor
timestamp: October 24, 2014, 09:46:30 pm
content: I installed a sound system into an Airport the AWOS (Automated Weather Observing System) is getting into the sound system. It is running at 118.775 VHF. I figured out that it is entering in the system through the automatic timed message machine. It is a ISP 4500http://www.premtech.com/isp-4500.htmlI want to put a choke on to the DC power to see if it is comming in through the power cable. I cannot find a choke that will filter out 118.775 VHF.If the above does not work I will try a voice activated module and a relay so I can cut off the signal when it is not playing. I know the AWOS will still be there during the recordings but it is so faint that I don't think it will be a problem. If it is a problem I will have to look for a module that can take in the AWOS and the recording that has the AWOS in it and subtract it out. Adding shielding is not possible in the setup. I cannot use another timed message machine I am stuck with it (I did not spec out the equipment).Anyone have a suggestion on a choke since it will be the cheapest place to start?Edit: Fixing link in post.

author: tggzzz
author_rank: Super Contributor
timestamp: October 24, 2014, 10:26:30 pm
content: Quote from: WakeUpWolfgang on October 24, 2014, 09:46:30 pmIt is difficult to give useful suggestions without knowing what EMI precautions have been tried and been insufficient.Presumably you don't want to filter out only 118.775MHz, but want to prevent all RF entering your system. In that case have you tried wrapping the input leads (certainly audio, but possibly power) through a ferrite toroid, or even clipping a ferrite toroid around them. Quick, dirty, cheap, sometimes effective.You could always ask your local radio hams; they will have dealt with this kind of thing many times before.

author: Marco
author_rank: Super Contributor
timestamp: October 24, 2014, 10:28:55 pm
content: Won't pretty much all Ni-Zn ferrites provide significant impedance at 100 MHz?Most of the snap on chokes at Digikey seem to be tested at 100 MHz ... likethis one.

author: tggzzz
author_rank: Super Contributor
timestamp: October 24, 2014, 10:56:37 pm
content: Quote from: Marco on October 24, 2014, 10:28:55 pmJust so. I suspect the OP was using "choke" in the old sense of "inductor".

author: WakeUpWolfgang
author_rank: Contributor
timestamp: October 27, 2014, 01:07:22 pm
content: Thank you I will give that a try. I will let you know if that works or not it might be a week or so before we can get in to try it. (Federal regulations have to wait for TSA)EDIT: Anyone know of a transformer filter I can use? I have some fairate filters here in shop but we have a feeling that will not work. I think the device in picking it up but my boss want to try filtering it out.

author: Marco
author_rank: Super Contributor
timestamp: October 27, 2014, 04:48:17 pm
content: Could you use speaker cross over filters?Whatever you do, bring two dozen of those clip on cores ... it's just too cheap to put 3 or so on every cable in sight just to test if it will work (you can use toroids if you need something more compact but then you need to cut the cable so the clip ons are good to test).

author: WakeUpWolfgang
author_rank: Contributor
timestamp: October 27, 2014, 09:32:37 pm
content: I had some in the shop and I was able to get in today. I removed the unit and placed ferrite cores along the power cord. (started with one and worked up testing as I went along) I used my butt set on the output and it did not help any. I tried it at the other end of the airport as far away from the transmitter as I could get and still b e in the airport. I was still picking it up. I think something in the unit in picking it up.My boss seems convinced that it is still going through the AC. Anyone know of any AC transformer chokes that I could try?

author: WakeUpWolfgang
author_rank: Contributor
timestamp: November 07, 2014, 02:18:54 pm
content: I found out that the power supply that comes with the unit is an unregulated power supply. I replaced it with a regulated power supply with 2 ferrite toroid on it. I also added a bogen WMT-1A to change it from a 8 OHM to a 600 OHM signal. We were getting an amplifier rush on the unit. The output volume of the unit was high but if we turned it down the recordings were inaudible. Going to send the unit back we should not have to have the volume so loud we are getting an amplifier rush just to hear the recordings. Yes I did try to turn the output of the device down and then amplify it later down the stream.