Does anyone in this group happen to have knowledge, tips, experience, suggestions,... about how to cancel RF signal (FM broadcast radio) interference? Our not-for-profit translator service has a severe interference problem and the locals (donors) are clamoring for it to be corrected.
We've tried the "obvious" filtering techniques to no avail. Some of the more drastic ideas such as dynamiting the interfering station's transmitter have been ruled out. It has been thought that some sort of echo cancellation or adaptive filtering might work but we don't have the expertise to judge. Subsisting on donations, we don't have a substantial budget to purchase a solution.
I have a reference to an article from a 1976 IEEE conference by F.A.Cassara, T.S.Sundresh and H.Schachter "Surpression of Interchannel interference in FM receivers"
I works in the HF frequency domain. To give an outline of the system:
The incoming signal is split in 2 brances, and every branche has a PLL locking to the signal. The oscillator signal of one PLL is 90 deg shifted and subtracted from the input of the other PLL (and vice versa), and the result will be that one PLL locks to the wanted and the other PLL to the unwanted station
More something for the rec.radioamateur.homebrew group or so.
A social engineering solution is to spread the rumour that the electromagnetic waves of the offending station will cause cancer (and impotence of course). It will be forced to close or be razed by the locals (at least in the UK where they have a good Luddite tradition).
My admittedly limited RF experience tells me that translators usually don't convert the signals to baseband, thus limiting what you could do with a DSP.
You are pretty much left with filters and antenna orientation. So if it were me (and I'm glad it's not), I'd look at highly direct- ional antennas pointing at your sources with filters and attenuators to limit the translater input to non-distorting levels.
This is the kind of job that generally *will* eat up time and money, especially if you aren't an expert and don't have the proper tools.
Try to find out if there is an Amateur Radio club in your area. If so there is likely to be a member that has the right tools and know-how to help you. You're likely to find an experienced RF engineer in such a group.
If you don't know how to find them send me an email with some particulars and we will try to locate the closest one to you.
Your domain name in your email address suggests you're in the USA. In the USA, broadcast radio stations are licensed users of their bands. One legal unlicensed use of a frequency that a licensed user uses, but not all frequencies, is under FCC Part 15 rules. In that case, your equipment should not interfere with licensed users, which might happen if you use directional aerials on the transmitter or too much power, and it should accept interference from licensed users, that is, you have no right to complain to the FCC. Your receivers are allowed to use directional or high gain aerials, but not the transmitter if the signal would interfere with a licensed user. And in any case, is Part 15 sufficient? What kind of range are you trying to achieve? Part 15 is good only for short distances when two way communication is required: e.g., personal wireless Internet for home use is Part 15 and at least one channel on 802.11b is in an amateur band. If Part 15 is sufficient, I suggest you go to the FCC's site and do a search for permitted Part 15 frequencies and change your equipment to use a quieter frequency, certainly not in an FM broadcast band.
Everything that's happening is legal. We're trying to receive a distant station well outside its protected area. We're so close to the local station that its "slop" over into the adjacent channel is also legal.
Changing frequencies is not an option since we only receive what's there and have no control over either station's transmissions. The local station's owner has told me that they would like to increase their power but doing so would cause them to interfere with the adjacent channel in the distant station's protected broadcast area.
Now we know why +/- 20 KHz (or so) modulation in a
200 KHz channel (in the U.S.) results in only alter- nate channels being allocated in a given area.
A technique that has been used by radio amateurs on HF is to receive the interfering signal with a separate antenna, amplify it and use it to null the interfering signal on the main antenna. There used to be a commercial product that used this technique some years ago. It was quite effective, by all accounts. I think some phase shifting was involved, which might be difficult at VHF.
A directional YAGI, albeit very short for FM, can be used very effectively to null the offending station rather than for maximum smoke. What are the polarisations of the two transmitters?
We've tried and/or are presently using these techniques and they are proving to not be effective enough. Trying to null or cancel the interfering station is complicated by the numerous reflections we receive from other antennas and structures.
Its really a job for rec.audio.pro NG I dont think a DSP is the best way to go, just another can of worms
I'm a much better audio engineer than an embedded programmer, so.......
I'm not quite sure what a "translator" service is, but I assume it is a microphone(s), mixer/ switching system, a distribution amp (DA) and a headphone (HP) setup.
Work out where the RF is getting in, start at the end of the system, ie the distribution amp and headphones, disconnect all inputs , short the input(s) to the DA. check each output at a time, at the back of the DA with headphones. If you still get the RF, you need to sort out DA.
If it is clean, reconnect the HP cables, one at a time, and listen for RF, at the far end of the cable. If you get RF now , it is probably getting in through the DA output cables, they need to be GOOD quality screened cable, VanDamme/Belden, etc, ie balanced mic cable. NOT the thin audio cable that you use on hifi interconnects . Use only the signal pair for feeding the headphones, with the screen tied to a GOOD ground at the distribution amp end, only. The chassis of the DA should be grounded.
Put ferrite beads/chokes on the signal leads as they leave the DA.
Disconnect the shorted inputs to he DA, if you now get RF, change/repair the DA.
Connect the mixer to the DA, if you get RF now, check the output of the mixer with a pair of headphones, if it is clean. you need to sort the cable between the mixer and the DA. I'm assuming that the mixer has unbalanced outputs ( bad ) but again try balanced cable, with the screen tied to ground at the mixer end only. There are several tricks, if you still get RF, with good cable.
1) put a 100 ohm resistor in the signal lead at the mixer end, this will stop RF getting into the mixer
2) use 1:1 audio transformers on the output of the mixer, to the DA this is the best option.
Go through the mic amp system in the same way
Basically everything in a high RF enviroment MUST be balanced, not unbalanced. All cables, even speaker/headphone leads should be screened, and the screen grounded at one end. The screen should not carry any signal. The far end of the screen can be connected to ground, through a small capacitor, say 100nF. Microphones are the exception.
The big problem is that Ground at audio frequencies is Not A Ground At RF , so this is why I say ground a screen only at one end, and the system ground might itself be RF contaminated, try to get it tied to a copper waterpipe, or dig a hole in the basement and put a big metal stake in the earth, make sure it is well watered, dry earth aint much good
1:1 600 ohm audio transformers are much better than electronically balanced output and input stages for RF rejection, check out jensen,lundhal, sowter and OEM transformer websites for info
martin
"When all else fails, digitize everything, use fiber optic cable and enter a whole new realm of problems."
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On Tue, 9 Dec 2003 10:45:13 PST, Everett M. Greene wrote: : and they are proving to not be effective enough. Trying : to null or cancel the interfering station is complicated : by the numerous reflections we receive from other antennas : and structures.
Sounds like you're screwed with the simple approaches then. On the assumption your front end isn't overloaded and the desired signal is not a reflection then ISTM the only viable approach is DSP, simply (!) filter out the later-arriving reflections keeping on eye the adjustments you'll need to make for the variations over time between arrival of the primary signal and the reflection(s) that are ruining your day.
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