Speaking of DSP...

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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.

Re: Speaking of DSP...

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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).

Wim



Re: Speaking of DSP...
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An interesting thought but probably won't work in our case --
too many technophiles in the area.

Re: Speaking of DSP...
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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.



Re: Speaking of DSP...

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Have you tried using a more directional antenna?

Ian


Re: Speaking of DSP...
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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.
--
Scott
ExoTech R&D, Inc.



Re: Speaking of DSP...
If this is in the U.S., have you considered complaining to FCC?

If you are operating on your own band or if it is a shared band,
you should not be seeing interference from other stations.

Sandeep
--
http://www.EventHelix.com/EventStudio
EventStudio 2.0 - System Architecture Design CASE Tool

Re: Speaking of DSP...

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  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.

Ian
--
Ian Stirling, G4ICV, AB2GR.

Re: Speaking of DSP...
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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.

Re: Speaking of DSP...


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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.

Leon


Re: Speaking of DSP...

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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?



Re: Speaking of DSP...
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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.

Re: Speaking of DSP...
 :  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.



Re: Speaking of DSP...
On Fri, 5 Dec 2003 09:32:12 PST, snipped-for-privacy@iwvisp.com (Everett M.

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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."

"We won’t use the words Microsoft and reliability in the same sentence."

<Found on the Rane tech pages>

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