Steve Nosko wrote:
I believe he is talking about adjacent channel signals which are much
stronger than the desired and trying to see if there is a way to "fix"
that. Capture does not apply for this situation. Capture is a co-channel
effect and is better the wider the whole system is, not just the IF.

Capture effect:

http://en.wikipedia.org/wiki/Capture_effect

It's an intersting phenomena, and potentially useful. If I could phase null
(at the antenna) the local stronger signal then it would dissappear as
inteference if it was a few db below the wanted signal.

Only problem is, is that if you cannot get the inteferring signal below the
wanted one, it's works very bad for you, because you cannot listen to an
inteferred with signal. Which is better than nothing I guess in some cases.

Pity that somehow you cannot design circuitry which recognises a 100Khz
difference between the wanted FM station and the unwanted FM station and
proceed to demodulate the wanted one only. If it were possible, it would
have been done by now. Maybe you could do it digitally,I dunno.

"Richard" wrote in message
...
Steve Nosko wrote:
I believe he is talking about adjacent channel signals which are much
stronger than the desired and trying to see if there is a way to "fix"
that. ...
Capture effect:
http://en.wikipedia.org/wiki/Capture_effect
It's an intersting phenomena, and potentially useful.

But I believe not here. Only for co-channel interference.

If I could phase null
(at the antenna) the local stronger signal ... if you cannot get the
inteferring signal below the

This is a sticky problem if the undesired is only 100kHz off center with
the standard 200kHz system design. I this case you can have significant
energy within the Rx passband coming from the undesired. I can't say how
low the undesired needs to be.


Pity that somehow you cannot design circuitry which recognises a 100Khz
difference between the wanted FM station and the unwanted FM station and
proceed to demodulate the wanted one only. If it were possible, it would
have been done by now. Maybe you could do it digitally,I dunno.

As you have surmised, the "null in the antenna pattern" is probably your
best bet. This is done in some rather dificult situations. One is on a
radio equiped copter where the undesides TX is on board! ( the TX output
itself can be sampled) The antenna with a null would be my first path. I
also have an FM band adjacent channel problem I'd like to solve and will
(when I get time) try a nulling antenn.

The "adcock" type would be one of the easiest. Two vertical dipoles
(probably folded, but not necessary) fed out of phase. There are two nuls
toward the "flat side" or broad-side. Point the null at the undesired.
The "two vertical cardioid" is just as easy. I think it is two vertical
dipoles spaced (I think) 1/4 wave apart with a 3/4 wave coax (electrical)
between and fed at one of the dipoles. The 1/4 wave (free-space) physical s
pacing gives 90 degrees and the 3/4 (in coax) gives another 90 for a total
of 180 - thus cancelling. 3/4 coax is needed because 1/4 will be shorter
that the free-space 1/4 spacing of the dipoles.

I recently saw both of these in the Ham transmitter hunting book. They are
probably both on the net if you know where to look. Try amateur radio DF or
ARDF.
http://members.aol.com/BmgEngInc/Adcock.html
Probably the transformer is not needed.

I took a quick look and can't find more, so Here's a web ring on DF:
http://m.webring.com/hub?ring=foxhunt

Other DF sites, but I don't know if they have for U. Some have more links.
http://www.panix.com/clay/ham/rdf.html
http://www.ardf-uk.co.uk/index2.html
http://members.aol.com/fdecker/rdf.htm
http://members.aol.com/homingin/index.html
Dual null & cardioid figures, prpbably not helpful.
http://members.aol.com/homingin/hfinderfix.html
Lota' links
http://members.aol.com/homingin/links.html



Another technique would also handle the afore mentined multipath problem.
This I will call the "Secondary Antenna & Canceller" (SAC) technique. It is
used in the military and commercial arena to solve some sticky strong signal
problems.
A secondary antenna is used and fed to an amplifier which has phase and
amplitute adjustments (under computer control for automatic tuning). The
output of this is fed back into the RX line (where the desired and undesired
signal are) and tuned for minimum interference. This amounts to a custom
antenna null. MFJ makes two for HF

http://www.mfjenterprises.com/contact.php

MFJ-1025 1.5 - 30 MHz Noise Canceling Antenna
MFJ-1026 1.5-30 MHz Deluxe Noise Canceller

http://www.mfjenterprises.com/produc...rodid=MFJ-1025
http://www.mfjenterprises.com/produc...rodid=MFJ-1026


--
Steve N, K,9;d, c. i My email has no u's.

"Richard" wrote in message
...
Steve Nosko wrote:
I believe he is talking about adjacent channel signals which are much
stronger than the desired and trying to see if there is a way to "fix"
that. ...
Capture effect:
http://en.wikipedia.org/wiki/Capture_effect
It's an intersting phenomena, and potentially useful.

But I believe not here. Only for co-channel interference.

If I could phase null
(at the antenna) the local stronger signal ... if you cannot get the
inteferring signal below the

This is a sticky problem if the undesired is only 100kHz off center with
the standard 200kHz system design. I this case you can have significant
energy within the Rx passband coming from the undesired. I can't say how
low the undesired needs to be.


Pity that somehow you cannot design circuitry which recognises a 100Khz
difference between the wanted FM station and the unwanted FM station and
proceed to demodulate the wanted one only. If it were possible, it would
have been done by now. Maybe you could do it digitally,I dunno.

As you have surmised, the "null in the antenna pattern" is probably your
best bet. This is done in some rather dificult situations. One is on a
radio equiped copter where the undesides TX is on board! ( the TX output
itself can be sampled) The antenna with a null would be my first path. I
also have an FM band adjacent channel problem I'd like to solve and will
(when I get time) try a nulling antenn.

The "adcock" type would be one of the easiest. Two vertical dipoles
(probably folded, but not necessary) fed out of phase. There are two nuls
toward the "flat side" or broad-side. Point the null at the undesired.
The "two vertical cardioid" is just as easy. I think it is two vertical
dipoles spaced (I think) 1/4 wave apart with a 3/4 wave coax (electrical)
between and fed at one of the dipoles. The 1/4 wave (free-space) physical s
pacing gives 90 degrees and the 3/4 (in coax) gives another 90 for a total
of 180 - thus cancelling. 3/4 coax is needed because 1/4 will be shorter
that the free-space 1/4 spacing of the dipoles.

I recently saw both of these in the Ham transmitter hunting book. They are
probably both on the net if you know where to look. Try amateur radio DF or
ARDF.
http://members.aol.com/BmgEngInc/Adcock.html
Probably the transformer is not needed.

I took a quick look and can't find more, so Here's a web ring on DF:
http://m.webring.com/hub?ring=foxhunt

Other DF sites, but I don't know if they have for U. Some have more links.
http://www.panix.com/clay/ham/rdf.html
http://www.ardf-uk.co.uk/index2.html
http://members.aol.com/fdecker/rdf.htm
http://members.aol.com/homingin/index.html
Dual null & cardioid figures, prpbably not helpful.
http://members.aol.com/homingin/hfinderfix.html
Lota' links
http://members.aol.com/homingin/links.html



Another technique would also handle the afore mentined multipath problem.
This I will call the "Secondary Antenna & Canceller" (SAC) technique. It is
used in the military and commercial arena to solve some sticky strong signal
problems.
A secondary antenna is used and fed to an amplifier which has phase and
amplitute adjustments (under computer control for automatic tuning). The
output of this is fed back into the RX line (where the desired and undesired
signal are) and tuned for minimum interference. This amounts to a custom
antenna null. MFJ makes two for HF

http://www.mfjenterprises.com/contact.php

MFJ-1025 1.5 - 30 MHz Noise Canceling Antenna
MFJ-1026 1.5-30 MHz Deluxe Noise Canceller

http://www.mfjenterprises.com/produc...rodid=MFJ-1025
http://www.mfjenterprises.com/produc...rodid=MFJ-1026


--
Steve N, K,9;d, c. i My email has no u's.

In article , "Richard"
writes:

Steve Nosko wrote:
I believe he is talking about adjacent channel signals which are much
stronger than the desired and trying to see if there is a way to "fix"
that. Capture does not apply for this situation. Capture is a co-channel
effect and is better the wider the whole system is, not just the IF.

Capture effect:

http://en.wikipedia.org/wiki/Capture_effect

It's an intersting phenomena, and potentially useful. If I could phase null
(at the antenna) the local stronger signal then it would dissappear as
inteference if it was a few db below the wanted signal.

Only problem is, is that if you cannot get the inteferring signal below the
wanted one, it's works very bad for you, because you cannot listen to an
inteferred with signal. Which is better than nothing I guess in some cases.

Simplistic explanations of "capture effect" don't help here.

The main, major cause of such strong signal interference is due to
the FM receiver's LIMITER stage. That stage (usually there are 2 to
3 of them) is biased such that a strong signal is going to clip or that
it will actually develop enough self-bias to clip even further. The end
result is that a limiter output is primarily composed of the strongest
signal's RF energy.

The action of a limiter is basically that of accepting only the strongest
signal and "pushing down" the energy of weaker signals, including
noise at the noise floor of the front end. THAT is what insures the FM
"clean signal" quality. It is also a sort of automatic AGC so that all
signals of the same modulation index will be demodulated to the same
audio level...even though the antenna input signal can vary considerably.

Such limiters have been used for some pulsed RF reception with the
ability to "push down" offending pulsed carrier frequencies' envelopes.

Pity that somehow you cannot design circuitry which recognises a 100Khz
difference between the wanted FM station and the unwanted FM station and
proceed to demodulate the wanted one only. If it were possible, it would
have been done by now. Maybe you could do it digitally,I dunno.

Not with "phasing" alone and not with limiters still in the IF chain. But,
it has been done by other means -

The brute force method is to use a many-pole crystal bandpass filter
ahead of all limiters. Even Heathkit did that. This cuts out very strong
adjacent signals and the limiters will be less affected by such. With
steep skirt selectivity, offending adjacent channel attenuation can be
60 db or greater.

A second method is to discard the limiter stages and try for a Ratio
detector instead of the Foster-Seeley discriminator variants plus AGC
on the non-limiting IF stages. Not a perfect solution but does work to
some extent. Most NTSC TV receivers use a variation of that for TV
sound, originally started many years ago as the "intercarrier sound"
system to replace a separate limiter-discriminator IF strip; the first
TV receivers actually had TWO IF strips...:-)

The most obvious way, and least familiar, is to use a large-bit digital
A-to-D system and high-speed microprocessor to (1) Do the channel
bandpass filtering with a "brick-wall" filter routine; (2) Do the FM
demodulation directly after digital bandpass filtering by summing the
sideband components of the FM signal; (3) Converting the demodulated
IF in digital form back to analog audio. That's been done but the
production costs for consumer electronics systems has only lately
come down enough to warrant such use.

All of the methods I've given are prior art.

Len Anderson
retired (from regular hours) electronic engineer person

In article , "Richard"
writes:

Steve Nosko wrote:
I believe he is talking about adjacent channel signals which are much
stronger than the desired and trying to see if there is a way to "fix"
that. Capture does not apply for this situation. Capture is a co-channel
effect and is better the wider the whole system is, not just the IF.

Capture effect:

http://en.wikipedia.org/wiki/Capture_effect

It's an intersting phenomena, and potentially useful. If I could phase null
(at the antenna) the local stronger signal then it would dissappear as
inteference if it was a few db below the wanted signal.

Only problem is, is that if you cannot get the inteferring signal below the
wanted one, it's works very bad for you, because you cannot listen to an
inteferred with signal. Which is better than nothing I guess in some cases.

Simplistic explanations of "capture effect" don't help here.

The main, major cause of such strong signal interference is due to
the FM receiver's LIMITER stage. That stage (usually there are 2 to
3 of them) is biased such that a strong signal is going to clip or that
it will actually develop enough self-bias to clip even further. The end
result is that a limiter output is primarily composed of the strongest
signal's RF energy.

The action of a limiter is basically that of accepting only the strongest
signal and "pushing down" the energy of weaker signals, including
noise at the noise floor of the front end. THAT is what insures the FM
"clean signal" quality. It is also a sort of automatic AGC so that all
signals of the same modulation index will be demodulated to the same
audio level...even though the antenna input signal can vary considerably.

Such limiters have been used for some pulsed RF reception with the
ability to "push down" offending pulsed carrier frequencies' envelopes.

Pity that somehow you cannot design circuitry which recognises a 100Khz
difference between the wanted FM station and the unwanted FM station and
proceed to demodulate the wanted one only. If it were possible, it would
have been done by now. Maybe you could do it digitally,I dunno.

Not with "phasing" alone and not with limiters still in the IF chain. But,
it has been done by other means -

The brute force method is to use a many-pole crystal bandpass filter
ahead of all limiters. Even Heathkit did that. This cuts out very strong
adjacent signals and the limiters will be less affected by such. With
steep skirt selectivity, offending adjacent channel attenuation can be
60 db or greater.

A second method is to discard the limiter stages and try for a Ratio
detector instead of the Foster-Seeley discriminator variants plus AGC
on the non-limiting IF stages. Not a perfect solution but does work to
some extent. Most NTSC TV receivers use a variation of that for TV
sound, originally started many years ago as the "intercarrier sound"
system to replace a separate limiter-discriminator IF strip; the first
TV receivers actually had TWO IF strips...:-)

The most obvious way, and least familiar, is to use a large-bit digital
A-to-D system and high-speed microprocessor to (1) Do the channel
bandpass filtering with a "brick-wall" filter routine; (2) Do the FM
demodulation directly after digital bandpass filtering by summing the
sideband components of the FM signal; (3) Converting the demodulated
IF in digital form back to analog audio. That's been done but the
production costs for consumer electronics systems has only lately
come down enough to warrant such use.

All of the methods I've given are prior art.

Len Anderson
retired (from regular hours) electronic engineer person

Steve Nosko wrote:
"Richard" wrote in message
...
Steve Nosko wrote:
I believe he is talking about adjacent channel signals which are much
stronger than the desired and trying to see if there is a way to "fix"
that. ...
Capture effect:
http://en.wikipedia.org/wiki/Capture_effect
It's an intersting phenomena, and potentially useful.

But I believe not here. Only for co-channel interference.

If I could phase null
(at the antenna) the local stronger signal ... if you cannot get the
inteferring signal below the

This is a sticky problem if the undesired is only 100kHz off center
with the standard 200kHz system design. I this case you can have
significant energy within the Rx passband coming from the undesired. I
can't say how low the undesired needs to be.


In the UK, where I am, the spacing between channels is 100Khz. And that is a
real problem, because, as you say a lot of RF energy from the undesired
station gets into the IF passband.

And so basically I was wondeting if the technolgy exists for the RX to know
that energy from the undesired station, 100Khz off frequency, is to be
ignored in the demodulation. I don't think it can be done, but if it
could somehow the electronics would have to associate the unwanted energy
with a non wanted station, and in a sense, ignore it. Just wondering if the
technoloy is around to do that. Probably not. Only likely possible thru
digital computing methods maybe.

Steve Nosko wrote:
"Richard" wrote in message
...
Steve Nosko wrote:
I believe he is talking about adjacent channel signals which are much
stronger than the desired and trying to see if there is a way to "fix"
that. ...
Capture effect:
http://en.wikipedia.org/wiki/Capture_effect
It's an intersting phenomena, and potentially useful.

But I believe not here. Only for co-channel interference.

If I could phase null
(at the antenna) the local stronger signal ... if you cannot get the
inteferring signal below the

This is a sticky problem if the undesired is only 100kHz off center
with the standard 200kHz system design. I this case you can have
significant energy within the Rx passband coming from the undesired. I
can't say how low the undesired needs to be.


In the UK, where I am, the spacing between channels is 100Khz. And that is a
real problem, because, as you say a lot of RF energy from the undesired
station gets into the IF passband.

And so basically I was wondeting if the technolgy exists for the RX to know
that energy from the undesired station, 100Khz off frequency, is to be
ignored in the demodulation. I don't think it can be done, but if it
could somehow the electronics would have to associate the unwanted energy
with a non wanted station, and in a sense, ignore it. Just wondering if the
technoloy is around to do that. Probably not. Only likely possible thru
digital computing methods maybe.

On Sat, 14 Feb 2004 16:39:52 -0000, "Richard"
wrote:

Do we
have technology these days to do what we thought was impossible in this
case?

While the sidebands above the wanted nominal carrier are mixed with
the lower sidebands of the unwanted signal, but if the lower sidebands
of the wanted signal and the upper sidebands of the unwanted signal
are free from any interference, it might be possible to determine
which sideband belongs to which signal and then demodulate them
separately.

This would at least require that all IF sidebands of both the wanted
and unwanted sidebands are digitised (prior to the limiter) and then
do some frequency domain processing after FFT.

When looking at an ordinary FM signal in frequency domain, there are
an infinite number of sidebands on each side of the nominal carrier
frequency (while in AM there is exactly one pair of sidebands) at
integer multiples of the baseband (audio) frequency. However, the
amplitude for each sideband is determined by the Bessel function of
the modulation index of each baseband signal. While the modulation
index for most audio tones is in practice quite low (below 5), there
are only going to be a few sidebands with any significant amplitude
around the nominal carrier. Thus, the bandwidth of an FM signal is not
infinite in practice.

For instance, in a stereo transmission, there are quite a lot of
signal amplitude around the baseband 38 kHz subcarrier frequency in
the area 36-40 kHz, thus there are going to be strong FM sidebands
around 36-40, 76-80 and possible 118-120 kHz around the carrier on
both sides.

Since there is some symmetry between the upper and lower sidebands, so
this may help reconstruct the missing sidebands if the other side of
the carrier is clean, while the other side is contaminated.

However, I have no idea if this has been attempted in practice.

Paul OH3LWR

On Sat, 14 Feb 2004 16:39:52 -0000, "Richard"
wrote:

Do we
have technology these days to do what we thought was impossible in this
case?

While the sidebands above the wanted nominal carrier are mixed with
the lower sidebands of the unwanted signal, but if the lower sidebands
of the wanted signal and the upper sidebands of the unwanted signal
are free from any interference, it might be possible to determine
which sideband belongs to which signal and then demodulate them
separately.

This would at least require that all IF sidebands of both the wanted
and unwanted sidebands are digitised (prior to the limiter) and then
do some frequency domain processing after FFT.

When looking at an ordinary FM signal in frequency domain, there are
an infinite number of sidebands on each side of the nominal carrier
frequency (while in AM there is exactly one pair of sidebands) at
integer multiples of the baseband (audio) frequency. However, the
amplitude for each sideband is determined by the Bessel function of
the modulation index of each baseband signal. While the modulation
index for most audio tones is in practice quite low (below 5), there
are only going to be a few sidebands with any significant amplitude
around the nominal carrier. Thus, the bandwidth of an FM signal is not
infinite in practice.

For instance, in a stereo transmission, there are quite a lot of
signal amplitude around the baseband 38 kHz subcarrier frequency in
the area 36-40 kHz, thus there are going to be strong FM sidebands
around 36-40, 76-80 and possible 118-120 kHz around the carrier on
both sides.

Since there is some symmetry between the upper and lower sidebands, so
this may help reconstruct the missing sidebands if the other side of
the carrier is clean, while the other side is contaminated.

However, I have no idea if this has been attempted in practice.

Paul OH3LWR