I have a very technical fundamental question about how radio works.
And no, I'm not ignorant of physics or the basics (I don't think,
anyway).. Here goes:

Say you have for simplicity two radio stations (AM for simplicity).
One is talk radio and the other plays Led Zeppelin all day. I
understand that "Talk" uses a different carrier frequency than
"Zeppelin" and that this allows my radio to tune in and detect one
without the other interfering. I also understand how a carrier wave
is modulated in AM and FM..

BUT, what I don't get is why the two do not interfere. Let me
explain.. Take a timeslice of EM radiation hitting my recieving
antenna at some moment. Some electrons in the antenna move up in
response to experiencing some energy from "Talk" station that
corresponds to a high point in the sine-wave. The same electron,
though, is pulled down a bit in response to some EM hitting it from
"Zeppelin". How does "Talk" not affect "Zeppelin" if both are shoving
the same electron in my antenna? How does my radio figure out that an
effect at the antenna is NOT an ordinary modulation of the "Talk"
carrier wave but rather of some other one and therefore to be ignored?

Thanks for answers folks!

Same way sound works with shared air : it's linear so whatever filter
you apply to get rid of stuff you don't want, gets rid of it regardless
whether you have stuff you want as well.

If you raise the amplitude, you can possibly get nonlinear effects, and
then you do hear the unwanted signal as well, for instance if you're
living next to a religious broadcast station and listening to some
distant devil-worship music, you can hear a sermon squawking in
one sideband or the other of the devil music.
--
Ron Hardin


On the internet, nobody knows you're a jerk.

Clay Denski wrote:

BUT, what I don't get is why the two do not interfere. Let me
explain.. Take a timeslice of EM radiation hitting my recieving
antenna at some moment. Some electrons in the antenna move up in
response to experiencing some energy from "Talk" station that
corresponds to a high point in the sine-wave. The same electron,
though, is pulled down a bit in response to some EM hitting it from
"Zeppelin". How does "Talk" not affect "Zeppelin" if both are shoving
the same electron in my antenna? How does my radio figure out that an
effect at the antenna is NOT an ordinary modulation of the "Talk"
carrier wave but rather of some other one and therefore to be ignored?



Get a long bladed handsaw. Lay it sideways on a piece of paper and trace
out the teeth with a pencil. Now get another long handsaw with a
completely different tooth spacing (pitch).

Lay it over the pencil pattern and retrace with a coloured pencil.
remove saw and compare wave patterns. See how the peaks hardly ever
coincide? At a few million teeth (hertz) per second, these coincidences
will be even fewer per unit time.

I was going to say this equates to the carrier frequency spacing of
radio stations, but then it occurred to me I just wanted to talk saw
blades and pencils, so we'll let it all drop now...



mike


--
__ __ __ __ __ __ __ __
/ /\ / /\ / /\ / /\ / /\ / /\ / /\ / /
/ /\ \/ /\ \/ /\ \/ /
/_/ \/_/ \/_/ \/_/ \/_/ \/_/ \/_/ \/_/

..let the cat out to reply..

©Densa International
'Think tanks cleaned cheap'

Excellent analogy!

"m II" wrote in message
news:XqvUc.21372$S55.11690@clgrps12...
Clay Denski wrote:

BUT, what I don't get is why the two do not interfere. Let me
explain.. Take a timeslice of EM radiation hitting my recieving
antenna at some moment. Some electrons in the antenna move up in
response to experiencing some energy from "Talk" station that
corresponds to a high point in the sine-wave. The same electron,
though, is pulled down a bit in response to some EM hitting it from
"Zeppelin". How does "Talk" not affect "Zeppelin" if both are shoving
the same electron in my antenna? How does my radio figure out that an
effect at the antenna is NOT an ordinary modulation of the "Talk"
carrier wave but rather of some other one and therefore to be ignored?



Get a long bladed handsaw. Lay it sideways on a piece of paper and trace
out the teeth with a pencil. Now get another long handsaw with a
completely different tooth spacing (pitch).

Lay it over the pencil pattern and retrace with a coloured pencil.
remove saw and compare wave patterns. See how the peaks hardly ever
coincide? At a few million teeth (hertz) per second, these coincidences
will be even fewer per unit time.

I was going to say this equates to the carrier frequency spacing of
radio stations, but then it occurred to me I just wanted to talk saw
blades and pencils, so we'll let it all drop now...



mike


--
__ __ __ __ __ __ __ __
/ /\ / /\ / /\ / /\ / /\ / /\ / /\ / /
/ /\ \/ /\ \/ /\ \/ /
/_/ \/_/ \/_/ \/_/ \/_/ \/_/ \/_/ \/_/

..let the cat out to reply..

©Densa International
'Think tanks cleaned cheap'

"Clay Denski" wrote in message
om...
I have a very technical fundamental question about how radio works.
And no, I'm not ignorant of physics or the basics (I don't think,
anyway).. Here goes:

Say you have for simplicity two radio stations (AM for simplicity).
One is talk radio and the other plays Led Zeppelin all day. I
understand that "Talk" uses a different carrier frequency than
"Zeppelin" and that this allows my radio to tune in and detect one
without the other interfering. I also understand how a carrier wave
is modulated in AM and FM..

BUT, what I don't get is why the two do not interfere. Let me
explain.. Take a timeslice of EM radiation hitting my recieving
antenna at some moment. Some electrons in the antenna move up in
response to experiencing some energy from "Talk" station that
corresponds to a high point in the sine-wave. The same electron,
though, is pulled down a bit in response to some EM hitting it from
"Zeppelin". How does "Talk" not affect "Zeppelin" if both are shoving
the same electron in my antenna? How does my radio figure out that an
effect at the antenna is NOT an ordinary modulation of the "Talk"
carrier wave but rather of some other one and therefore to be ignored?

Thanks for answers folks!

You are thinking in the time domain where the signals are combined and hard
to separate. If you think in the frequency domain the signals are easy to
separate because of the difference in the carrier frequencies.

The filters in your radio act in the frequency domain. Their behaviour is
independant of time, but not frequency. Signals at one frequency are allowed
to pass while at another they are attenuated.

craigm

(Clay Denski) wrote in message . com...
I have a very technical fundamental question about how radio works.
And no, I'm not ignorant of physics or the basics (I don't think,
anyway).. Here goes:

Say you have for simplicity two radio stations (AM for simplicity).
One is talk radio and the other plays Led Zeppelin all day. I
understand that "Talk" uses a different carrier frequency than
"Zeppelin" and that this allows my radio to tune in and detect one
without the other interfering. I also understand how a carrier wave
is modulated in AM and FM..

BUT, what I don't get is why the two do not interfere.
......snip........................................ ...................
It is quite easy to build a receiver that will give you all the
stations in your area all mixed together. It is called a crystal
detector. The thing it needs to start separating the stations is to
add a tuned circuit composed of a coil and a variable condenser. When
the tuned circuit is added to the crystal detector, the stations that
were on top of each other will appear at different points as the
condenser is turned. Some sound from adjacent stations will still be
heard as this arrangement is not very selective. When good coils,
filters, and digital filters are used in a radio, it is very selective
and you only hear the station you are tuned to. Actually, there is
much more to this topic, but a good place to start is "The Radio
Amateur's Handbook" found at any library. "Resonant Circuits" will
probably tell you more than you want to know.....ken.....

In article ,
(Clay Denski) wrote:

I have a very technical fundamental question about how radio works.
And no, I'm not ignorant of physics or the basics (I don't think,
anyway).. Here goes:

Say you have for simplicity two radio stations (AM for simplicity).
One is talk radio and the other plays Led Zeppelin all day. I
understand that "Talk" uses a different carrier frequency than
"Zeppelin" and that this allows my radio to tune in and detect one
without the other interfering. I also understand how a carrier wave
is modulated in AM and FM..

BUT, what I don't get is why the two do not interfere. Let me
explain.. Take a timeslice of EM radiation hitting my recieving
antenna at some moment. Some electrons in the antenna move up in
response to experiencing some energy from "Talk" station that
corresponds to a high point in the sine-wave. The same electron,
though, is pulled down a bit in response to some EM hitting it from
"Zeppelin". How does "Talk" not affect "Zeppelin" if both are shoving
the same electron in my antenna? How does my radio figure out that an
effect at the antenna is NOT an ordinary modulation of the "Talk"
carrier wave but rather of some other one and therefore to be ignored?

Thanks for answers folks!

The induced "Talk" electrons are separate from the "Zeppelin" electrons.
The two different EM waves generate two different RF currents or two
different flows of electrons in the antenna wire. If you look at the
generated voltage on the wire with a oscilloscope in the time domain
then you are going to see the sum of both generated RF currents.
However, if you looked at it with a spectrum analyzer you would still
see them as separate signals in the frequency domain. The radio receiver
is designed to see one narrow band of frequencies at a time so only one
of the two is received while the other is rejected.

--
Telamon
Ventura, California

Telamon wrote:
(Clay Denski) wrote:
BUT, what I don't get is why the two do not interfere. Let me
explain.. Take a timeslice of EM radiation hitting my recieving
antenna at some moment. Some electrons in the antenna move up in
response to experiencing some energy from "Talk" station that
corresponds to a high point in the sine-wave. The same electron,
though, is pulled down a bit in response to some EM hitting it from
"Zeppelin". How does "Talk" not affect "Zeppelin" if both are shoving
the same electron in my antenna? How does my radio figure out that an
effect at the antenna is NOT an ordinary modulation of the "Talk"
carrier wave but rather of some other one and therefore to be ignored?

The induced "Talk" electrons are separate from the "Zeppelin" electrons.
The two different EM waves generate two different RF currents or two
different flows of electrons in the antenna wire. If you look at the
generated voltage on the wire with a oscilloscope in the time domain
then you are going to see the sum of both generated RF currents.
However, if you looked at it with a spectrum analyzer you would still
see them as separate signals in the frequency domain. The radio receiver
is designed to see one narrow band of frequencies at a time so only one
of the two is received while the other is rejected.

"Pushing the electrons" is a red herring. All electromagnetic and
electrostatic forces in the universe contribute to pushing those
electrons. Remember that those electrons come from your antenna wire,
not the distant forces; the forces move *your* electrons, they don't
send electrons to your antenna.

Remember also that electricity isn't electrons, it's a flow of energy
that can move electrons. So it's not necessarily the electron movement
that we're interested in, it's the flow. Overquantize this concept and
you won't recognize the forest because you're looking at a few trees.

Two stations, "Talk" and "Zeppelin", induce flows of two different
frequencies in your antenna. We use the principle of resonance to
separate them; we tune a resonant tuned circuit to one frequency or
the other. Hold a 600 Hz tuning fork to an electric razor, and if
there's any 600 Hz component in the vibration the tuning fork will
show the principle; you'll hear 600 Hz louder than most other
frequencies in the resulting co-vibration. All the other frequencies
sort of disappear in the noise.

Tuned to "Talk", we get a whole lot of "Talk" frequency flow, just
like with the tuning fork. Other frequencies like "Zeppelin" aren't
resonated here; they get lost in the noise.

--
It's unfair to characterize Kerry as a flip-flopper. He's consistently in
favor of marrying a rich widow and buying your way into the White House.
-- Rex Tincher

Clay Denski wrote:
I have a very technical fundamental question about how radio works.
And no, I'm not ignorant of physics or the basics (I don't think,
anyway).. Here goes:

Say you have for simplicity two radio stations (AM for simplicity).
One is talk radio and the other plays Led Zeppelin all day. I
understand that "Talk" uses a different carrier frequency than
"Zeppelin" and that this allows my radio to tune in and detect one
without the other interfering. I also understand how a carrier wave
is modulated in AM and FM..

BUT, what I don't get is why the two do not interfere. Let me
explain.. Take a timeslice of EM radiation hitting my recieving
antenna at some moment. Some electrons in the antenna move up in
response to experiencing some energy from "Talk" station that
corresponds to a high point in the sine-wave. The same electron,
though, is pulled down a bit in response to some EM hitting it from
"Zeppelin". How does "Talk" not affect "Zeppelin" if both are shoving
the same electron in my antenna? How does my radio figure out that an
effect at the antenna is NOT an ordinary modulation of the "Talk"
carrier wave but rather of some other one and therefore to be ignored?

Thanks for answers folks!

Tuned circuits. The stations transmit on different frequencies, the
radio wave from "Talk" induces a current (vibrates the electrons)at
whatever carrier frequency the station transmits on, and likewise the
station transmitting "Zeppelin", but at a different frequency. The tuned
circuits in the receiver are selective and will pass the frequency they
are tuned to and reject the other frequencies. In the case of a parallel
tuned circuit, (coil and capacitor connected in parallel), the circuit
presents a high impedance at the resonant frequency (the frequency it is
tuned to) and a low impedance to other frequencies. A series tuned
circuit (coil and capacitor in series) is just the opposite, low
impedance to the resonant frequency and high impedance to other frequencies.

In article ,
clifto wrote:

Telamon wrote:
(Clay Denski) wrote:
BUT, what I don't get is why the two do not interfere. Let me
explain.. Take a timeslice of EM radiation hitting my recieving
antenna at some moment. Some electrons in the antenna move up in
response to experiencing some energy from "Talk" station that
corresponds to a high point in the sine-wave. The same electron,
though, is pulled down a bit in response to some EM hitting it from
"Zeppelin". How does "Talk" not affect "Zeppelin" if both are shoving
the same electron in my antenna? How does my radio figure out that an
effect at the antenna is NOT an ordinary modulation of the "Talk"
carrier wave but rather of some other one and therefore to be ignored?

The induced "Talk" electrons are separate from the "Zeppelin" electrons.
The two different EM waves generate two different RF currents or two
different flows of electrons in the antenna wire. If you look at the
generated voltage on the wire with a oscilloscope in the time domain
then you are going to see the sum of both generated RF currents.
However, if you looked at it with a spectrum analyzer you would still
see them as separate signals in the frequency domain. The radio receiver
is designed to see one narrow band of frequencies at a time so only one
of the two is received while the other is rejected.

"Pushing the electrons" is a red herring. All electromagnetic and
electrostatic forces in the universe contribute to pushing those
electrons. Remember that those electrons come from your antenna wire,
not the distant forces; the forces move *your* electrons, they don't
send electrons to your antenna.

Where did I say that the electrons came from the EM wave? I used the
word "induced." This discussion is about two forces acting on electrons
in a wire. Let's not make it more complicated than necessary to convey
the concept of how they simultaneously interact.

Remember also that electricity isn't electrons, it's a flow of energy
that can move electrons. So it's not necessarily the electron movement
that we're interested in, it's the flow. Overquantize this concept and
you won't recognize the forest because you're looking at a few trees.

DC or AC a "current" is a flow or movement of electrons.

Two stations, "Talk" and "Zeppelin", induce flows of two different
frequencies in your antenna. We use the principle of resonance to
separate them; we tune a resonant tuned circuit to one frequency or
the other. Hold a 600 Hz tuning fork to an electric razor, and if
there's any 600 Hz component in the vibration the tuning fork will
show the principle; you'll hear 600 Hz louder than most other
frequencies in the resulting co-vibration. All the other frequencies
sort of disappear in the noise.

Tuned to "Talk", we get a whole lot of "Talk" frequency flow, just
like with the tuning fork. Other frequencies like "Zeppelin" aren't
resonated here; they get lost in the noise.

--
Telamon
Ventura, California