Do magnetars emit audible LW AM radio waves that can be heard on receivers?
Radium
Do magnetars emit AM radio waves below the medium-wave range? If so,
how do we detect these waves? Can these waves be heard on the AM
radio? If so, what do they sound like?
Thanks,
Radium
Sam Wormley
Why would you expect a star to generate amplitude modulation?
Chris L Peterson
wrote:
> Why would you expect a star to generate amplitude modulation?
Why would you expect it to generate anything else? These sorts of
objects are rotating at high speed, which modulates the amplitude we
receive. While there are probably other types of modulation as well, the
amplitude variation is the dominant effect. Of course, magnetars are
emitting mainly hard x-rays. I don't know that there's enough long
wavelength energy to detect on any kind of ordinary radio.
_________________________________________________
Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com
ji...@specsol.spam.sux.com
Frequencies above approximately 100 MHz almost always get through
the ionization layers.
Frequencies in the approximate range of 10 MHz to 100 MHz sometimes
get through
Frequencies below approximately 10 MHz almost never get through.
So, if by "the AM radio" you mean a Broadcast Band radio which
runs from about .5 MHz to 1.2 MHz, not a chance in hell of ever
hearing anything from off the planet.
Try again.
--
Jim Pennino
Remove .spam.sux to reply.
Radium
> In rec.radio.amateur.space Radium <gluceg...@gmail.com> wrote:
>
> > Hi:
> > Do magnetars emit AM radio waves below the medium-wave range? If so,
> > how do we detect these waves? Can these waves be heard on the AM
> > radio? If so, what do they sound like?
>
> Frequencies above approximately 100 MHz almost always get through
> the ionization layers.
>
> Frequencies in the approximate range of 10 MHz to 100 MHz sometimes
> get through
>
> Frequencies below approximately 10 MHz almost never get through.
>
> So, if by "the AM radio" you mean a Broadcast Band radio which
> runs from about .5 MHz to 1.2 MHz, not a chance in hell of ever
> hearing anything from off the planet.
>
> Try again.
Okay. But what if this is a supercooled AM radio receiver on a
spaceship orbiting Earth? If I am on a space station like MIR and this
station has a supercooled AM radio 44.1 KHz frequency receiver, will I
hear anything specific of magnetars?
44.1 KHz is the carrier-frequency this hypothetical receiver receives.
I place the frequency of this hypothetical AM radio carrier wave at
44.1 KHz for the same reason CDs use a sample rate of 44.1 KHz -- it
is the minimum required to prevent aliasing.
AFAIK, space station orbit earth above the ionosphere so the
limitations [preventing long-waves from outer space from reaching the
Earth's surface] do not apply.
ji...@specsol.spam.sux.com
> > In rec.radio.amateur.space Radium <gluceg...@gmail.com> wrote:
> >
> > > Hi:
> > > Do magnetars emit AM radio waves below the medium-wave range? If so,
> > > how do we detect these waves? Can these waves be heard on the AM
> > > radio? If so, what do they sound like?
> >
> > Frequencies above approximately 100 MHz almost always get through
> > the ionization layers.
> >
> > Frequencies in the approximate range of 10 MHz to 100 MHz sometimes
> > get through
> >
> > Frequencies below approximately 10 MHz almost never get through.
> >
> > So, if by "the AM radio" you mean a Broadcast Band radio which
> > runs from about .5 MHz to 1.2 MHz, not a chance in hell of ever
> > hearing anything from off the planet.
> >
> > Try again.
> Okay. But what if this is a supercooled AM radio receiver on a
> spaceship orbiting Earth? If I am on a space station like MIR and this
> station has a supercooled AM radio 44.1 KHz frequency receiver, will I
> hear anything specific of magnetars?
Generally, super cooled electronic components stop working.
Try again.
> 44.1 KHz is the carrier-frequency this hypothetical receiver receives.
A super stupid frequency to pick. Generally for listening for natural
phenomena, you want a wide as possible bandwidth given the noise
floor.
> I place the frequency of this hypothetical AM radio carrier wave at
> 44.1 KHz for the same reason CDs use a sample rate of 44.1 KHz -- it
> is the minimum required to prevent aliasing.
Yeah, for digitized, audible music, you twit.
Are you expecting to hear alien rock and roll?
An AM receiver isn't digitizing anything, sample rates don't apply,
and aliasing doesn't apply.
Try again, idiot.
> AFAIK, space station orbit earth above the ionosphere so the
> limitations [preventing long-waves from outer space from reaching the
> Earth's surface] do not apply.
Probably the only thing you got right.
Radium
> In rec.radio.amateur.space Radium <gluceg...@gmail.com> wrote:
> > Okay. But what if this is a supercooled AM radio receiver on a
> > spaceship orbiting Earth? If I am on a space station like MIR and this
> > station has a supercooled AM radio 44.1 KHz frequency receiver, will I
> > hear anything specific of magnetars?
> Generally, super cooled electronic components stop working.
Isn't a super cooled receiver less vulnerable to thermal noise than a
receiver of a higher temperature? This is why SETI super-cools their
radio receivers. So that the heat will not generate electric currents
that would drown-out the intended signals in hiss.
> > 44.1 KHz is the carrier-frequency this hypothetical receiver receives.
> A super stupid frequency to pick. Generally for listening for natural
> phenomena, you want a wide as possible bandwidth given the noise
> floor.
But humans only hear from 20 to 20,000 Hz. So why use a higher
frequency?
>From what you think, what is the best frequency for listening to
magnetars and other natural phenomena?
> > I place the frequency of this hypothetical AM radio carrier wave at
> > 44.1 KHz for the same reason CDs use a sample rate of 44.1 KHz -- it
> > is the minimum required to prevent aliasing.
> An AM receiver isn't digitizing anything, sample rates don't apply,
> and aliasing doesn't apply.
Isn't it true that the carrier-frequency must be at least 2x the
highest intended frequency of the modulator signal?
I am not talking about sample rates. I am talking about carrier
frequency. From the answers to my previous questions regarding carrier
frequencies, I thought it was established that you mathematically
can't have a modulator frequency more than 0.5x the carrier-frequency.
What happened?
Since humans hear up to 20 KHz it is mathematically-required that the
carrier frequency be at least 40 KHz or 2 x 20 KHz. Due to physical
factors it would be most practical to use a carrier frequency slightly
higher than 2x the maximum intended modulator frequency -- hence 44.1
KHz.
ji...@specsol.spam.sux.com
If you had the slightest bit of education or common sense, you would
be asking a question that makes sense instead of going on forever
about the minutiae of what makes your questions idiotic.
Here's a question that makes sense:
Are there any sources of RF energy outside our solar system that
could possiblely be detected given an environment free of the
constraints of the ionsphere and man made noise?
If so, what frequecy or frequencies would they be seen at and what
would be the general characteristics of such a signal?
Also, the same question but operating from the Earth's surface.
And this is an astronomy/astro-physics question, not an electromagnetics
question.
Notice there is nothing in there about "supercooling", modulation
type, sampling rates, or any of the other nonsense in your question.
Babbling twit.
George Dishman
"Radium" <gluc...@gmail.com> wrote in message
news:1184373407.5...@m37g2000prh.googlegroups.com...
..
> Isn't it true that the carrier-frequency must be at least 2x the
> highest intended frequency of the modulator signal?
No.
> I am not talking about sample rates. I am talking about carrier
> frequency. From the answers to my previous questions regarding carrier
> frequencies, I thought it was established that you mathematically
> can't have a modulator frequency more than 0.5x the carrier-frequency.
No. You can't have a sampling rate less than twice the highest
frequency in the source without aliasing, but that refers only
to sampling.
> What happened?
At a guess, you misunderstood the context of the answers to
your previous questions, or those who answered misunderstood
the context of your questions.
HTH
George
Andrew Smallshaw
>
> If you had the slightest bit of education or common sense, you would
> be asking a question that makes sense instead of going on forever
> about the minutiae of what makes your questions idiotic.
This is Radium's usual posting style - to produce some random,
obvious ideas as if no one has ever considered them before, and
then to lightly dismiss any practical difficulties anyone points
out as trivial. After all, he's the genius, you're expected to
put in all the leg work to make a ridiculous idea actually work.
I've seen him all to many times before on the likes of
alt.comp.hardware.homebuilt. It's worth looking at the Goggle
archives to see just what nonsense he comes up with. My favourite
example is Radium's understanding of semiconductors:
> Why is silicon needed in the 1st place? For that matter, why any semi-
> conductor? Why not just use the copper electric circuits? Semi-
> conductors are half-way between conductor and insulator?
I think that that says it all.
--
Andrew Smallshaw
and...@sdf.lonestar.org
Radium
> "Radium" <gluceg...@gmail.com> wrote in message
>
> news:1184373407.5...@m37g2000prh.googlegroups.com...
> ..
>
> > Isn't it true that the carrier-frequency must be at least 2x the
> > highest intended frequency of the modulator signal?
>
> No.
Karl Uppiano sharply disagrees.
Karl Uppiano explained in http://groups.google.com/group/sci.energy/msg/a74d4d8ddcea47a5?hl=en&
:
> The highest modulating frequency for AM must be less than 1/2 the carrier
> frequency. Conversely, the lowest carrier frequency must be twice the
> highest modulating frequency. Period. I don't care what specific frequencies
> and/or energies and/or colors you propose.
>
> If you want to modulate at 20KHz, the carrier must be at least 40KHz. It is
> no coincidence that CD audio uses a 44.1KHz sample rate. It is essentially
> the same principle. If you exceed the Nyquist criterion, the sidebands
> overlap the baseband (i.e., aliasing occurs) and you cannot unambiguously
> decode the original modulation.
So who is right and who is wrong? I am so interested yet so frustrated
over this!
I keep getting conflicting answers about this topic. Its driving me
crazy!!!!!!!!!!!!!!!!!
WTF is going on here??????????????????!!!!!!!!!!!!!!!!!!!!?!?!?!?!?!
ji...@specsol.spam.sux.com
> Karl Uppiano sharply disagrees.
WTF is going on is that you can't ask a meaningful question.
Here's some reality:
Q: Do extra terrestrial objects generate radio signals that can be heard
on Earth?
A. Yes, thousands and thousands of them. The field is called Radio
Astronomy. Google for more information.
Q: What frequency do they generate?
A: Basically, all of them. Most natural sources of RF are broad band
generators much like an electric arc.
Q: Where would one listen for signal?
A: Usually from around 1 GHz to hundreds of GHz. Some objects in the
solar system generate signals down into the tens of MHz but antenna
size and the ionosphere place a practical lower limit of around 100 MHz.
Q: Is the signal AM or FM or what?
A: None of the above. Modulation implies a carrier with information.
Natural objects generate broad band RF noise.
Q: Don't some of the sources vary in some way?
A: Some of them vary in magnitude over time, i.e. they get louder
and weaker periodically. Some sources are "bursty", i.e. most of
the time the are not there, then for some period of time they are.
Q: Do magnetars generate signals?
A: Some do, some don't seem to.
Q: Are these signals audible?
A: Depends on what you mean. If you hooked a speaker to a radio telescope,
you would hear white noise, i.e. a hissing sound much like what you
hear on an FM radio between stations.
George Dishman
> On Jul 14, 1:17 am, "George Dishman" <geo...@briar.demon.co.uk> wrote:
>> "Radium" <gluceg...@gmail.com> wrote in message
>>
>> news:1184373407.5...@m37g2000prh.googlegroups.com...
>> ..
>>
>> > Isn't it true that the carrier-frequency must be at least 2x the
>> > highest intended frequency of the modulator signal?
>>
>> No.
>
> Karl Uppiano sharply disagrees.
>
> Karl Uppiano explained in
> http://groups.google.com/group/sci.energy/msg/a74d4d8ddcea47a5?hl=en&
He is wrong. The basis of AM is that the sine wave
carrier is multiplied by another signal which can be
treated as a sum of sines. The relevant maths is:
http://www.sosmath.com/trig/prodform/prodform.html
If the carrier frequency if fc and the modulation has
frequencies up to fm then you get sidebands like
this:
http://en.wikipedia.org/wiki/Image:Am-sidebands.png
If you multiply 44.1kHz by a band from 20Hz to 20kHz,
you get an upper sideband given 44.12kHz to 64.1kHz
and a lower sideband from 44.08kHz down to 24.1kHz
>> The highest modulating frequency for AM must be less than 1/2 the carrier
>> frequency. Conversely, the lowest carrier frequency must be twice the
>> highest modulating frequency. Period. I don't care what specific
>> frequencies
>> and/or energies and/or colors you propose.
>>
>> If you want to modulate at 20KHz, the carrier must be at least 40KHz. It
>> is
>> no coincidence that CD audio uses a 44.1KHz sample rate. It is
>> essentially
>> the same principle. If you exceed the Nyquist criterion, the sidebands
>> overlap the baseband (i.e., aliasing occurs) and you cannot unambiguously
>> decode the original modulation.
Nyquist applies to sampling.
> So who is right and who is wrong?
Look at the maths, it is never wrong. Modulating fc
with fm gives a lowest frequency of fc-fm so as long
as fc > fm, you don't get aliasing.
George
Sam Wormley
Amplitude modulation, in the communications world, has a definite
structure--I suspect that magnetar spectra don't exhibit amplitude
modulation characteristics.
However, I see, that amplitude modulation is appropriate in
astrophysics--for example...
Double Mode Cepheids with Amplitude Modulation
http://sait.oat.ts.astro.it/MSAIt770106/PDF/2006MmSAI..77..563M.pdf
Chris L Peterson
wrote:
> Amplitude modulation, in the communications world, has a definite
> structure--I suspect that magnetar spectra don't exhibit amplitude
> modulation characteristics.
A magnetar is a type of pulsar. You have a signal at some frequency (or
range frequencies) that varies in amplitude with time (as the object
spins). That's the very definition of amplitude modulation. Nearly every
radio source around shows some degree of amplitude modulation as the
result of spin. This includes objects radiating well out of the radio
band, as well. Optical binaries exhibit AM. Starspots show up as AM.
Rotating asteroids are AM. Cepheids. Cataclysmic variables. Etc.
ji...@specsol.spam.sux.com
The definitions for all types of modulation involve a carrier frequency.
Since natural phenomena generate broad band noise, it is arm waving
at best to call the variations in amplitude "amplitude modulation".
What it is is a broad band source that periodically varies in signal
strength.
Chris L Peterson
>The definitions for all types of modulation involve a carrier frequency.
>
>Since natural phenomena generate broad band noise, it is arm waving
>at best to call the variations in amplitude "amplitude modulation".
>
>What it is is a broad band source that periodically varies in signal
>strength.
There's nothing that defines how narrow a band need be to qualify as a
"carrier". Many modern communication systems are spread spectrum, which
means the carrier may be very broad. Such systems are certainly
modulated. Also, many astronomical sources are not broadband at all, but
radiate across a narrow spectrum.
Radium
> Look at the maths, it is never wrong. Modulating fc
> with fm gives a lowest frequency of fc-fm so as long
> as fc > fm, you don't get aliasing.
So an fm of 10 KHz would work on an fc of 10 KHz?
ji...@specsol.spam.sux.com
> On Sun, 15 Jul 2007 00:55:01 GMT, ji...@specsol.spam.sux.com wrote:
> >The definitions for all types of modulation involve a carrier frequency.
> >
> >Since natural phenomena generate broad band noise, it is arm waving
> >at best to call the variations in amplitude "amplitude modulation".
> >
> >What it is is a broad band source that periodically varies in signal
> >strength.
> There's nothing that defines how narrow a band need be to qualify as a
> "carrier". Many modern communication systems are spread spectrum, which
> means the carrier may be very broad. Such systems are certainly
> modulated. Also, many astronomical sources are not broadband at all, but
> radiate across a narrow spectrum.
Spread spectrum technology uses discrete frequency hopping, not a
broad band signal as a carrier.
If I have a transmitter hooked to an antenna swaying in the breeze
such that the received signal strength is varying, would you call
that AM?
If the side lobes of a search radar are big enough, you can receive
them no matter where the radar points. The signal strength goes up
and down and goes up dramatically when you are swept by the main
lobe. Would you call that AM?
ji...@specsol.spam.sux.com
What part of "as long as fc is greater that fm" are you too blazingly
stupid to understand?
Is 10 KHz bigger than 10 KHz?
Idiot.
Chris L Peterson
>Spread spectrum technology uses discrete frequency hopping, not a
>broad band signal as a carrier.
That's one spread spectrum method. Not the only one. But regardless, it
still presents as a broad band carrier.
>If I have a transmitter hooked to an antenna swaying in the breeze
>such that the received signal strength is varying, would you call
>that AM?
Absolutely.
>If the side lobes of a search radar are big enough, you can receive
>them no matter where the radar points. The signal strength goes up
>and down and goes up dramatically when you are swept by the main
>lobe. Would you call that AM?
AM is a variation in amplitude of some signal- any signal- with time.
Yes, your radar is a type of amplitude modulation. If it were a deep
space signal, it is the modulation of amplitude that would most catch
our attention.
Certainly AM carries a somewhat narrower meaning when discussing
communications that it does when looking at astronomical signals. But
the same underlying theory works for analyzing any signal that varies in
intensity with time.
ji...@specsol.spam.sux.com
> >Spread spectrum technology uses discrete frequency hopping, not a
> >broad band signal as a carrier.
> That's one spread spectrum method. Not the only one. But regardless, it
> still presents as a broad band carrier.
Nope, all spread spectrum is based on discrete frequencies with
frequency hopping of some sort.
It is only "broad band" if you integrate over multiple hops.
The carrier at each frequency is quite conventional.
Google for it.
> >If I have a transmitter hooked to an antenna swaying in the breeze
> >such that the received signal strength is varying, would you call
> >that AM?
> Absolutely.
> >If the side lobes of a search radar are big enough, you can receive
> >them no matter where the radar points. The signal strength goes up
> >and down and goes up dramatically when you are swept by the main
> >lobe. Would you call that AM?
> AM is a variation in amplitude of some signal- any signal- with time.
Nope, mathematically AM is defined as a single carrier frequency
multipled by the modulation frequency. That you get a variation in
amplitude is an effect, not a definition.
It is a bit of a stretch to call a signal comprised of every frequency
over a 100 GHz span AM.
> Yes, your radar is a type of amplitude modulation. If it were a deep
> space signal, it is the modulation of amplitude that would most catch
> our attention.
> Certainly AM carries a somewhat narrower meaning when discussing
> communications that it does when looking at astronomical signals. But
> the same underlying theory works for analyzing any signal that varies in
> intensity with time.
I'm afraid my background IS communications so I have to say astronomers
are arm waving when they call astronomical signals AM unless ET is
phoning home.
Radium
> The relevant maths is:
>
> http://www.sosmath.com/trig/prodform/prodform.html
>
The above link says nothing about amplitude-modulation
KLM
Radium wrote:
> On Jul 13, 2:15 pm, j...@specsol.spam.sux.com wrote:
> > In rec.radio.amateur.space Radium <gluceg...@gmail.com> wrote:
> >
> > > Hi:
> > > Do magnetars emit AM radio waves below the medium-wave range? If so,
> > > how do we detect these waves? Can these waves be heard on the AM
> > > radio? If so, what do they sound like?
> >
> > Frequencies above approximately 100 MHz almost always get through
> > the ionization layers.
> >
> > Frequencies in the approximate range of 10 MHz to 100 MHz sometimes
> > get through
> >
> > Frequencies below approximately 10 MHz almost never get through.
> >
> > So, if by "the AM radio" you mean a Broadcast Band radio which
> > runs from about .5 MHz to 1.2 MHz, not a chance in hell of ever
> > hearing anything from off the planet.
> >
> > Try again.
>
> Okay. But what if this is a supercooled AM radio receiver on a
> spaceship orbiting Earth? If I am on a space station like MIR and this
> station has a supercooled AM radio 44.1 KHz frequency receiver, will I
> hear anything specific of magnetars?
Nothing "specific". Thats the whole point. What exactly did you
hope to hear that you think is significant? Obviously you have
something in mind.
KLM
George Dishman
<ji...@specsol.spam.sux.com> wrote in message
news:o8mom4-...@mail.specsol.com...
> In rec.radio.amateur.space Chris L Peterson <c...@alumni.caltech.edu>
> wrote:
>> On Sun, 15 Jul 2007 03:45:01 GMT, ji...@specsol.spam.sux.com wrote:
>
>> >Spread spectrum technology uses discrete frequency hopping, not a
>> >broad band signal as a carrier.
That is correct, it is a narrow band carrier which
moves.
>> That's one spread spectrum method. Not the only one. But regardless, it
>> still presents as a broad band carrier.
>
> Nope, all spread spectrum is based on discrete frequencies with
> frequency hopping of some sort.
Not always, consider the use of a frequency-shifted
fast PRBS as the carrier. Of course it is more
usual to use the PRBS to define the hop sequence in
the style you describe above but as you say your
background is communications, I'm sure you are aware
of the relationships between hop rate and carrier
spacing which lead to a band-limited white spectrum.
>> AM is a variation in amplitude of some signal- any signal- with time.
>
> Nope, mathematically AM is defined as a single carrier frequency
> multipled by the modulation frequency. That you get a variation in
> amplitude is an effect, not a definition.
>
> It is a bit of a stretch to call a signal comprised of every frequency
> over a 100 GHz span AM.
Consider applying audio (with a DC bias) to a light
bulb and receiving it with a photocell. The carrier
is much more than 100 GHz wide, but I would still
call that AM, YMMV.
> I'm afraid my background IS communications ...
Mine too ;-)
George
George Dishman
It says:
sin(a)sin(b) = 1/2 * [ cos(a-b) - cos(a+b) ]
Take a carrier at frequency fc:
Vc = sin(2*pi*fc*t)
and a typical modulating signal at fm:
Vm = sin(2*pi*fm*t)
Amplitude modulation involves multiplying those
together with an offset so that there is always
some level of carrier so the transmitted signal
is:
Vt = Vc * (1 + M * Vm)
where 0 < M < 1
You get components at cos(2*pi*(fc-fm)*t) and
cos(2*pi*(fc+fm)*t) as well as the carrier at fc.
George
George Dishman
fc > fm means fc should be greater than fm, not the same.
For fm = 10,000Hz and fc = 10,001Hz you get a lower
sideband at 1Hz and an upper sideband at 20,001Hz.
If you modulate 10kHz with 10Khz, the lower sideband
becomes 0Hz or DC. The value of that depends on the
phase of the modulating signal relative to the carrier
(which is now constant since they are at the same
frequency). Of course sending DC to an antenna won't
give you a transmitted signal but it doesn't produce
an alias either.
If you modulate 10,000Hz with 10,001Hz then your lower
sideband becomes -1Hz, and of course sin(-x) = sin(x)
so that is identical to a frequency of 1Hz which you
would get if you modulated with 9,999Hz. That ambiguity
is why we call such a signal an "alias", the 10,001Hz
signal appears after modulation then demodulation
masquerading as a signal of 9,999Hz.
George
Chris L Peterson
>Nope, all spread spectrum is based on discrete frequencies with
>frequency hopping of some sort.
I would not consider that to be an accurate description for a variety of
direct sequence techniques, where the carrier itself is phase or
frequency modulated by a pseudorandom sequence. Such a signal, viewed on
a spectrum analyzer, looks no different than many natural, moderately
narrowband sources. And there would be nothing stopping somebody from
amplitude modulating such a signal.
>Nope, mathematically AM is defined as a single carrier frequency
>multipled by the modulation frequency. That you get a variation in
>amplitude is an effect, not a definition.
Reference? I think you are confusing a single definition of AM with all
other definitions of the term.
>I'm afraid my background IS communications so I have to say astronomers
>are arm waving when they call astronomical signals AM unless ET is
>phoning home.
As an astronomer, I could as easily say you are arm waving by trying to
restrict the meaning of AM to a narrow definition used within your
field. The simple fact is that astronomers _do_ refer to signals that
vary in amplitude as a function of time as "amplitude modulated". Some
of those signals are broadband, and others are not (there are, for
example, amplitude modulated natural masers). This is normal usage
within the astronomical community, and it causes no confusion at all.
ji...@specsol.spam.sux.com
> <ji...@specsol.spam.sux.com> wrote in message
> news:o8mom4-...@mail.specsol.com...
> > In rec.radio.amateur.space Chris L Peterson <c...@alumni.caltech.edu>
> > wrote:
> >> On Sun, 15 Jul 2007 03:45:01 GMT, ji...@specsol.spam.sux.com wrote:
> >
> >> >Spread spectrum technology uses discrete frequency hopping, not a
> >> >broad band signal as a carrier.
> That is correct, it is a narrow band carrier which
> moves.
> >> That's one spread spectrum method. Not the only one. But regardless, it
> >> still presents as a broad band carrier.
> >
> > Nope, all spread spectrum is based on discrete frequencies with
> > frequency hopping of some sort.
> Not always, consider the use of a frequency-shifted
> fast PRBS as the carrier. Of course it is more
> usual to use the PRBS to define the hop sequence in
> the style you describe above but as you say your
> background is communications, I'm sure you are aware
> of the relationships between hop rate and carrier
> spacing which lead to a band-limited white spectrum.
If you frequency or phase modulate, the resultant spectrum may be
quite broad, but the carrier frequency is still discreate no matter
what you modulate with.
It may look like band limited white noise on a spectrum analyser,
but if you decompose in the time domain, it isn't.
> >> AM is a variation in amplitude of some signal- any signal- with time.
> >
> > Nope, mathematically AM is defined as a single carrier frequency
> > multipled by the modulation frequency. That you get a variation in
> > amplitude is an effect, not a definition.
> >
> > It is a bit of a stretch to call a signal comprised of every frequency
> > over a 100 GHz span AM.
> Consider applying audio (with a DC bias) to a light
> bulb and receiving it with a photocell. The carrier
> is much more than 100 GHz wide, but I would still
> call that AM, YMMV.
Got me there.
> > I'm afraid my background IS communications ...
> Mine too ;-)
> George
--
ji...@specsol.spam.sux.com
> >Nope, all spread spectrum is based on discrete frequencies with
> >frequency hopping of some sort.
> I would not consider that to be an accurate description for a variety of
> direct sequence techniques, where the carrier itself is phase or
> frequency modulated by a pseudorandom sequence. Such a signal, viewed on
> a spectrum analyzer, looks no different than many natural, moderately
> narrowband sources. And there would be nothing stopping somebody from
> amplitude modulating such a signal.
If you frequency or phase modulate, the spectrum does get broad, but
there is still a discreat carrier frequency.
While it may look like band limited white noise on a vanilla spectrum
analyzer, it isn't.
> >Nope, mathematically AM is defined as a single carrier frequency
> >multipled by the modulation frequency. That you get a variation in
> >amplitude is an effect, not a definition.
> Reference? I think you are confusing a single definition of AM with all
> other definitions of the term.
How about this one:
http://www.rfcafe.com/references/electrical/amplitude_modulation.htm
> >I'm afraid my background IS communications so I have to say astronomers
> >are arm waving when they call astronomical signals AM unless ET is
> >phoning home.
> As an astronomer, I could as easily say you are arm waving by trying to
> restrict the meaning of AM to a narrow definition used within your
> field. The simple fact is that astronomers _do_ refer to signals that
> vary in amplitude as a function of time as "amplitude modulated". Some
> of those signals are broadband, and others are not (there are, for
> example, amplitude modulated natural masers). This is normal usage
> within the astronomical community, and it causes no confusion at all.
So, is the square root of -1 j or i?
ji...@specsol.spam.sux.com
> Radium wrote:
What makes you think there is anything in that mind other than a
bunch of technical words and terms strung together in a random manner?
Radium
> fc > fm means fc should be greater than fm, not the same.
> For fm = 10,000Hz and fc = 10,001Hz you get a lower
> sideband at 1Hz and an upper sideband at 20,001Hz.
Sorry. I didn't read it correctly.
> If you modulate 10kHz with 10Khz, the lower sideband
> becomes 0Hz or DC. The value of that depends on the
> phase of the modulating signal relative to the carrier
> (which is now constant since they are at the same
> frequency). Of course sending DC to an antenna won't
> give you a transmitted signal but it doesn't produce
> an alias either.
>
> If you modulate 10,000Hz with 10,001Hz then your lower
> sideband becomes -1Hz, and of course sin(-x) = sin(x)
> so that is identical to a frequency of 1Hz which you
> would get if you modulated with 9,999Hz. That ambiguity
> is why we call such a signal an "alias", the 10,001Hz
> signal appears after modulation then demodulation
> masquerading as a signal of 9,999Hz.
Does this mean an fm of 10 KHz would work on an fc of
10.0000000000000000000001 KHz?
If so, then the minimum frequency required for my "project" would be
only 20.0000000000000000001 KHz. Or just anything above 20 KHz, even
if it's just an extremely extremely small number above 20,000. Right?
I apologize if readers find my question annoying.
To all:
I have a neurological disability called Asperger's Syndrome.
I would like to give you some information about my disability. The
reason I am posting this message about Asperger's is to help avoid any
potential misunderstandings [though it's probably too late].
I have been diagnosed with Asperger's Syndrome (AS). AS is a
neurological condition that causes significant impairment in social
interactions. People with AS see the world differently and this can
often bring them in conflict with conventional ways of thinking. They
have difficulty in reading body language, and interpreting subtle
cues. In my situation, I have significant difficulty with natural
conversation, reading social cues, and maintaining eye contact. This
can lead to a great deal of misunderstanding about my intent or my
behavior. For example, I may not always know what to say in social
situations, so I may look away or may not say anything. I also may not
always respond quickly when asked direct questions, but if given time
I am able express my ideas.
On Usenet, the text-equivalent of my disability is probably noticed. I
do apologize profusely, for any inconvenience it causes.
Thank you very much in advance for your understanding, cooperation,
and assistance.
ji...@specsol.spam.sux.com
Since all you are going to hear from astronomical sources is white
noise, it doesn't matter.
Aliasing is irrelevant.
The bigger issue with the frequency (other than plain stupidity) you
have choosen is that to be able to say you are hearing signals from
something specific in the sky as opposed to everything in the sky,
you need to have antenna directivity.
At a frequency of 44 KHz that means an antenna about 500 miles in
diameter and there isn't much that can be done to make it smaller.
Radium
> The bigger issue with the frequency (other than plain stupidity) you
> have choosen is that to be able to say you are hearing signals from
> something specific in the sky as opposed to everything in the sky,
> you need to have antenna directivity.
>
> At a frequency of 44 KHz that means an antenna about 500 miles in
> diameter and there isn't much that can be done to make it smaller.
Why does it need to be around 500 miles? Atomic-clock wrist-watches
receive extremely long wavelengths and are able to do so with their
tiny sizes. Couldn't something similar be done for my 'application'?
If not, then why?
ji...@specsol.spam.sux.com
You need directivity otherwise you are "listening" to everything, not
just one source or even a small number of sources.
Directivity is directly related to antenna physical size in wavelengths.
The wavelength of 44 KHz is roughly 7,000 meters.
You can't get around it.
George Dishman
> On Jul 15, 3:35 am, "George Dishman" <geo...@briar.demon.co.uk> wrote:
>
>> fc > fm means fc should be greater than fm, not the same.
>> For fm = 10,000Hz and fc = 10,001Hz you get a lower
>> sideband at 1Hz and an upper sideband at 20,001Hz.
>
> Sorry. I didn't read it correctly.
No problem, it's an easy point to miss and in fact
you are right in that it doesn't produce aliases.
>> If you modulate 10kHz with 10Khz, the lower sideband
>> becomes 0Hz or DC. The value of that depends on the
>> phase of the modulating signal relative to the carrier
>> (which is now constant since they are at the same
>> frequency). Of course sending DC to an antenna won't
>> give you a transmitted signal but it doesn't produce
>> an alias either.
>>
>> If you modulate 10,000Hz with 10,001Hz then your lower
>> sideband becomes -1Hz, and of course sin(-x) = sin(x)
>> so that is identical to a frequency of 1Hz which you
>> would get if you modulated with 9,999Hz. That ambiguity
>> is why we call such a signal an "alias", the 10,001Hz
>> signal appears after modulation then demodulation
>> masquerading as a signal of 9,999Hz.
>
> Does this mean an fm of 10 KHz would work on an fc of
> 10.0000000000000000000001 KHz?
Yes.
> If so, then the minimum frequency required for my "project" would be
> only 20.0000000000000000001 KHz. Or just anything above 20 KHz, even
> if it's just an extremely extremely small number above 20,000. Right?
In theory, but you are missing the bigger picture. Space
is not a perfect vacuum and in the vicinity of the Solar
System, the Solar wind pushes back the inter-stellar
medium. You can read more on that here:
http://en.wikipedia.org/wiki/Heliosphere
Both the material within the region and the ISM are almost
entirely ionised so are in a form known as "plasma", a
soup of charged particles. One of the feautures of plasma
is that is absorbs low frequency signals below what is
called the "plasma cutoff frequency". Basically below
the VHF band, space becomes increasingly opaque and signals
from stars don't reach us.
> I apologize if readers find my question annoying.
No problem.
George
Radium
wrote:
> "Radium" <gluceg...@gmail.com> wrote in message
> > If so, then the minimum frequency required for my "project" would be
> > only 20.0000000000000000001 KHz. Or just anything above 20 KHz, even
> > if it's just an extremely extremely small number above 20,000. Right?
> In theory, but you are missing the bigger picture. Space
> is not a perfect vacuum and in the vicinity of the Solar
> System, the Solar wind pushes back the inter-stellar
> medium. You can read more on that here:
> http://en.wikipedia.org/wiki/Heliosphere
Thanks for the link
> Both the material within the region and the ISM are almost
> entirely ionised so are in a form known as "plasma", a
> soup of charged particles. One of the feautures of plasma
> is that is absorbs low frequency signals below what is
> called the "plasma cutoff frequency". Basically below
> the VHF band, space becomes increasingly opaque and signals
> from stars don't reach us.
Do you think the SHF band [at most 30 GHz but at least 3 GHz] would be
better suited for this? Would it be the ideal range of frequencies for
detecting AM radio signals emitted by magnetars?
http://en.wikipedia.org/wiki/Super_high_frequency
SHF seems to be the sweet spot between frequencies that are high-
enough to rip through charged particles yet low-enough to be wireless
and long-distance. At EHF and above, it starts to get into the IR
range where long-distance wireless reception is not possible.
Dead Paul
> In rec.radio.amateur.space Chris L Peterson <c...@alumni.caltech.edu>
> wrote:
>> On Sun, 15 Jul 2007 03:45:01 GMT, ji...@specsol.spam.sux.com wrote:
>
>> >Spread spectrum technology uses discrete frequency hopping, not a broad
>> >band signal as a carrier.
>
>> That's one spread spectrum method. Not the only one. But regardless, it
>> still presents as a broad band carrier.
>
> Nope, all spread spectrum is based on discrete frequencies with frequency
> hopping of some sort.
>
> It is only "broad band" if you integrate over multiple hops.
>
> The carrier at each frequency is quite conventional.
>
> Google for it.
>
>> >If I have a transmitter hooked to an antenna swaying in the breeze such
>> >that the received signal strength is varying, would you call that AM?
>
>> Absolutely.
>
>> >If the side lobes of a search radar are big enough, you can receive
>> >them no matter where the radar points. The signal strength goes up and
>> >down and goes up dramatically when you are swept by the main lobe.
>> >Would you call that AM?
>
>> AM is a variation in amplitude of some signal- any signal- with time.
>
> Nope, mathematically AM is defined as a single carrier frequency multipled
> by the modulation frequency.
He is correct. AM is a variation in amplitude of some signal (usually
a carrier wave).
> That you get a variation in amplitude is an
> effect, not a definition.
So what, you can define it how you wish, the effect is still one of
variation in amplitude of the carrier. Definitions do not dictate what
happens.
> It is a bit of a stretch to call a signal comprised of every frequency
> over a 100 GHz span AM.
Such a signal may still be demodulated with an am receiver. Of course that
will not yield the full extent of the information from the signal that may
be had from it but just for the sake of "listening" to it as some want to
do there is nothing wrong with that.
>> Yes, your radar is a type of amplitude modulation. If it were a
deep
>> space signal, it is the modulation of amplitude that would most catch
>> our attention.
>
>> Certainly AM carries a somewhat narrower meaning when discussing
>> communications that it does when looking at astronomical signals. But
>> the same underlying theory works for analyzing any signal that varies
>> in intensity with time.
>
> I'm afraid my background IS communications so I have to say astronomers
> are arm waving when they call astronomical signals AM unless ET is
> phoning home.
Astronomical signals aren't derived from PLL's or crystals, they aren't
synthesized for receiving on radios with parameters tailored for speech or
music even if they are sometimes or often demodulated by them given the
right circumstances. I think you need to understand that radio sources in
space are complex and may have several facets operating simultaneously
each of which may tell a physicist a part of a story. You seem to be stuck
in some sort of pedantic communications rut.
--
___ _______ ___ ___ ___ __ ____
/ _ \/ __/ _ | / _ \ / _ \/ _ |/ / / / /
/ // / _// __ |/ // / / ___/ __ / /_/ / /__
/____/___/_/ |_/____/ /_/ /_/ |_\____/____/
Chris L Peterson
>> >Nope, mathematically AM is defined as a single carrier frequency
>> >multipled by the modulation frequency. That you get a variation in
>> >amplitude is an effect, not a definition.
>
>> Reference? I think you are confusing a single definition of AM with all
>> other definitions of the term.
>
>How about this one:
>
>http://www.rfcafe.com/references/electrical/amplitude_modulation.htm
That isn't a mathematical definition of amplitude modulation. It's an
example of an implementation of amplitude modulation for the purpose of
deliberately encoding information. And in fact, it's idealized since no
carrier frequency can ever be perfectly discrete. I've never seen a rule
for how narrow or broad a carrier needs to be in order for an amplitude
modulated form of it to be allowed to carry the term "amplitude
modulation". And I'm sure you haven't either.
Like I said, you're confusing one particular usage of the term (one that
isn't even relevant to astronomy) with the more general meaning- a
signal that is modulated in intensity as a function of time.
In astronomy, we are concerned mostly with two types of modulation.
Amplitude modulation is what we usually observe with a range of
phenomena that show a variation in intensity with time, where the
carrier signal may range from hard x-rays through radio waves (and be
either narrowband or broadband), and the modulation frequency varies
from a few kilohertz to many years per cycle. Frequency modulation is
what we observe in spectroscopic binaries, large rotating objects, and
other sources that have a line-of-site motion component. The carrier
signal again ranges from x-rays through radio, but the modulation cycle
time is measured in hours or more.
Chris L Peterson
>Since all you are going to hear from astronomical sources is white
>noise, it doesn't matter.
There are many astronomical sources that are not white noise. There are
masers. There are natural filters that convolve some background signal
that may or may not itself have structure. There are signals that are
modulated in amplitude, phase, or frequency.
If the Universe consisted of nothing but noise sources, there wouldn't
be much value in radio astronomy.
Andrew Smallshaw
> On Jul 15, 10:05 am, j...@specsol.spam.sux.com wrote:
>
>> The bigger issue with the frequency (other than plain stupidity) you
>> have choosen is that to be able to say you are hearing signals from
>> something specific in the sky as opposed to everything in the sky,
>> you need to have antenna directivity.
> Why does it need to be around 500 miles? Atomic-clock wrist-watches
> receive extremely long wavelengths and are able to do so with their
> tiny sizes. Couldn't something similar be done for my 'application'?
> If not, then why?
Since when have radio controlled watches indicated the direction
to their transmitters?
There are ways of avoiding the need for such a large antenna, but
it isn't straightforward. Basically you'd work from two sites at
least that far apart and combine the signals via interferometry.
But the maths for this will be above your head. If you are interested
in this then change the habit of a lifetime and research the area
thoroughly for yourself _before_ spouting forth yet more ill-conceived
pseudoscientific "theories".
--
Andrew Smallshaw
and...@sdf.lonestar.org
Rich
> On Sun, 15 Jul 2007 00:55:01 GMT, j...@specsol.spam.sux.com wrote:
> >The definitions for all types of modulation involve a carrier frequency.
>
> >Since natural phenomena generate broad band noise, it is arm waving
> >at best to call the variations in amplitude "amplitude modulation".
>
> >What it is is a broad band source that periodically varies in signal
> >strength.
>
> There's nothing that defines how narrow a band need be to qualify as a
> "carrier". Many modern communication systems are spread spectrum, which
> means the carrier may be very broad. Such systems are certainly
> modulated. Also, many astronomical sources are not broadband at all, but
> radiate across a narrow spectrum.
Bet he feels less superior now....10:1 he slithers off.
ji...@specsol.spam.sux.com
You ignored the most important question; is the square root of -1 i or j?
Chris L Peterson
>You ignored the most important question; is the square root of -1 i or j?
I use i. But I certainly recall electronics classes where j was used to
avoid confusion. I don't worry too much about the choice of symbol.
ji...@specsol.spam.sux.com
> >You ignored the most important question; is the square root of -1 i or j?
> I use i. But I certainly recall electronics classes where j was used to
> avoid confusion. I don't worry too much about the choice of symbol.
There you have it.
The basic problem is we speak two different languages; it's j.
It is like telling the Marine Corp and the Air Force to "secure a building".
The Corp orders air and artillery strikes, does a frontal infantry
assualt and establishes a defensive perimeter inside the building.
The Air Force takes out a three year lease with an option to buy.
Sam Wormley
> On Sat, 14 Jul 2007 23:36:08 GMT, Sam Wormley <swor...@mchsi.com>
> wrote:
>
>> Amplitude modulation, in the communications world, has a definite
>> structure--I suspect that magnetar spectra don't exhibit amplitude
>> modulation characteristics.
>
> A magnetar is a type of pulsar. You have a signal at some frequency (or
> range frequencies) that varies in amplitude with time (as the object
> spins). That's the very definition of amplitude modulation. Nearly every
> radio source around shows some degree of amplitude modulation as the
> result of spin. This includes objects radiating well out of the radio
> band, as well. Optical binaries exhibit AM. Starspots show up as AM.
> Rotating asteroids are AM. Cepheids. Cataclysmic variables. Etc.
>
> _________________________________________________
>
> Chris L Peterson
> Cloudbait Observatory
> http://www.cloudbait.com
1E 1048.1-5937 (Magnitar) Spectra
http://images.google.com/images?q=1E%201048.1-5937%20spectra
SGR 1806-20 (Magnitar) spectra
http://images.google.com/images?q=SGR+1806-20+spectra
Radium
wrote:
> http://en.wikipedia.org/wiki/Heliosphere
>
> Both the material within the region and the ISM are almost
> entirely ionised so are in a form known as "plasma", a
> soup of charged particles. One of the feautures of plasma
> is that is absorbs low frequency signals below what is
> called the "plasma cutoff frequency". Basically below
> the VHF band, space becomes increasingly opaque and signals
> from stars don't reach us.
Okay so it seems like a no win-situation here. Too low and plasmas
will cut it off. Too high and you lose strength really fast.
http://www.terabeam.com/support/calculations/free-space-loss.php
>From the above link, it seems that a higher-frequency radio wave would
lose its strength faster than a lower-frequency radio wave of the same
original strength would.
At the start of the transmission, both the higher and lower
frequencies maybe at the same power [watts-per-meter-squared].
However, the higher frequency will lose its strength -- and likely
dissipate into the noise floor -- quicker than the lower-frequency.
Hence, reception of higher-frequencies requires that the transmitter
and receiver be closer together.
Chris L Peterson
>There you have it.
>
>The basic problem is we speak two different languages; it's j.
No, I'm bilingual. You apparently are not <g>.
ji...@specsol.spam.sux.com
> >There you have it.
> >
> >The basic problem is we speak two different languages; it's j.
> No, I'm bilingual. You apparently are not <g>.
Blasphemy; unbelievers are not allowed to speak the one true language.
In a broad band astronomical signal, where does one find the upper and
lower side bands of AM?
Are they standing tall and proud as they should, or are they lost in
the hiss and spit of a signal that is best described as "shit happens"?
KLM
Rozagy
> potential misunderstandings [though it's probably too late].
>
> neurological condition that causes significant impairment in social
> interactions. People with AS see the world differently and this can
> often bring them in conflict with conventional ways of thinking. They
> have difficulty in reading body language, and interpreting subtle
> cues. In my situation, I have significant difficulty with natural
> conversation, reading social cues, and maintaining eye contact. This
> can lead to a great deal of misunderstanding about my intent or my
> behavior. For example, I may not always know what to say in social
> situations, so I may look away or may not say anything. I also may not
> always respond quickly when asked direct questions, but if given time
> I am able express my ideas.
>
> On Usenet, the text-equivalent of my disability is probably noticed. I
> do apologize profusely, for any inconvenience it causes.
>
> Thank you very much in advance for your understanding, cooperation,
> and assistance.
Good on you, mate!
Welcome to the Aspie community!!!!!
You'll fit right in - make yourself at home!
The world is chaninng :-)
Roza
xxx
www.myspace.com/rozagy
George Dishman
The sweet spot is the waterhole:
http://www.setileague.org/general/waterhol.htm
Pulsars are detected at somewhat lower frequencies
and quite a bit of work has been done studying the
background at 408 MHz:
http://www.jb.man.ac.uk/research/cmb/foreground.html
I assume you are expecting a spinning magnetar to
give a signal simliar to a pulsar.
George
George Dishman
George Dishman
> wrote:
>
> > http://en.wikipedia.org/wiki/Heliosphere
>
> > Both the material within the region and the ISM are almost
> > entirely ionised so are in a form known as "plasma", a
> > soup of charged particles. One of the feautures of plasma
> > is that is absorbs low frequency signals below what is
> > called the "plasma cutoff frequency". Basically below
> > the VHF band, space becomes increasingly opaque and signals
> > from stars don't reach us.
>
> Okay so it seems like a no win-situation here. Too low and plasmas
> will cut it off. Too high and you lose strength really fast.
>
> http://www.terabeam.com/support/calculations/free-space-loss.php
>
> From the above link, it seems that a higher-frequency radio wave would
> lose its strength faster than a lower-frequency radio wave of the same
> original strength would.
That link is only relevant for a fixed size antenna. See
my other post on the waterhole and the 408 MHz band.
George