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Author: Chris L PetersonChris L Peterson Date: Jul 14, 2007 21:24
>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.
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Author: jimpjimp Date: Jul 14, 2007 22:05
In rec.radio.amateur.space Chris L Peterson alumni.caltech.edu> 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?
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Author: RadiumRadium Date: Jul 14, 2007 22:05
On Jul 14, 4:11 pm, "George Dishman" briar.demon.co.uk> wrote:
The above link says nothing about amplitude-modulation
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Author: KLMKLM Date: Jul 15, 2007 01:58
Radium wrote:
>> In rec.radio.amateur.space Radium gmail.com> wrote:
>>>>> Hi:
>>> Do magnetars emit AM radio waves below the medium-wave range...
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Author: George DishmanGeorge Dishman Date: Jul 15, 2007 03:12
> In rec.radio.amateur.space Chris L Peterson alumni.caltech.edu>
> 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.
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Author: George DishmanGeorge Dishman Date: Jul 15, 2007 03:24
> On Jul 14, 4:11 pm, "George Dishman" briar.demon.co.uk> wrote:
>>
> The above link says nothing about amplitude-modulation
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)
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Author: George DishmanGeorge Dishman Date: Jul 15, 2007 03:35
> On Jul 14, 4:11 pm, "George Dishman" briar.demon.co.uk> wrote:
>>> 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?
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.
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Author: Chris L PetersonChris L Peterson Date: Jul 15, 2007 07:36
>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.
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Author: jimpjimp Date: Jul 15, 2007 09:05
In rec.radio.amateur.space George Dishman briar.demon.co.uk> wrote:
> specsol.spam.sux.com> wrote in message
> news:o8mom4-on8.ln1@mail.specsol.com...>> In rec.radio.amateur.space Chris L Peterson alumni.caltech.edu>
>> 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.
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Author: jimpjimp Date: Jul 15, 2007 09:15
In rec.radio.amateur.space Chris L Peterson alumni.caltech.edu> wrote:>>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.
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