AM electromagnetic waves: 20 KHz modulationfrequencyonanastronomically-low carrier frequency
 

isw wrote:

What is the difference between AM and DSB?




AM is a process. DSB (double sideband), with carrier, is it's most
simple
result. DSB without carrier (suppressed carrier dsb) requires using, at
least, a balanced mixer as the AM multiplier.

And requires, for proper reception, that a carrier be recreated at the
receiver which has not only the amplitude of the original, but also its
exact phase. Absent some sort of "pilot" to get things synchronized,
this makes reception very difficult.

Isaac


Try a Costas loop.

AM electromagnetic waves: 20 KHz modulation frequencyonanastronomically-low carrier frequency
 

On Thu, 05 Jul 2007 00:00:45 -0700, Ron Baker, Pluralitas! wrote:

Suppose you have a 1 MHz sine wave whose amplitude
is multiplied by a 0.1 MHz sine wave.
What would it look like on an oscilloscope?

This is close, but not to scale:
http://en.wikipedia.org/wiki/Amplitude_modulation
The animation shows the "envelope".

What would it look like on a spectrum analyzer?

One vertical "spike" at 1 MHz with smaller spikes at .9 and 1.1 MHz. The
height of the two side spikes, depends on the depth of modulation.
In this case, the carrier is in the middle, and the sidebands are on the
sides.

Then suppose you have a 1.1 MHz sine wave added
to a 0.9 MHz sine wave.
What would that look like on an oscilloscope?

whatever 0.9 MHz superimposed on 1.1 MHz looks like. ;-)

What would that look like on a spectrum analyzer?

One spike at each input frequency, 0.9 and 1.1 MHz. If they're mixed
nonlinearly, then you get modulation, as above.

Hope This Helps!
Rich

AM electromagnetic waves: 20 KHzmodulationfrequencyonanastronomically-low carrier frequency
 

On 7/6/07 12:15 PM, in article
, "isw"
wrote:

In article ,
Don Bowey wrote:

On 7/6/07 9:36 AM, in article
, "isw"
wrote:

In article ,
Don Bowey wrote:

On 7/5/07 10:27 PM, in article ,
"Ron Baker, Pluralitas!" wrote:


"Don Bowey" wrote in message
...
On 7/5/07 12:00 AM, in article ,
"Ron Baker, Pluralitas!" wrote:


"Don Bowey" wrote in message
...
On 7/4/07 8:42 PM, in article ,
"Ron
Baker, Pluralitas!" wrote:


"Don Bowey" wrote in message
...
On 7/4/07 10:16 AM, in article
,
"Ron Baker, Pluralitas!" wrote:


"Don Bowey" wrote in message
...
On 7/4/07 7:52 AM, in article
,
"Ron
Baker, Pluralitas!" wrote:

snip


cos(a) * cos(b) = 0.5 * (cos[a+b] + cos[a-b])

Basically: multiplying two sine waves is
the same as adding the (half amplitude)
sum and difference frequencies.

No, they aren't the same at all, they only appear to be the same
before
they are examined. The two sidebands will not have the correct
phase
relationship.

What do you mean? What is the "correct"
relationship?


One could, temporarily, mistake the added combination for a full
carrier
with independent sidebands, however.




(For sines it is
sin(a) * sin(b) = 0.5 * (cos[a-b]-cos[a+b])
= 0.5 * (sin[a-b+90degrees] -
sin[a+b+90degrees])
= 0.5 * (sin[a-b+90degrees] +
sin[a+b-90degrees])
)

--
rb





When AM is correctly accomplished (a single voiceband signal is
modulated

The questions I posed were not about AM. The
subject could have been viewed as DSB but that
wasn't the specific intent either.

What was the subject of your question?

Copying from my original post:

Suppose you have a 1 MHz sine wave whose amplitude
is multiplied by a 0.1 MHz sine wave.
What would it look like on an oscilloscope?
What would it look like on a spectrum analyzer?

Then suppose you have a 1.1 MHz sine wave added
to a 0.9 MHz sine wave.
What would that look like on an oscilloscope?
What would that look like on a spectrum analyzer?




So the first (1) is an AM question and the second (2) is a non-AM
question......

What is the difference between AM and DSB?




AM is a process. DSB (double sideband), with carrier, is it's most simple
result. DSB without carrier (suppressed carrier dsb) requires using, at
least, a balanced mixer as the AM multiplier.

And requires, for proper reception, that a carrier be recreated at the
receiver which has not only the amplitude of the original,

There is no need at all to match the carrier amplitude of the original
signal. You can use an excessively high carrier injection amplitude with no
detrimental affect, but if the injected carrier is too little, the
demodulated signal will be over modulated and sound distorted.

but also its exact phase.

Exact, not required. The closer the better, however.

Well, OK, the phase must at least bear a constant relationship to the
one that created the signal. If you inject a carrier that has a
quadrature relationship to the one that created the DSB signal, the
output will be PM (phase modulation). In between zero and 90 degrees,
the output is a combination of the two. If the injected carrier is not
at precisely the proper frequency, the phase will roll around and the
output will be unintelligible.

Not unintelligible.... Donald Duckish.

On a more practical side, however, most receiver filters for ssb will
essentially remove one sideband if there are two, and can attenuate a
carrier so the local product detector can do it's job resulting in improved
receiving conditions. But this is more advanced than the Ops questions.

Don

Isaac

AM electromagnetic waves: 20 KHz modulation frequency on an astronomically-low carrier frequency
 

On Thu, 05 Jul 2007 20:02:15 -0600, Bob Myers wrote:
"John Fields" wrote in message

You missed my point, which was that in a mixer (which the ear is,
since its amplitude response is nonlinear) as the two carriers
approach each other the difference frequency will go to zero and the
sum frequency will go to the second harmonic of either carrier,
making it largely appear to vanish into the fundamental.

Sorry, John - while the ear's amplitude response IS nonlinear, it
does not act as a mixer. "Mixing" (multiplication) occurs when
a given nonlinear element (in electronics, a diode or transistor, for
example) is presented with two signals of different frequencies.
But the human ear doesn't work in that manner - there is no single
nonlinear element which is receiving more than one signal.

Sure there is - the cochlea. (well, the whole middle ear/inner ear
system.)

What would the output look like if you summed a 300Hz tone and a 400Hz
tone and sent the sum to a log amp and spectrum analyzer/fft?

Thanks,
Rich

AM electromagnetic waves: 20 KHz modulation frequencyonanastronomically-low carrier frequency
 

On 7/4/07 8:42 PM, in article , "Ron
Baker, Pluralitas!" wrote:


"Don Bowey" wrote in message
...
On 7/4/07 10:16 AM, in article ,
"Ron Baker, Pluralitas!" wrote:


"Don Bowey" wrote in message
...
On 7/4/07 7:52 AM, in article ,
"Ron
Baker, Pluralitas!" wrote:

snip


cos(a) * cos(b) = 0.5 * (cos[a+b] + cos[a-b])

Basically: multiplying two sine waves is
the same as adding the (half amplitude)
sum and difference frequencies.

No, they aren't the same at all, they only appear to be the same before
they are examined. The two sidebands will not have the correct phase
relationship.

What do you mean? What is the "correct"
relationship?


One could, temporarily, mistake the added combination for a full carrier
with independent sidebands, however.




(For sines it is
sin(a) * sin(b) = 0.5 * (cos[a-b]-cos[a+b])
= 0.5 * (sin[a-b+90degrees] - sin[a+b+90degrees])
= 0.5 * (sin[a-b+90degrees] + sin[a+b-90degrees])
)

--
rb





When AM is correctly accomplished (a single voiceband signal is modulated

The questions I posed were not about AM. The
subject could have been viewed as DSB but that
wasn't the specific intent either.

You should take some time to more carefully frame your questions.

Do you understand that a DSB signal *is* AM?

Post your intention; it might help.


onto a carrier via a non-linear process), at an envelope detector the two
sidebands will be additive. But if you independe ntly place a carrier at
frequency ( c ), another carrier at ( c-1 khz) and another carrier at (c+
1
kHz), the composite can look like an AM signal, but it is not, and only by
the most extreme luck will the sidebands be additive at the detector.
They
would probably cycle between additive and subtractive since they have no
real relationship and were not the result of amplitude modulation.

AM electromagnetic waves: 20 KHz modulation frequency on an astronomically-low carrier frequency
 

"Rich Grise" wrote in message
...
Sorry, John - while the ear's amplitude response IS nonlinear, it
does not act as a mixer. "Mixing" (multiplication) occurs when
a given nonlinear element (in electronics, a diode or transistor, for
example) is presented with two signals of different frequencies.
But the human ear doesn't work in that manner - there is no single
nonlinear element which is receiving more than one signal.

Sure there is - the cochlea. (well, the whole middle ear/inner ear
system.)

Nope - the point had to do with the inner workings of the cochlea.
You can't consider it as a single element, as the inner workings
consists of what are essentially thousands of very narrowband
individual sensors. There is no *single* nonlinear element in which
mixing of, say, the hypothetical 300 Hz and 400 Hz tones would
take place. John responded that the eardrum (typmanic membrane)
would act as such an element, but I would suggest that any mixing
which might in theory go on here is not a signifcant factor in how we
perceive such tones. The evidence for this is obvious - if presented
with, say, a pure 440 Hz "A" from a tuning fork, and the note from the
slightly flat instrument we're trying to tune (let's say 438 Hz), we DO
hear the 2 Hz "beat" that results from the interference (in the air)
between these two sounds. What we do NOT hear to any significant
degree is the 878 Hz sum that would be expected if there were much
contribution from a multiplicative ("mixing") process.

Bob M.

AM electromagnetic waves: 20 KHz modulation frequency on an astronomically-low carrier frequency
 

Jim Kelley wrote:
On Jul 5, 9:38 pm, John Fields wrote:

Sure enough, I heard the beat even though it came from different
sources, but I couldn't quite get it down to DC even with the
scope's trace at 0V.

Of course you heard beats. What you didn't hear is the sum of the
frequencies. I've had the same setup on my bench for several months.
It's also one of the experiments the students do in the first year
physics labs. Someone had made the claim a while back that what we
hear is the 'average' of the two frequencies. Didn't make any sense
so I did the experiment. The results are as I have explained.

We hear the average of two frequencies if both frequencies
are indistinguishably close, say with a difference of some few
hertz. For example, the combination of a 220 Hz signal and
a 224 Hz signal with the same amplitude will be perceived as
a 4 Hz beat of a 222 Hz tone.

gr, Hein

AM electromagnetic waves: 20 KHz modulation frequencyonanastronomically-lowcarrier frequency
 

Tommy Tootles wrote:

Uh, John...respectfully, I have to wonder just who is on drugs.

The original poster *ASKED* about *DSB* vs. AM

YOU *ANSWERED* about *SSB*. Here is the correct answer...

There are two broad types of DSB (double sideband) transmission:

DSB-RC and DSB-SC, meaning BOTH sidebands are transmitted, but with
either a (R)educed (C)arrier or a (S)urpressed (C)arrier. AM sends
both sidebands and full carrier.

Hope that answers the OP's question

John Smith I wrote:

Yeah, you just discovered that for all intents and purposes double
sideband is am, and suppressed carrier is just like suppressed
carrier am?

Oh well, better late than never ...

JS

What *I* discovered is -not- the point. And for all intents and
purposes, "AM" and DSB are two distinct (but certainly related) things.
Different hardware to create (balanced modulator for DSB vs high level
plate modulation for 'classic' AM), more power required for AM and
finally, back in the day, the FCC had -different- emission designators
for AM vs DSB. Now, if they were the same, why would you think the FCC
gave them -different- emission designators?

What IS the point is:

1) The original poster asked a question about "x".

2) You gave a half-assed answer to "y".

And then, you had the bare faced gall to accuse the original poster of
being on drugs!

Look at the good news--even though you gave a partially wrong answer to
a question that wasn't even asked, you at least resolved the issue of
which of the two of you is on drugs... ;-)

AM electromagnetic waves: 20 KHz modulationfrequencyonanastronomically-lowcarrier frequency
 

Ron Baker, Pluralitas! wrote:

What is the difference between AM and DSB?

The two actually describe different properties, so a signal can be be
AM, DSB, neither, or both.

And here we run into some trouble between technical correctness and
common usage.

DSB stands for Double SideBand. Although I suppose an FM signal could be
called DSB because it has two *sets* of sidebands, and a narrowband FM
signal has only one significant pair like an AM signal, in my experience
the term DSB virtually always refers to a signal generated by amplitude
modulation.

AM is Amplitude Modulation. Straightforward amplitude modulation such as
done for AM broadcasting produces a carrier and two sidebands, or DSB
with carrier. Either the carrier or one sideband, or both, can be
suppressed. If you suppress the carrier (or don't generate it in the
first place), you get DSB with suppressed carrier, or DSB-SC. If you
suppress one sideband, you get SSB. Usually, but not always, the carrier
is also suppressed along with the one sideband, resulting in SSB-SC.
NTSC television transmission is VSB -- AM with a carrier and "vestigial"
or partially suppressed sideband and a full second sideband. Partial
suppression of the carrier is also done for some broadcast purposes.

So a commercial AM broadcast station broadcasts a signal that's both AM
and DSB. A typical amateur or military SSB transmission is AM but not
DSB. A QPSK signal is neither. And, as I mentioned, some signals like FM
could be considered DSB but not AM (although this isn't common usage).

In common amateur parlance, however,

"AM" usually means AM with two sidebands and carrier.
"DSB" usually means AM with two sidebands and suppressed carrier
"SSB" usually means AM with a single sideband and suppressed carrier

Roy Lewallen, W7EL

AM electromagnetic waves: 20 KHz modulation frequency on an astronomically-low carrier frequency
 

"isw" wrote in message
...
In article ,
"Ron Baker, Pluralitas!" wrote:

"isw" wrote in message
...
In article ,
"Ron Baker, Pluralitas!" wrote:

"isw" wrote in message
...
In article ,
"Ron Baker, Pluralitas!" wrote:

"isw" wrote in message
...

snip


After you get done talking about modulation and sidebands,
somebody
might want to take a stab at explaining why, if you tune a
receiver
to
the second harmonic (or any other harmonic) of a modulated
carrier
(AM
or FM; makes no difference), the audio comes out sounding exactly
as
it
does if you tune to the fundamental? That is, while the second
harmonic
of the carrier is twice the frequency of the fundamental, the
sidebands
of the second harmonic are *not* located at twice the frequencies
of
the
sidebands of the fundamental, but rather precisely as far from
the
second harmonic of the carrier as they are from the fundamental.

Isaac

Whoa. I thought you were smoking something but
my curiosity is piqued.
I tried shortwave stations and heard no harmonics.
But that could be blamed on propagation.
There is an AM station here at 1.21 MHz that is s9+20dB.
Tuned to 2.42 MHz. Nothing. Generally the lowest
harmonics should be strongest. Then I remembered
that many types of non-linearity favor odd harmonics.
Tuned to 3.63 MHz. Holy harmonics, batman.
There it was and the modulation was not multiplied!
Voices sounded normal pitch. When music was
played the pitch was the same on the original and
the harmonic.

One clue is that the effect comes and goes rather
abruptly. It seems to switch in and out rather
than fade in an out. Maybe the coming and going
is from switching the audio material source?

This is strange. If a signal is multiplied then the sidebands
should be multiplied too.
Maybe the carrier generator is generating a
harmonic and the harmonic is also being modulated
with the normal audio in the modulator.
But then that signal would have to make it through
the power amp and the antenna. Possible, but
why would it come and go?
Strange.

Hint: Modulation is a "rate effect".

Isaac

Please elaborate. I am so eager to hear the
explanation.

The sidebands only show up because there is a rate of change of the
carrier -- amplitude or frequency/phase, depending; they aren't
separate, stand-alone signals. Since the rate of change of the
amplitude
of the second harmonic is identical to that of the fundamental, the
sidebands show up the same distance away, not twice as distant.

Isaac

That doesn't explain why the effect would come and go.

I don't understand what effect you're referring to here.

When I was tuned to the 3rd harmonic sometimes
I would hear it and sometimes not.
It would come and go rather abruptly. It didn't seem
to be gradual fading.


But once again you have surprised me.
Your explanation of the non-multiplied sidebands,
while qualitative and incomplete, is sound.

I'm a physicist/engineer, and have been for a long time. I have always

The you understand Fourier transforms and convolution.

maintained that if the only way one can understand physical phenomena is
by solving the differential equations that describe them, then one does
not understand the phenomena at all. If you can express a thing in
words, such that a person with little mathematical ability can
understand what's going on, *then* you have a good grasp of it.

I too am a fan of the intuitive approach.
But I find that theory is often irreplacable.


It looks to me that the tripple frequency sidebands
are there but the basic sidebands dominate.
Especially at lower modulation indexes.

I don't understand what you are saying here either. And in my
experience, the term "modulation index" is more likely to show up in a
discussion of FM or PM than AM; are you using it interchangeably with
"modulation percentage"?

http://en.wikipedia.org/wiki/Amplitu...dulation_index



Isaac