RadioBanter - View Single Post - AM electromagnetic waves: 20 KHz modulation frequency on an astronomically-low carrier frequency

isw

In article ,
Don Bowey wrote:

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.

I think you are confusing *single* sideband, for which that is correct,
and *double* sideband (which we were discussing), for which it is not
true.

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.

Doing it that way will work, but it's not "fair", because you are not
actually demodulating a DSB signal (which was the subject of the
discussion).

Isaac