"Ian Jackson" wrote in message
...
In message , cledus
writes

snip


The fundamental answer is no, it is not possible to generate AM where the
baseband signal is a pure 20 kHz sinewave and Fc20kHz. The reason is
that the modulated waveform consists of the sum of a sinewave at Fc, a
sinewave at Fc+20kHz, and a sinewave at Fc-20kHz. If Fc20kHz then one of
the components becomes a "negative" frequency. So the carrier must be
greater than the baseband signal to prevent this.

I'm afraid that this is not correct. The 'laws of physics' don't suddenly
stop working if the carrier is lower than the modulating frequency.
However, there's no need to get into complicated mathematics to illustrate
this. Here is a simple example:

(a) If you modulate a 10MHz carrier with a 1MHz signal, you will produce
two new signals (the sidebands) at the difference frequency of 10 minus 1
= 9MHz, and the sum frequency of 10 plus 1 = 11MHz. So you have the
original carrier at 10MHz, and sideband signals at 9 and 11MHz (with a
balanced modulator - no carrier - only 9 and 11MHz).

(b) If you modulate a 1MHz carrier with a 10MHz signal, you will produce
two new signals (the sidebands) at the difference frequency of 1 minus 10
= minus 9MHz, and the sum frequency of 1 plus 10 = 11MHz. The implication
of the negative 'minus 9' MHz signal is that the phase of the 9MHz signal
is inverted, ie 180 degrees out-of-phase from 9MHz

Actually there would be no phase flip.
cos(-a) = cos(a)

produced in (a). So you have the original carrier at 1MHz, and sidebands
at 9 and 11MHz (again, with a balanced modulator - no carrier - only 9 and
11MHz).

The waveforms of the full composite AM signals of (a) and (b) will look
quite different. The carriers are at different frequencies, and the phase
of the 9MHz signal is inverted. However, with a double-balanced modulator,
you will only have the 9 and 11MHz signal so, surprisingly, the resulting
signals of (a) and (b) will look the same.

A double-balanced mixer is a multiplier. A * B = B * A


[Note that, in practice, many double-balanced modulators/mixers put loads
of unwanted signals - mainly due the effects of harmonic mixing. However,
the basic 'laws of physics' still apply.]

Finally, although I have spoken with great authority, when I get a chance
I WILL be doing at test with a tobacco-tin double-balanced mixer,

What's a tobacco-tin double-balanced mixer?

a couple of signal generators and a spectrum analyser - just to make sure
that I'm not talking rubbish. In the meantime, I'm sure that some will
correct me if I'm wrong.

You did pretty good.


Ian.
--


--
rb

Jeff Liebermann wrote:

40KHz thru 285Khz? Great idea. Just one minor problem. A 1/4 wave
antenna at about 100Khz frequency is 750 meters long. That's going to
be a rather large antenna for literally dragging behind you.

Well, your math is correct. However, the so-called "atomic" wrist
watches receive their time signal from WWVB which transmits at 60kHz.

How do they get that 1250 meter long antenna ( 1/4 wave at 60 kHz)
inside that itty bitty wrist watch case? ;-)

Don Bowey wrote:

AM is a process of frequency multiplication. Now you tell me where you
think such multiplication takes place on a phone line, and I'll follow-uo by
telling why you're full of crap.

SIMECS!


It is all right before your eyes, if you can't see it by now, forget it
.... perhaps at a later date. I know your frustration, I have seen the
mentally handicapped attempt to deal with the real world and it end only
in frustration ... perhaps a change of meds is in order ...

JS

Radium wrote:
...

You and your buddies are a hopeless mess, a thread plonk is in order here.

PLONK!

JS

On 7/1/07 2:57 PM, in article , "John Smith I"
wrote:

Don Bowey wrote:

AM is a process of frequency multiplication. Now you tell me where you
think such multiplication takes place on a phone line, and I'll follow-uo by
telling why you're full of crap.

SIMECS!


It is all right before your eyes, if you can't see it by now, forget it
... perhaps at a later date. I know your frustration, I have seen the
mentally handicapped attempt to deal with the real world and it end only
in frustration ... perhaps a change of meds is in order ...

JS

I see..... You finally admit you don't understand AM at all and can't
justify your statement. It's what I expected.

Now, run off and play in the street with your tinker toys.

Ian Jackson wrote:
In message , cledus
writes
Radium wrote:
Hi:
Please don't be annoyed/offended by my question as I decreased the
modulation frequency to where it would actually be realistic.
I have a very weird question about electromagnetic radiation,
carriers, and modulators.





No offense but please respond with reasonable answers & keep out the
jokes, off-topic nonsense, taunts, insults, and trivializations. I am
really interested in this.
Thanks,
Radium



The fundamental answer is no, it is not possible to generate AM where
the baseband signal is a pure 20 kHz sinewave and Fc20kHz. The
reason is that the modulated waveform consists of the sum of a
sinewave at Fc, a sinewave at Fc+20kHz, and a sinewave at Fc-20kHz.
If Fc20kHz then one of the components becomes a "negative"
frequency. So the carrier must be greater than the baseband signal to
prevent this.

I'm afraid that this is not correct. The 'laws of physics' don't
suddenly stop working if the carrier is lower than the modulating
frequency. However, there's no need to get into complicated mathematics
to illustrate this. Here is a simple example:

(a) If you modulate a 10MHz carrier with a 1MHz signal, you will produce
two new signals (the sidebands) at the difference frequency of 10 minus
1 = 9MHz, and the sum frequency of 10 plus 1 = 11MHz. So you have the
original carrier at 10MHz, and sideband signals at 9 and 11MHz (with a
balanced modulator - no carrier - only 9 and 11MHz).

(b) If you modulate a 1MHz carrier with a 10MHz signal, you will produce
two new signals (the sidebands) at the difference frequency of 1 minus
10 = minus 9MHz, and the sum frequency of 1 plus 10 = 11MHz. The
implication of the negative 'minus 9' MHz signal is that the phase of
the 9MHz signal is inverted, ie 180 degrees out-of-phase from 9MHz
produced in (a). So you have the original carrier at 1MHz, and sidebands
at 9 and 11MHz (again, with a balanced modulator - no carrier - only 9
and 11MHz).

The waveforms of the full composite AM signals of (a) and (b) will look
quite different. The carriers are at different frequencies, and the
phase of the 9MHz signal is inverted. However, with a double-balanced
modulator, you will only have the 9 and 11MHz signal so, surprisingly,
the resulting signals of (a) and (b) will look the same.

[Note that, in practice, many double-balanced modulators/mixers put
loads of unwanted signals - mainly due the effects of harmonic mixing.
However, the basic 'laws of physics' still apply.]

Finally, although I have spoken with great authority, when I get a
chance I WILL be doing at test with a tobacco-tin double-balanced mixer,
a couple of signal generators and a spectrum analyser - just to make
sure that I'm not talking rubbish. In the meantime, I'm sure that some
will correct me if I'm wrong.

Ian.


Ian,

I believe your analysis is correct. But if you expect to build a
receiver that uses a filter centered at 1 MHz with a BW of 20+ MHz to
recover a DSB AM signal, I don't believe that the DBM approach will
accomplish this. With your approach, you could filter out the sidebands
by centering a filter around 10 MHz (the baseband freq). This could be
used to recover the baseband 10 MHz signal. But the OP asked about AM
of a carrier at very low frequencies. Good explanation of what happens
when using a DBM, though.

Regards,
-C

In article ,
cledus wrote:

Snip

Would you please have the decency to snip rec.radio.shortwave and other
groups from the newsgroup header. Thanks.

--
Telamon
Ventura, California

"John Smith I" wrote in message
...
Listen to a "strong--pure am signal" on an fm receiver, turn up the volume
on the fm receiver, something is responsible for that ... repeat
experiment with the reverse ... "imperfect world theory" proof!

What is responsible for that is not that AM somehow also
produces FM, but simply that the type of demodulator used
by the FM receiver in question will also demodulate AM to
a usable degree. Ditto the reverse (look up "slope detection"
for an example of how a very common AM demodulator
can also demodulate FM).

Bob M.

Jeff Liebermann wrote:

I found a photo of the insides of a watch. See:
http://tf.nist.gov/general/pdf/1877.pdf
on page 11-12. It's a small 2cm internal rod antenna.

A ferrite loop that small for VLF must be very inefficient. I guess they
used a high gain RF stage to make the VLF signal usable.

Jeff Liebermann wrote:

Watch antennas:
http://www.c-max-time.com/products/productsOverview.php?catID=5
See the photos of the various antennas. Too bad there's no specs.

I'll grind out the field strength numbers later. I've been living in
the microwave region for so long, that I'm having problems with LF
calcs.

http://www.c-max-time.com/downloads/getFile.php?id=423
Gives dimensions,No of turns,Inductance etc.