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.
--

Radium wrote:
"I have a very weird question about electromagnetic radiation, carriers,
and modulators."

It makes no difference which signal you call the modulating signal and
which you call the carrier. Modulation is simply mixing the two together
at a nonlinear point to create new frequencies along with the originals.

The same modulation products are produced by 20 Hz on a 20 KHz carrier
or 20 KHz on a 20 Hz carrier.

The nonlinear point may be a single diode, a diode bridge, or a
high-level plate modulator. The signals out can be the same except for
amplitudes.

I think Ian Jackson has also given a correct answer that is more
complete though less simple.

Best regards, Richard Harrison, KB5WZI

"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

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

On Jul 1, 10:06 pm, Telamon
wrote:
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

Telamon,

Off-Topic + Cross-Posting -and- 'Decency'
http://en.wikipedia.org/wiki/Oxymoron
Wow - Now There Is A Real Oxymoron !

~ RHF

On 7/1/07 10:06 PM, in article
,
"Telamon" wrote:

In article ,
cledus wrote:

Snip

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

Would you please come and ask nicely. I don't like how you put your order.

On Jul 2, 6:16 am, Don Bowey wrote:
On 7/1/07 10:06 PM, in article
,

"Telamon" wrote:
In article ,
cledus wrote:

Snip

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

- Would you please come and ask nicely.
- I don't like how you put your order.

don bowey, Don Bowey. DON BOWEY !

Oh Please with Sugar and Spice and Everything Nice
snip, Snip. SNIP ! the "Rec.Radio.Shortwave" Group
from the Newsgroups {Distribution} Header when you
Post your Reply - It would be ever so decent of you
Kind and Wonder Sir. ;-)

thank you very much - most respectfully ~ RHF

On Mon, 02 Jul 2007 14:42:49 -0700, RHF
wrote:

On Jul 2, 6:16 am, Don Bowey wrote:
On 7/1/07 10:06 PM, in article
,

"Telamon" wrote:
In article ,
cledus wrote:

Snip

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

- Would you please come and ask nicely.
- I don't like how you put your order.

don bowey, Don Bowey. DON BOWEY !

Oh Please with Sugar and Spice and Everything Nice
snip, Snip. SNIP ! the "Rec.Radio.Shortwave" Group
from the Newsgroups {Distribution} Header when you
Post your Reply - It would be ever so decent of you
Kind and Wonder Sir. ;-)

thank you very much - most respectfully ~ RHF

---
Seems to me his posts are on topic for rrs, so why don't you just
learn how to use a filter?


--
JF

In article ,
Don Bowey wrote:

On 7/1/07 10:06 PM, in article
,
"Telamon" wrote:

In article ,
cledus wrote:

Snip

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

Would you please come and ask nicely. I don't like how you put your order.

This is a stupid cross posted Troll thread so pretty please with sugar
on it snip the other news groups it does not originate from. Thank you
very, very much in advance.

--
Telamon
Ventura, California