Ian Jackson wrote:
> In message <EXChi.23350$C96.1422@newssvr23.news.prodigy.net >, cledus
> <cledus@noemail.net> 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 Fc<20kHz. 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 Fc<20kHz 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