Re: AM/FM on one carrier

On Tue, 14 Jun 2005 00:31:13 -0700, Don Bowey <dbowey@xxxxxxxxxxx>
wrote:

>On 6/13/05 8:42 PM, in article cmsre.75$j85.6092@xxxxxxxxxxxxxxx, "Larry
>Brasfield" <donotspam_larry_brasfield@xxxxxxxxxxx> wrote:
>
>> "Don Bowey" <dbowey@xxxxxxxxxxx> wrote in message
>> news:BED39AE3.5A21%dbowey@xxxxxxxxxxxxxx
>>> On 6/13/05 4:24 PM, in article lp4sa155402uh70aen5m09h85lho9okqvg@xxxxxxx,
>>> "John Fields" <jfields@xxxxxxxxxxxxxxxxxxxxx> wrote:
>>>
>>>> Since the amplitude of the carrier will be varying because of the AM
>>>> and
>>>
>>> Tjhat is not correct. The carrier amplitude is constant with modulation.
>>
>>
>> If the carrier amplitude is constant, it is not being amplitude modulated.
>> This is so obvious as to be tautological. "AM" = "amplitude modulation".
>
>Thanks for adding an opinion on the subject. Your assumption is obviously
>in error.

---
Considering what happens in amplitude modulation, that is that the
amplitude of the carrier is forced to vary with the modulating
waveform, I'd say that the error was yours.

Consider CW, where the carrier is either on or off. Is CW amplitude
modulation? Definitely. Does the amplitude of the carrier vary
during the process of modulation? Definitely.

Let's look at a slightly more involved example/thought experiment.
Imagine you have a rig set up so that you can plate modulate the final
but that instead of varying the final's B+ by speaking into the
microphone you vary the amplitude of the plate supply by using a pot.
Rotate the pot counter-clockwise and the plate voltage decreases;
rotate it clockwise and it increasese.

Will rotating the pot CCW decrease the amplitude of the carrier? Yes.
Will rotating the pot CW increase the amplitude of the carrier? Yes.

If a modulating waveform causes the plate voltage to decrease will the
amplitude of the carrier decrease? Yes.

If a modulating waveform causes the plate voltage to increase will the
amplitude of the carrier increase? Yes.


--
John Fields
Professional Circuit Designer
.

Relevant Pages

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  • Re: Are purely-analog audio devices immune to aliasing?
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    (rec.audio.tech)
  • Re: AM/FM on one carrier
    ... >>carrier, you will observe that the carrier amplitude does not change. ... They got injected through the miracle of modulation. ... The demodulator consists of an I.F. AGC amplifier feeding a Linear tech ...
    (sci.electronics.design)
  • Re: AM/FM on one carrier
    ... >>> If the carrier amplitude is constant, it is not being amplitude modulated. ... > Considering what happens in amplitude modulation, ... > Will rotating the pot CCW decrease the amplitude of the carrier? ... > If a modulating waveform causes the plate voltage to decrease will the ...
    (sci.electronics.design)
  • Re: Modulate HF onto LF?
    ... J.A. Legris wrote: ... frequency carrier than the modulation. ... amplitude modulation and other techniques associated with modems, ... Thus reducing the channel width for a given amount of transferred info. ...
    (sci.electronics.design)
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