Re: AM/FM on one carrier

On 6/14/05 6:03 AM, in article 4alta15alp8egiqopnmvg59ljtjp2qt643@xxxxxxx,
"John Fields" <jfields@xxxxxxxxxxxxxxxxxxxxx> wrote:

> 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.

I do believe we have been through all this before, and I didn't change your
mind then, nor probably now.

There are probably many methods of "AM" that will result in carrier voltage
modulation while also creating sidebands, they are not a usable form of AM,
as they result in godawful distortion with wideband splatter. Meaningful
discussions of AM by inquiring minds should be oriented toward meaningful
processes, not processes that generate unusable signals. My dabble into
this was in 1954, and was short-lived because the result was what I
anticipated.... But I just had to see it. If someone starts a thread with
their interesting modulation method test results, I will post mine to it
also.

Getting to the bottom line, if you use a selective frequency voltmeter to
observe an unmodulated carrier, and then you (correctly) AM modulate the
carrier, you will observe that the carrier amplitude does not change.

And the modulation power is all in the sidebands.

Don

Don

.

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