Re: AM/FM on one carrier

On 6/14/05 9:16 AM, in article xpDre.24$Lg5.2880@xxxxxxxxxxxxxxx, "Larry
Brasfield" <donotspam_larry_brasfield@xxxxxxxxxxx> wrote:

> "John Larkin" <jjlarkin@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx> wrote
> in message news:6irta1p3g1nlsddu7s591m2pekqtuqdiv8@xxxxxxxxxx
>> On Mon, 13 Jun 2005 20:42:04 -0700, "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".
>>
>> If you define "carrier" as the amplitude of the central spectral line,
>> excluding the sidebands, then an AM carrier is constant irrespective
>> of AM modulation.
>
> You are mixing domains here. "Constant" refers to a time
> domain concept. "Spectral line" refers to a frequency
> domain concept. If you are saying the set of AM spectra
> starting with the same unmodulated carrier all have the
> same line height at the carrier frequency, I would agree
> with an exception for the results of DC modulations.

DC modulation is not AM modulation (in it's classic, intended sense). It is
coded changes; turning the carrier on and off as in international Morse
code. I imagine one could use a code with multiple levels of carrier
amplitude along with signal on-off code, but it would not be AM either.

Don


>
>> The carrier amplitude of an FM signal varies with
>> modulation index according to a Bessel function, and has nulls at
>> various points.
>>
>> So if you FM modulate a carrier with the right sinewave signal at the
>> right deviation, the carrier disappears completely, which has got to
>> confuse an AM detector.
>>
>> I'm guessing that AM and FM modulation can be used fairly orthogonally
>> at very low modulation levels.
>
> Sure. The crux is how low the modulation
> levels need to be given the OP's requirement
> and receiver attributes.

.

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