kronecker@yahoo.co.uk wrote:
> On Apr 29, 11:55 am, John Monro <johnmo...@optusnet.removethis.com.au>
> wrote:
>> Green Xenon [Radium] wrote:
>>> Hi:
>>> Most cell phones and wireless internet routers, modems, and access
>>> points that use spread spectrum usually broadcast and receive their data
>>> on FM-radio waves. Just out of curiosity, I ask, why not use AM?
>>> Let's say a DSSS/FHSS type of spread-spectrum is transmitted and
>>> received using the AM radio waves in the UHF spectrum [i.e. spread info
>>> for transmission throughout the UHF band and receive AM radio waves
>>> throughout all UHF frequencies]. What would be the disadvantages of this?
>>> Normally DSSS and FHSS are transmitted/received on FM radio waves. So I
>>> ask what would be the disadvantages of using AM instead of FM for this?
>>> AM radio tends to be more vulnerable to unwanted magnetic disruptions
>>> than FM, however this only affects analog reception. Digital reception
>>> on AM should be unaffected even by the strongest-interfering analog RF
>>> magnetic signal. Right?
>>> Thanks,
>>> Radium
>> Hi,
>>
>> FM demodulation produces a handy 'capture' effect. If a weak and a
>> strong signal are present together, then the recovered baseband signal
>> from the weaker FM signal is greatly reduced. This is useful in
>> rejecting interference from adjacent stations on the same frequency.
>>
>> With AM, the demodulated baseband signals are present in amplitudes that
>> are proportional to their RF amplitudes. In addition, the AM carriers
>> will 'beat' together to produce an additional and unwanted
>> tone-modulation of the received RF signal.
>>
>> Regards,
>> John
>
> Only if the power of the co-channel is very muchless otherwise you
> get complete destruction of both to a point when they flip over and
> you hear the other channel.

Neither AM nor FM can cope with co-channel interference when the powers
are roughly equal.

Power saving is the big thing with FM.
> K.

It all depends on what baseband Signal-to-Noise ratio is acceptable.
Very high S/N applications (e.g. Hi-Fi music) require an AM RF-level
that is high relative to the FM RF-level. For low S/N applications
(e.g. voice communication) the positions are reversed.

Regards,
John