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 much less otherwise you
get complete destruction of both to a point when they flip over and
you hear the other channel. Power saving is the big thing with FM.

K.