On Apr 29, 12:52 pm, "Green Xenon [Radium]" <gluceg...@excite.com>
wrote:
> John Monro 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
>
> But if the signal is digital, won't it remain immune to EMI/RFI [analog
> disruption] even if received on AM? DSSS and FHSS and digital. So I
> would think that the analog magnetic interferences wouldn't affect it.
>
> Also, doesn't FM have the disadvantages in that it hogs more bandwidth
> than AM?

Ye cannae re-write the laws of physics captain - digital or analogue
FM is the same. You do the Maths and see.

Digital comms still breaks down except it has error-correcting codes
built in - so you don't see it! Ocassionally you may well find that
your cell-phone losses connection too. It's all or nothing with
digital.

K.