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?