Michael Rigby-Jones wrote:
>
>> Manchester encoding has no effect on RF bandwidth unless you are using
>> PM or FM. It's a frequency shift.
>>     
>
> If you are suggesting that amplitude modulation does not affect the
> spectrum of an RF carrier then you are mistaken. Simple AM modulation
> produces sidebands above and below the carrier frequency, the width of
> which will depend on the frequency of the modulating signal.
>
>   
No, I'm not suggesting that.

The ASK/OOK receiver will however work with one sideband or both as it's 
only on/off keying, not AM. Basic NRZ on long runs will either have 
twice average power at carrier frequency, or no signal at all. It's 
inherently unsuitable for OOK, better suited to PM/FM. But if you are 
using FSK/PM/FM you don't need NRZ. Basic AM wastes a lot of power in 
the carrier. Manchester encoded ASK/OOK, even if you don't remove one 
sideband, has power only in the sidebands, thus has better SNR for the 
max permitted power. The  power of Manchester Encoded is exactly 1/2 
peak power averaged over 4 bits, no matter the data content. The NRZ 
power is dependent on data pattern, thus SNR varies considerably with 
content.

If bandwidth is an issue you can use either filter or dual modulators 
and quadrature to cancel extra sideband with ASK/OOK as it's not AM in 
the conventional sense.

The off the shelf ASK/OOK ICs used as per datasheet or cheap 
pre-assembled modules do need Manchester Encoded OOK. That's what they 
are designed for. Basic NRZ will not operate the data slicer properly 
and is more awkward to retrieve  clock. Manchester is self clocked.









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