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RE: Manchester FSK vs FSK



> Would someone explain the difference between "Manchester FSK"
> and "FSK"?

Don -- one way to look at the real difference is to look at the modulation
spectrum of the signals.

With "straight FSK", the raw data is applied directly to the FSK modulator.
Therefore the (fundamental) spectrum of modulating frequencies goes from
zero up to B/2, where B is the Bit rate. For 1200 bit/sec data, the highest
modulating frequency happens when the bits alternate 0/1, i.e. B/2. Of
course for square-edged bits, all the odd harmonics are also present --
that's why I used the word fundamental.

The problem with this in the real world is that the modulator has to be
capable of going all the way down to DC to accommodate data with all zeroes
or all ones. For that reason, the scheme that was invented eons ago was
Manchester encoding. This mixes the modulation spectrum up to a higher
frequency to get rid of the DC problem. The simple implementation is to take
the B=1200 bit/sec data and feed it into one leg of an exclusive-or gate.
The other input to the XOR is a clock at 1200 Hz with the phase of its edges
set to coincide with the transitions in the data.

The effect of this modulation "carrier" is to shift the 0-B/2 spectrum up in
frequency. The output of the XOR gate is a DSB signal with the LSB extending
from B/2 up to B Hz (600-1200 Hz in this example) and the USB going from B
up to 3B/2 Hz (1200-1800 Hz). Because the two sidebands are mirrors, the
1200 Hz "carrier" is suppressed. But now notice that the (fundamental)
spectrum now extends from B/2 to 3B/2, i.e. 600-1800 Hz which will "fit"
into an ordinary audio channel.



Another way to look at the difference: Some folks who prefer to think about
bit recovery prefer to think of this as a way of transmitting the data clock
along with the data. This alternative viewpoint is merely the Fourier
transform of what I described above ;<}



A couple of web sites that offer a similar (but more technical) explanation
include
  http://www.erg.abdn.ac.uk/users/gorry/course/phy-pages/man.html
  http://homepage.ntlworld.com/matthew.rowe/micros/virbook/manchest.htm

73, Tom

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