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*Subject*: RE: [amsat-bb] Manchester FSK vs FSK*From*: "Tom Clark, W3IWI" <w3iwi@xxxxxxxx>*Date*: Fri, 21 Mar 2003 22:54:37 -0500*In-Reply-To*: <29b401c2f015$0fae5d20$0300000a@abraxis.com>

> 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 ---- Sent via amsat-bb@amsat.org. Opinions expressed are those of the author. Not an AMSAT member? Join now to support the amateur satellite program! To unsubscribe, send "unsubscribe amsat-bb" to Majordomo@amsat.org

**References**:**Manchester FSK vs FSK***From:*Don Woodward

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