[GreenKeys] FSK vs. AFSK

[Bob Camp]

Hi,

There are an almost infinite number of ways to generate any signal. 
Rather than classifying everything based on the way we generate it we 
usually classify it based on the result.

AFSK *should* be an audio tone shifted on an AM or FM voice 
transmission. You could generate the AM signal by phasing techniques 
but it would still be AM AFSK. If you look at an AM AFSK signal on a 
good spectrum analyzer you will see a carrier and sets of sidebands 
both above and below the carrier. Same thing is true for FM AFSK except 
the carrier and sideband relationship is a bit more complex.

The signal we generate with an audio tone into an SSB transmitter 
*looks* just like a FSK transmission (provided we do it right ....). If 
you put it next to a frequency shifted VFO the two signals will both 
flip back and forth between the mark and space frequencies the same 
way. On our good spectrum analyzer you will see the two tones (mark and 
space) and a group of sidebands around each one. In this case you won't 
see the carrier that you had with the AM AFSK and you only get half the 
number of sidebands.

If you want to go nuts about it a "normal" fsk signal looks just like a 
pair of 100% modulated AM transmissions. You could generate it that way 
but the result would still be called FSK.

I don't know if that helps or not.

	Enjoy!

		Bob  Camp
		KB8TQ


On Apr 20, 2004, at 5:44 PM, Tom Elmore wrote:

>   I know this question will generate a lot of email so I have my Model 
> 15 hard hat in place just in case. Is FSK
> that is used on HF really AFSK let me explain. If we are modulating an 
> HF transmitter with a mark tone and then a space tone and using the 
> difference as the shift isn't this a form of AFSK since we are 
> modulating the transmitter with an audio frequency. I'm sure some fine 
> greenkeyer will set me straight :)
>
> Thank You
> Tom Elmore KA1NVZ
> Anchorage  Alaska
>
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