[GreenKeys] FSK vs. AFSK

[Gary Chatters]

On Tue, 20 Apr 2004, 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 :)


You're right.  This could generate a lot of email.  :-)

There is more than one way to generate an RTTY signal on HF.

Way back when I first got on RTTY my transmitter was a Heath DX-60.  I
built a little add on circuit that installed in the VFO that could shift
its frequency by 850Hz.  This was real direct FSK and was rather common.
It works with most any transmitter.  I later used the same circuit on a
Collins 32S1.

The alternative, if you have an SSB transmitter, is the scheme you
describe.  Feed an AFSK generator in to the mike connection.

I'd call it AFSK if I had to label it.  But what comes out the RF
connector on the back of the transmitter is essentially the same, to the
extent you can suppress carrier and alternate sideband. I think there is a
specific modulation specifier for this that is different than for direct
FSK.  J2B perhaps?


Gary