[Elecraft] [K3] FSK Help

[Ed Muns]

> But, I have a question for you, being a neophyte.  Why are 
> you not interested in AFSK?  Does FSK work better in some 
> way?  Let me say, I have been very pleased with AFSK so far.  
> I have worked almost any station that I could hear/print so far.
> 
> Is it worth the bother to wire up a cable for FSK?  Why is it 
> better than AFSK?

RTTY transmission using FSK or AFSK is a personal preference and some people
are very outspoken with their opinion about which is "best".  There are
advantages of each method and equally excellent results have been achieved
with either, i.e., equally difficult DX has been worked with either method
and major RTTY contests have been won with either method.

AFSK uses the PC soundcard which is often being used anyway for reception
with software decoders.  A standard audio cable is used where FSK requires a
special keying cable identical to that used for CW keying.  The audio level
of the soundcard audio tones must be set to drive the transmitter adequately
but not over-drive it.  Since the MARK and SPACE RF frequencies are derived
from audio tones run through the SSB portion of the radio, the software can
dither those audio frequencies to automatically change the MARK/SPACE RF
frequencies providing dynamic zero-beating.  In most radios, the VFO dial
frequency is the suppressed carrier frequency of the SSB signal.  Thus, the
actual MARK frequency will differ from this by whatever the current audio
tone frequency is being used, which may be moving around due to the
automatic frequency feature.  Since PSK31 typically uses soundcard audio
generation for transmission via the SSB mode, the same hardware (cabling)
setup can be used for RTTY.  Finally, some older radios do not have a FSK
keying input, so the only choice is AFSK.  This typically means that the IF
filtering available on receive is the wider SSB filters.

FSK is direct keying of the transmitter just like CW.  The VFO dial
frequency is the MARK frequency and unambiguous when exchanging QRV
frequencies.  Since the transmitter creates the two MARK/SPACE RF
frequencies, there is no worry about audio level adjustments or remembering
to turn SSB compression off.  Some will argue that the FSK signal is cleaner
than AFSK, but with modern radios this difference is seldom an issue.  Since
automatic TX frequency dithering is not possible with FSK, there is less
chance to inadvertently transmit at an offset.  Radios with FSK keying
typically provide narrow IF filtering similar to CW.

FSK is clearly simpler from an overall hardware/software system standpoint,
and therefore less likely to have unintentional problems.  But many argue
that AFSK is "simpler" because it doesn't require the special keying cable.
Yet another religious debate within our hobby.

73,
Ed - W0YK