[Elecraft] FSK on a K2?

[Ron D'Eau Claire]

FSK is FSK. That means the frequency of the signal shifts to encode the
data. The only difference is in how the FSK is created. 

Half a century ago FSK was generated by shifting the frequency of the
oscillator that determined the transmitter's frequency. The usual approach
was to use a reactance tube or even a relay to switch a capacitor or
inductor in and out of the oscillator that shifted the frequency of the
oscillator slightly. (Think of very slow frequency modulation <G>). 

The K2 is not designed for that form of FSK (or frequency modulation
either). Indeed the whole design is to prevent such frequency shifts
because, when they happen very slowly, we call it "drift"!!

But, by driving the K2 in SSB mode with an audio tone, the K2 generates
essentially *one* RF frequency: the carrier offset by the tone either above
(USB) or below (LSB) the carrier frequency. By shifting the audio tone the
one RF frequency produced by the K2 shifts accordingly. This produces FSK
that, at the rig's output, is exactly the same as that produced by the old
rig's shifting oscillator.

At the receiving end, nothing has changed. In the old days an FSK signal was
received and the beat-frequency oscillator (which made a CW signal audible
as a tone) was adjusted so you'd hear in the speaker an audio tone that
shifted in frequency. 

In modern rigs designed for SSB, the product detector produces those tones
as well. There is *no* difference between the audio output of a K2 listening
to another K2 develop FSK using SSB mode and the audio output of a 1950's
receiver listening to a 1950's transmitter producing FSK by shifting the
oscillator frequency. 

AFSK, Audio Frequency Shift Keying, is something quite different. It was
commonly used on VHF/UHF half a century ago and some use might still exist
today. It had a huge advantage for VHF/UHF because it did not require a very
stable transmitter or receiver. In the 1930's, through perhaps the 1970's it
was enough of a challenge to produce a stable enough carrier frequency on
the HF bands for critical tuning, much less on VHF or UHF. So, up on the
higher frequencies it was common to us an AM transmitter. The AM transmitter
produced a carrier and two sidebands just like a conventional SW broadcaster
or any broadcast band AM station does today. When tuning across such a
station, the only thing that changes is the strength of the signal. Unlike
SSB, tuning across an AM signal does not cause the modulation tones to
shift. A 1 KHz tone modulating an AM signal will be a 1 kHz tone at the
receiver, no matter the tuning. For that reason AFSK was very popular on the
higher frequencies back then. Conventional transmitters and receivers which
might drift hundreds of Hz during a single transmission could be used easily
to receive radio teletype (RTTY) using AFSK. 

Today, some folks mistakenly call what we do on the HF bands with an SSB rig
(like a K2 equipped with the SSB module) "AFSK". They are partially right.
The output of their source, usually a computer, is an audio tone that shifts
in frequency. But their rigs produce true FSK as a result: an RF frequency
that shifts according to the modulation. 

So the way to do FSK on a K2 is to develop audio tones within the normal
voice bandpass of 300 to about 2.1 KHz whose separation is equal to the
shift you need in frequency. Apply those to the K2's mic input and it'll
produce a shifting RF output or FSK. Today, those tones normally come from a
PC or special controller of some sort. 

Ron AC7AC

-----Original Message-----
From: elecraft-bounces at mailman.qth.net
[mailto:elecraft-bounces at mailman.qth.net] On Behalf Of Ken Alexander
Sent: Wednesday, January 31, 2007 6:46 PM
To: Elecraft
Subject: [Elecraft] FSK on a K2?


Hi folks,

I'm thinking of getting a MicroKeyer from MicroHam for
my K2.  One of the attractions is that it claims I can
run FSK RTTY instead of AFSK.  Really?  I don't find
any mention of FSK with a K2 in anything I've
read...so either I don't know enough about FSK or I
don't know enough about my K2.  How do you do FSK on a
K2?

I'd also be interested in hearing from anyone using a MicroKeyer with their
K2.

Thanks and 73,

Ken Alexander, VE3HLS _______________________________________________
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