[Elecraft] K3 and Datamodes

[Bob Nielsen]

On Jun 6, 2007, at 7:52 AM, Vic K2VCO wrote:

> Julian G4ILO wrote:
>> On 6/5/07, FISCHER,GREG <ab7r at cablespeed.com> wrote:
>>> I believe the only time you will need some other external
>>> interface would be for true FSK.  The line for this is on
>>> the ACC connector and should be keyed via an open
>>> collector at less than 5 volts.
>> Why would one want to use true FSK if you are using for receive a
>> modern datamode program that is capable of AFSK anyway? With AFSK you
>> can net on a station by clicking on the waterfall. With FSK you have
>> to tune the radio to the exact frequency.
>
> I must be missing something. I thought AFSK was an FSK signal  
> produced by feeding an audio tone which is switched between mark  
> and space frequencies to an SSB transmitter, resulting in a single  
> RF carrier whose frequency changes.
>
> This signal, if the tones are clean sine waves and the transmitter  
> has good carrier and opposite sideband suppression, should be  
> *identical* to a 'true' FSK signal, which is produced by varying  
> the frequency of a VFO in the exciter.
>
> If you mean that it's easier to tune an AFSK signal that has some  
> residual carrier, that's a poor signal.

It isn't necessary to have any carrier leakage.  With the modern  
software demodulators using a waterfall display one can fine-tune the  
received signal by clicking on the waterfall display.  This adjusts  
the mark/space frequencies rather than tuning the VFO.  The  
transmitter mark/space audio frequencies are then matched to those  
used to receive. Having previously (40 years ago) used a fixed-filter  
TU with a "green eye" tuning indicator and a diode-switched shift  
capacitor on my transmit VFO, I was a bit skeptical until I tried it,  
but it works quite well.

73 - Bob, N7XY