[Elecraft] K3 AFSK TX Filter

[Don Wilhelm]

David and all,

I would beg to differ, AFSK starts as external audio tones into an SSB 
transmitter.
How those tones are modulated into an RF signal is not dependent on 
whether the transceiver is DSP or analog, the output should be the 
same.  It may be digitized in the middle of the transceiver, but the 
input and the RF output should be the same.  The input is pure analog 
and not digital sine waves.

FSK is a slightly different 'animal' and in its simplistic form is the 
shift of a VFO frequency to create the mark and space tones. That too 
can be handled internally by various DSP algorithms, but the end result 
should be the same - an RF output that shifts in frequency between the 
mark and space frequencies.

Both are demodulated by the receiving station as tones to be decoded.  
Theoretically, there is no difference on the receiving end by an RTTY 
signal generated by AFSK or FSK means.

73,
Don W3FPR



On 12/4/2014 7:17 PM, David Woolley wrote:
> On 04/12/14 18:20, Joe Subich, W4TV wrote:
>
>> The K3 does not generate AFSK internally. AFSK is only transmitted
>> in response to external audio.
>
> But that is exactly how SDR architecture transmitters do generate FSK. 
> The only difference from what is popularly called AFSK is that the 
> tone starts as digital sine waves.
>
> The ones may be a bit higher pitch than one normally expects from 
> AFSK, but they are still audio tones.
>
> (As the K3 uses an I/Q structure, you could, theoretically, generate 
> the two tones as positive and negative tones at the same frequency, 
> but the problems with handling the DC crossover means that it is 
> unlikely that anyone would do that.)
>
> In case it is not clear, I consider both the K3 and KX3 to be SDR 
> designs.
>