[Elecraft] K3 AFSK TX Filter

[Joe Subich, W4TV]

While the K3 is an SDR, it generates FSK Mark and Space tones directly
at IF.  There is no digitizing of audio tones from the input and
numerically mixing them to IF and filtering (or numerically cancelling)
the unwanted products.

The fact that the tones are created in response to a DC control input
- not an audio input - is what makes FSK_D FSK not AFSK.

73,

    ... Joe, W4TV



On 2014-12-04 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.
>