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K3 AFSK TX Filter

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Dave Barr-2

K3 AFSK TX Filter

I have heard that the configuration of AFSK TX - FIL ON/FIL OFF affects
afsk transmitted by feeding computer sound card generated afsk audio
tones. The entry in the manual seems to indicate the filter is only
active on afsk generated by the K3 internal afsk capability. Which is
correct? (I think I already know.)

Dave, K2YG
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Joe Subich, W4TV-4

Re: K3 AFSK TX Filter

> The entry in the manual seems to indicate the filter is only
> active on afsk generated by the K3 internal afsk capability.

The K3 does not generate AFSK internally. AFSK is only transmitted
in response to external audio.

73,

... Joe, W4TV

On 2014-12-04 12:41 PM, Dave Barr wrote:

>
> I have heard that the configuration of AFSK TX - FIL ON/FIL OFF affects
> afsk transmitted by feeding computer sound card generated afsk audio
> tones. The entry in the manual seems to indicate the filter is only
> active on afsk generated by the K3 internal afsk capability. Which is
> correct? (I think I already know.)
>
> Dave, K2YG
> ______________________________________________________________
> Elecraft mailing list
> Home: http://mailman.qth.net/mailman/listinfo/elecraft
> Help: http://mailman.qth.net/mmfaq.htm
> Post: mailto:[hidden email]
>
> This list hosted by: http://www.qsl.net
> Please help support this email list: http://www.qsl.net/donate.html
> Message delivered to [hidden email]
>

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David Woolley (E.L)

Re: K3 AFSK TX Filter

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.

--
David Woolley
Owner K2 06123
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Joe Subich, W4TV-4

Re: K3 AFSK TX Filter

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.
>

Lyle Johnson

Re: K3 AFSK TX Filter

In reply to this post by David Woolley (E.L)

In the case of the K3, the FSK mode uses DSP-generated IF frequencies in
the 15 kHz range for the Tx, and simultaneously creates audio tones at
the selected pitch-pair.

In AFSK mode, the radio requires that you supply audio to a Mic
connector or the Line In connector. The AFSK Tx filter may optionally
be employed, but its use is not required and with later versions of DSP
code is probably of marginal utility.

In the case of the KX3, the tones are generated at baseband.

73,

Lyle KK7P (who emphatically agrees these products are SDRs

> 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.
>

Don Wilhelm-4

Re: K3 AFSK TX Filter

In reply to this post by David Woolley (E.L)

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.
>

David Woolley (E.L)

Re: K3 AFSK TX Filter

In reply to this post by Lyle Johnson

Bottom quote below:

> In the case of the K3, the FSK mode uses DSP-generated IF frequencies in
> the 15 kHz range for the Tx, and simultaneously creates audio tones at

15kHz is audio frequency.

> In AFSK mode, the radio requires that you supply audio to a Mic
> connector or the Line In connector. The AFSK Tx filter may optionally

That audio is really an IF of about 1kHz. The same probably applies to
the KX3.

The actual difference here isn't between audio and non-audio, it is
between pure real and complex. The algorithm that generates the IF
level FSK signal within the DSP has access to both I and Q channels, so
the image can be phased out when upconverting. Although the PC may well
use almost the same algorithm, it only has access to the I channel,
which means that the DSP has to do what it does with audio, either
generate the Q channel, using a Hilbert transform or equivalent, or
filter out the image.

(Interestingly, whilst there is a big demand for I and Q outputs, there
doesn't seem to be a corresponding one for I and Q inputs, although that
is what one wants for efficient processing of data modes.)

--
David Woolley
Owner K2 06123

On 05/12/14 01:28, Lyle Johnson wrote:

> In the case of the K3, the FSK mode uses DSP-generated IF frequencies in
> the 15 kHz range for the Tx, and simultaneously creates audio tones at
> the selected pitch-pair.
>
> In AFSK mode, the radio requires that you supply audio to a Mic
> connector or the Line In connector. The AFSK Tx filter may optionally
> be employed, but its use is not required and with later versions of DSP
> code is probably of marginal utility.
>
> In the case of the KX3, the tones are generated at baseband.
>
> 73,
>
> Lyle KK7P (who emphatically agrees these products are SDRs
>
>> 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.
>>
>
>