[Elecraft] K3 - FT8

[Gordon LaPoint]

The problem was discussed in this thread  last year.
http://elecraft.365791.n2.nabble.com/K3-Odd-ALC-behavior-td7633052i20.html

Gordon - N1MGO
On 05/14/2018 03:05 AM, Alan Ibbetson wrote:
> Don,
>
> I'm puzzled you when you say
>
> "If it were a K3 problem, that aliasing would also be happening with 
> other  data modes as well. Other soundcard data modes do not have a 
> problem with a TUNE at 1000 Hz"
>
> I can reproduce this problem with my K3 with MMVARI on other data 
> modes. I have tried it with another PC and sound card and again I can 
> still see the problem. So I suspect that feeding a pure sine wave from 
> any source into LINE IN can cause ALC bargraph artefacts at certain 
> specific audio frequencies.
>
> This is not FT8 specific, nor is it audio harmonics or IMD. It is 
> simply an unfortunate choice of audio tone frequency interacting with 
> the K3 internals. Or at least that is my best guess from the evidence 
> I see here. The only constant throughout these tests is my K3. If it 
> is faulty then OK, no big deal.
>
> I hoped by reporting this effect on this mailing list I might help the 
> others who have also commented about it. If there is a consensus that 
> it's specific to my setup then OK. I can reliably avoid my problem by 
> changing the audio tone frequency just a few Hz so I will just do that 
> and get on with my life.
>
> 73, Alan G3XAQ
>
> On 13/05/2018 23:20, Don Wilhelm wrote:
>> Alan,
>>
>> That is getting into issues associated with WSJT-X and FT8.
>> My website article is written to apply to all soundcard data modes and
>> not FT8 specifically.  There was no intention for addressing all the
>> various data modes that could be used.  All of them require the right
>> amount of audio drive.
>>
>> You should look to the FT8 forums for questions about that aliasing.  If
>> it were a K3 problem, that aliasing would also be happening with other
>> data modes as well.
>> Other soundcard data modes do not have a problem with a TUNE at 1000 Hz,
>> so why is FT8 any different?
>>
>> Of course, if your soundcard is generating harmonics or significant IMD
>> before it reaches the K3, there is not much one can do other than
>> getting a different soundcard.
>>
>> I don't know about the "moving 10kHz" thing, but I have heard reports of
>> similar symptoms.
>>
>>
>> 73,
>> Don W3FPR
>>
>> On 5/13/2018 4:22 PM, G3XAQ wrote:
>>> Don,
>>>
>>> I suggest you need to draw attention in your web page and general 
>>> advice on
>>> here to the requirement to choose the AUDIO FREQUENCY with some care 
>>> when
>>> setting up ALC for FT8 via the TUNE button in WSJT-X.
>>>
>>> The problem seems to be that the DSP sampling rate inside the K3 
>>> must not be
>>> related to the TUNE frequency. Ideally I expect they should be 
>>> co-prime, or
>>> something like that, to avoid aliasing artifacts in the ALC bargraph
>>> readout. The actual frequency you should choose is probably affected 
>>> by PC
>>> and K3 clock frequency accuracy becasue a change of just a few Hertz 
>>> can
>>> have a dramatic effect.
>>>
>>> In my case, the default WSJT-X tune frequency of 1000Hz is about as 
>>> bad as
>>> you can get. My ALC bargraph oscillates with a beat frequency of not 
>>> much
>>> more than 1Hz. And the ALC goes from zero to 6 bars! What is really
>>> confusing is that WSJT-X leaves the tune frequency at whatever it 
>>> was when
>>> you last transmitted, so if you tune around, click on some signals, 
>>> have a
>>> listen and then try using TUNE the audio frequency could be anywhere 
>>> unless
>>> you take specific action.
>>>
>>
>

-- 
Gordon - N1MGO