[Elecraft] K3 - FT8

[Alan Ibbetson]

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.
>>
> 

-- 

Alan Ibbetson
alan at g3xaq.net