[Elecraft] K3: Odd ALC behavior

[David Anderson]

I was thinking that as the "ALC" meter isn't really showing ALC, but audio level when below I believe 5 bars? - that the ALC isn't being modulated, rather what we may be seeing is an artefact caused by the way the peak level of the audio is detected by sampling the audio at a particular sampling rate.  

Because it isn't just a simple hardware analog peak detector that is used, the audio meter reading displayed will beat between the sampling rate of the audio detector and the actual frequency of the tone, even an absolutely pure tone with no harmonics. 

I have seen similar odd things happen with another make of radio with their power meter (which again is sampled) and AM modulation on the transmission. Putting a true analog power meter on the output showed no variation in power output, it was all an illusion.

It's just one of those annoying things that happen when we aren't dealing with purely analog methods of detection of frequencies. You see similar aliases when using modern Digital Sampling Oscilloscopes.

As you have pointed out the solution can only involve moving the software's tune frequency to a frequency that does not beat at such a visibly slow rate with the audio detector sampling rate. 

73 from David GM4JJJ

> On 5 Aug 2017, at 00:45, Wayne Burdick <n6kr at elecraft.com> wrote:
> 
> Is it possible that a beat note between tones (and their harmonics) is what’s modulating the ALC reading? If so, moving the tone slightly off the standard pitch may be an acceptable workaround.
> 
> Wayne
> N6KR
> 
> 
>> On Aug 4, 2017, at 1:42 PM, Wes Stewart <wes_n7ws at triconet.org> wrote:
>> 
>> Good find Brian!!! I see it too in FT8.
>> 
>> This probably explains the great variability in results reported. Some of us have a problem, others say, Huh, what the heck are you talking about?
>> 
>> Wes, N7WS
>> 
>> 
>>> On 8/4/2017 11:33 AM, Brian Hunt wrote:
>>> Here's some followup:
>>> FLDIGI shows the "pumping" affect with carrier tones of 500, 1000, 1500, and 2000 Hz.
>>> WSJT-X 1.8 shows the "pumping" affect with carrier tones of 1000, 1500 and 2000 Hz (couldn't select 500 Hz).
>>> 
>>> The affect is very narrow with frequency.  10 Hz either way and the output and ALC display and MON output is rock solid.  Changing the K3 filter center frequency has no effect on the above.
>>> 
>>> 73,
>>> Brian, K0DTJ
>>> 
>>>> On 8/4/2017 10:58, Brian Hunt wrote:
>>>> I spent some quality time with my K3, FLDIGI and an oscilloscope today.  I had the same problem as described.  Looking at my USB soundcard output to the K3, the amplitude of the audio from FLDIGI in TUNE mode was rock solid. Looking at the MON output from the K3, the level oscillated in time with the ALC "pumping" at about 2.4 Hz (timed with a stop watch).  At the same time the K3 power output into a dummy load was rock solid at a selected 10 watts.
>>>> 
>>>> Then I changed the carrier frequency in FLDIGI slightly off 1500 Hz and the "pumping" went away!  At 1515 Hz I could easily set the ALC to the desired 4 bars solid, 5th flickering.  Then I remembered (and verified) that the default filter center in DATA A mode is 1500 Hz. I changed the filter center frequency to 1450 Hz and moved the FLDIGI frequency around but couldn't reproduce the "pumping".
>>>> 
>>>> I plan to repeat the same tests with WSJT-X 1.8 but wanted to get this out to the group.
>>>> 
>>>> 73,
>>>> Brian, K0DTJ
>>>> 
>>> 
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