[Elecraft] K3 Harmonic Distortion

[Brendan Minish]

Jack 

I have seen postings on the main reflector recommending settings as low
as 3 to avoid distortion products that are being attributed to
transformer distortion. This did not really make sense to me, especially
at such low levels where consumer sound cards tend to get non-linear and
nasty.

I took a look at the specifications for the TTC-108 audio transformer
used in the line out of the K3 

Nominally it's specified as having an operating level of -45dBm to
+7dBm 
THD is specified at 0.5% @0dBm 300Hz to 3.5 Khz @ 0dBm
this would make the transformer related harmonics worst case at -46dB
relative to the fundamental  

I did some measurements with an s9 CW signal zero beat at various pitch
settings
I used an old copy of SMAART pro that I have from my Sound engineering
days. the sound card is a delta44 

Lin = 100       Pitch 300       pitch 550       pitch 800
DR3             47.3dB          51.4dB          54dB

Lin = 50        Pitch 300       Pitch 550       Pitch 800
DR3             43.1dB          47.6dB          50.6dB

Lin = 10        Pitch 300       Pitch 550       Pitch 800
DR3             35dB            39.7dB          43.1dB

Lin = 5         Pitch 300       Pitch 550       Pitch 800
DR3             31.9dB          37.7dB          42.7dB

Lin = 3         Pitch 300       Pitch 550       Pitch 800
DR3             31dB            40.3db          47.6dB

DR3 is the Dynamic range between the fundamental and the 3rd harmonic
(the most dominant harmonic by a wide margin)

I did a quick look at 5th and 7th orders too, again the best performance
here was found with LIN = 100  

LIN = 100 is an output level of approximately 700mv for an S9 CW sig on
my K3 

The Harmonic distortion does not increase with higher line out levels,
it actually improves. This does not appear to be a transformer
saturation issue 

With lin = 100 users will see a ghost on the waterfall and may risk
over-driving some consumer sound cards. lowering LIN may get rid of the
visible ghost by reducing the 3rd Harmonic to below the waterfall
threshold (& perhaps the sound card input noise floor ) but also reduces
the overall usable dynamic range  


73
Brendan EI6IZ 

-- 
Don‘t complain. Nobody will understand. Or care. And certainly don‘t try
to fix the situation yourself. It‘s dangerous. Leave it to a highly
untrained, unqualified, expendable professional.