[Elecraft] K3 Harmonic Distortion

[Stewart Baker]

I find virtually the same results here using  50uV RF from an 
Elecraft signal source, and measuring using Wave Spectra.
An offset of 43 to 45dB, and below LIN OUT = 5 the harmonics drop 
rapidly.

73
Stewart G3RXQ
On Tue, 02 Sep 2008 12:28:02 -0400, Jack Smith wrote:
> Brendan:
>
> You might wish to look at the measurements I made, with LIN OUT 
settings
> from 2 to 100.  There's an animated GIF that steps through the 
LIN OUT
> settings so you can see  the effect. The data was  taken at 600 
Hz.
> http://www.cliftonlaboratories.com/elecraft_k3_receive_audio.htm 
about
> half way down the page
>
> Like you, I see a relatively constant 45 dB or so offset between 
the
> fundamental and 3rd harmonic for LIN OUT values of 10 and over.
>
> However, setting the LIN OUT at 2 results in quite a bit better 
odd
> order harmonic suppression, on the order of nearly 70 dB. By the 
time
> LIN OUT is set to 5, however, the odd order suppression is 
reduced to a
> bit more than 60 dB. At LIN OUT = 10, it's 45 dB or so where it 
stays at
> all values of LIN OUT tested between 10 and 100.
>
> (For some reason, the 2nd harmonic pops up at certain settings, 
but not
> others higher or lower, but that's not of immediate concern.)
>
> There's a  related issue with the AGC. The audio output is 
proportional
> to RF signal level, as the AGC has a slope to it. Hence, even 
though the
> LIN OUT might be set to, say, 2, the actual audio level and 
hence the
> distortion, will vary as the RF signal level changes. The degree 
of
> audio level variation with RF input changes depends on how the 
user has
> set the K3's AGC slope and intercept parameters.
>
> I'll make  the measurements on a couple of TTC-108 parts when 
they
> arrive as a matter of trying to understand where the harmonic 
distortion
> arises. To duplicate the K3's audio chain, I'll drive the 
TTC-108s from
> an op-amp so that it will be hit with a low Z source, instead of 
the
> standard audio generators available to me, with 50 or 600 ohm 
output Z.
>
> Jack
>
>
> Brendan Minish wrote:
>> 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
>>
>>
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