[Elecraft] K3 Harmonic Distortion

[Jim Brown]

On Mon, 01 Sep 2008 18:45:36 -0400, Jack Smith wrote:

>I've found that the LIN OUT port has typical third harmonic levels 
>around -45 dB from the fundamental, with the LIN OUT level set at 10, 
>the factory default. The headphone port, under the same conditions, 
will 
>show the third harmonic down about 70 dB or so at levels appropriate 
for 
>my Sony high sensitivity headphones. Cranking the audio level up will 
>cause higher distortion on the headphone port, but only at levels that 
>are impossible to listen to as they are so loud as to  be unbearable.

>I've also seen some non-harmonic artifacts, particularly when very 
>strong signals are pumped into the K3's antenna port.

Jack,

I assembled the first of the following reports in June after getting my 
first K3 running, and sent privately to the guys at Elecraft. That's why 
the new default LIN OUT gain is 10 rather than 50. It's also why there's 
a note in the new manual about LIN OUT level.  

http://audiosystemsgroup.com/K3FilterStudy.pdf

A month or two later, I prepared a report on filters in my Yaesu 
FT1000MPs (mostly Inrads) and my K3.

http://audiosystemsgroup.com/FilterTestNotes.pdf

This second report describes the test methodology use for the earlier 
work, which uses rather a high quality pro audio FFT system (EASERA 
SysTun). 

My measurements compare pretty well with yours, but use a very different 
measurement method that is not dependent on the distortion of a signal 
generator. My method also makes the IM stick out like a sore thumb!

As you can see from the first graph in the first pdf, distortion at the 
headphone output (the black curve) is 0.02% 2nd harmonic, 0.014% 3rd 
harmonic, and IM on the order of 0.1%. This was done at about 1 VRMS, 
and was done after the LIN OUT measurements without changing levels. 

The only difference between Joe's measurements and mine is that my 
analyzer has a bit more resolution, but we're all getting the same 
result. The headphone out is VERY VERY VERY good, but the LiN Out starts 
producing distortion at a rather low level. BUT, that low level is quite 
sufficient to drive a sound card. 

As to the setting of 3 or 10. The distortion is mainly caused by the 
output transformer, and distortion in an output transformer increases 
rapidly at low frequencies when it approaches its amplitude limit. The 
data output is MOSTLY intended for decoding digital signals, especially 
RTTY, and the distortion at 2 kHz is a LOT less than at 400 Hz for the 
same levels. But distortion is excited not only from signal, but from 
noise, and impulse noise can get pretty high. 

Taking all of this into account, I run my LIN OUT gain at 3, and 
recommend that setting. 

73,

Jim K9YC