Thanks for your typically thorough testing and analysis, Jack. I would
add one comment to your tests as it relates to my testing during the
design of LP-PAN. You assume a 10K load Z for a typical sound card. I
have now characterized half a dozen "professional" sound cards suitable
for use with broadband "softrock" type receivers. In my testing I have
found substantial variation of input Z vs. frequency, ranging from under
500 ohms to almost 100K ohms depending on card and frequency. It should
be noted that my test equipment leaves something to be desired when
measuring Z at very low audio frequencies. I don't know that such loads
would affect the distortion of the circuit being tested, but I suspect
it could at certain frequencies.
I add a resistive load across the xfmr secondary to moderate the effects
of the varying loads, and set a lower limit for cards with extremely
high input Z. The optimum value changes from card to card, but I found
approx. 2K ohms to be a reasonable choice with LP-PAN (using SP70
xfmrs) where flat response out to 100 kHz is a primary design goal.
This, of course, would not be the case for a K3 audio output where 5 kHz
would be adequate response. The added load could probably be lower in
that case, but that may be counter productive if the sound card has a
low Z in the 400-800 Hz range used for CW. These things are never
simple, are they?
Larry N8LP
Date: Fri, 05 Sep 2008 21:22:33 -0400
From: Jack Smith <[EMAIL PROTECTED]>
Subject: [Elecraft] Harmonic Distortion and Audio Intermodulation in
K3's LIN OUT Port
To: [email protected]
Message-ID: <[EMAIL PROTECTED]>
Content-Type: text/plain; charset=ISO-8859-1; format=flowed
I've completed a series of detailed measurements trying to understand
why my K3's LIN OUT port exhibits greater distortion and anomalies than
found in the headphone and speaker ports.
Details are at
http://www.cliftonlaboratories.com/elecraft_k3_receive_audio.htm
What I found is that the 604 ohm resistor between U29's output amplifier
stage and the TTC-108 transformer primary winding forces the TTC-108
isolation transformer to operate in a regime where the performance
improving effects of a zero ohm driving source are not available. Hence,
the TTC-108's 0.5% THD performance significantly limits the LIN OUT
ports performance.
I've also provided a theoretical parts value change that should improve
the LIN OUT distortion and intermodulation. I have not implemented this
change in my K3, but the external circuit mockup I used for testing
strongly supports the change.
I've supplied my measurements to the gurus at Elecraft for their
consideration. If my data and recommendations make sense, I would
expect confirmation from Aptos, after they have time to think it over
and consider possible side effects.
Jack K8ZOA
www.cliftonlaboratories.com
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