About a half hour ago, I added frequency sweep data for the TTC-108
transformer to my
http://www.cliftonlaboratories.com/elecraft_k3_receive_audio.htm page.
The new data looks at frequency response with terminations of 620 ohm
and 10K, for both direct op-amp drive and drive through a 620R series
resistor. The good news is that even the worst case frequency response
(620R series to the primary and 620R termination) is more than adequate
for our purposes, 200 Hz to 10 KHz with reasonable flatness. Low
frequency response, of course, gets much better with direct drive from a
zero ohm source, and high frequency response improves with high Z loads,
but there's no real difference at the K3's maximum audio response. Of
course, that's far from the case with the LP-PAN where you seek flat
response up to 100 KHz and beyond.
Assuming your measured bounds of 500 ohms to 100K ohms is
representative of sound cards, I don't see a problem with the TTC-108,
assuming the TTC-108 is driven with a low impedance source, at least
from a frequency response point of view.
I have not yet looked at THD or IMD with varying terminations. I plan to
do that in the next week or so after my HP8903B audio test set arrives
and I can automate THD measurements. It's far too labor intensive to do
a lot of combinations at the moment.
Jack
Larry Phipps wrote:
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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