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
_______________________________________________
Elecraft mailing list
Post to: Elecraft@mailman.qth.net
You must be a subscriber to post to the list.
Subscriber Info (Addr. Change, sub, unsub etc.):
http://mailman.qth.net/mailman/listinfo/elecraft
Help: http://mailman.qth.net/subscribers.htm
Elecraft web page: http://www.elecraft.com
