I'm pasting this on behalf of Bob, K6XX, who was kind enough to send me a
personal reply
after Yahoo gave him guff when he tried to post. Now it's in the archives.
Thanks everyone else for replies as well. I've read them but, am now busily
getting my
work done like a good beaver so I can leave for Dayton in 2 days. See you at
FDIM!
I'm using some random SB card, but, before I leave for Dayton, I'll get a
Delta44 ordered
and maybe it'll arrive by the time I return.
I suspect I had this problems 2 years ago when I built the thing, I had this
problem, but, I
was a different ham then, and I also was just excited to have the thing hearing
signals! I
suspect that I didn't notice it with Rocky because I wasn't looking for it.
Joe, KC0VKN
----------------------
Excellent questions, Joe.
Hopefully a real expert will tell us both how this is best done. But
until
then, here's the procedure I use. That's USE in the present tense,
since I'm
doing this task over and over each time something changes here. That
measly
$10 investment in the SR hardware has incurred several hundred dollars
of
expense here buying a new, more powerful PC and several increasingly
capable
(but expensive) sound cards!
Instead of running Skimmer, use Rocky. It has the same type of
self-calibration routine, but its wideband spectrum analyzer display
helps
you see what is going on faster than the waterfall. Once Rocky is
ready,
we'll get back to Skimmer.
1. Connect the SR to your sound card.
2. Get a signal generator. I've been using an Elecraft KX1 into a
Ethernet
resistive termination so I can place it close to the SR antenna input.
But a
regular dummy load and transmitter has enough leakage to do the job. If
you
use the Ethernet terminator, use low power of course. Less than a half
watt.
(But ignore the 75 ohm value mismatch--its close enough to not hurt the
transmitter). My transmitter system works better than my signal
generator
because its output waveform is much, much cleaner. Remember, the
transmitter
is not connected to the SR antenna input. The dummy load is merely
placed
alongside the SR board.
3. Transmit a stream of dashes near the bottom of the SR tuning range.
Do
NOT tune it near midrange!
4. Vary the transmit frequency. If you QSY up, does the Rocky display
show
the dashes moving up? If so, your I/Q is correct. If the display shows
the
dashes moving down, the IQ is backward. Change it in VIEW:SETTINGS.
There is a third situation: where you see a strong image on both sides
of
center frequency and one moves up and the other moves down. This means
your
balance is way off and the SR hardware needs attention. For example, is
one
of the audio channels disconnected? This fortunately does not appear to
be
your situation.
After this point, the SR is ready for autocalibration in either Rocky
or
CWSkimmer. However, I'm not patient enough to await the dozens of
strong
signals distributed all across the band that are required for the
autocal to
do its job by itself. I "accelerate" the process a bit...
5. Move the dummy load ("probe") or increasing the signal output from
your
generator until you have >40dB of signal indication on Rocky. In fact,
set
it to 50dB or 60dB. Open the RX IQ Balance window (Tools). Sweep the
signal
across the band slowly. You should see red and blue dots appear and a
approximation line or curve connect their averages. I've found a stream
of
dashes works better than a solid carrier, but I'm not certain about
this
observation.
6. If your Phase samples are within a couple of degrees and your gain
is
rather flat or at least gently sloping, all the way across the band,
your
receiver is working fine. If the phase shows +/- 20 degrees of
variation,
and/or the gain or phase curve is piecewise continuous (broken into
separate, unconnected curves), you have a hardware problem. Do not
worry
about a big separation at midband--this is normal. Nothing seems to
work
right around the SR's center frequency. That's why we started this
procedure
near a band edge rather than near midband.
7. Now that Rocky is working and calibrated, close it. Open CW Skimmer
and
re-sweep the band using the same setup and signal levels. Although the
same
technique is employed with both Rocky and Skimmer, their Auto I/Q
balance
data is incompatible and you must re-do it for each program.
Fortunately it
only takes a few minutes with a local signal generator instead of the
hours
awaiting random, strong on-air signals.
8. Also, this procedure must be re-done each time you change sampling
rate
or sound cards. That means if you have calibrated your SR and soundcard
at
48kHz sampling, you must repeat the process when you switch to 96kHz.
When I
added a preamp (to eliminate the oscillator backfeed issue), I also saw
some
change in the Auto I/Q balance.
I hope this has been helpful. I developed this procedure after fussing
with
the autocal for many hours/days. Hopefully someone who really "knows"
how it
should be performed will speak up and let us all know!
73 de Bob, K6XX
www.k6xx.com
