Hi Eric,
Not being able to null the image has usually been associated with the I and
Q signals being crossed or one missing. Check to make sure the stereo cable
is connected to the board with the stereo connector tip to C18, the via
nearest the corner board mounting hole. Check with a meter to make sure
there is nothing to short out one of the I or Q signals. Also, if the QSD
is not getting one of the clock phases, open at a IC pin or short between
pins, it can result is such an unbalance between the I and Q signals such
that one can not get a null. Solder bridges between IC pins can sometimes
be hard to see without looking with good lighting and magnification.
Please let me know what you find or if you still have the problem. I may be
able to suggest other things to look at.
Good luch with the debug.
73,
Tony KB9YIG
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----- Original Message -----
From: "Eric" <eric_ke6us@...>
To: <softrock40@...>
Sent: Tuesday, September 20, 2005 8:10 PM
Subject: [softrock40] DSP Image Rejection problem and some thoughts
The radio went together fairly easily, though I'm at
a loss to figure out why an experimenter's radio is
jammed onto such a small board. This might be OK for
a production rig, but some room would be nice to
experiment.
That said, the radio works except there is no image
rejection and the DSP image rejection controls do
nothing. The instructions say, "If the image cannot
be nulled at least 50 db, then a problem exists..."
Well, some guidance would have been helpful.
Especially, given my opening criticism.
I am reluctant to tear into this think quite yet as
there is not a lot of room to work. I originally
thought it may be a transformer mis-wiring on my part.
Again, we're talking an experimenter's radio on 40
meters where things are relatively less critical than
other bands, so a little larger toroid with more room
to make changes would make it a lot easier.
I'm hoping when I fix the image problem that all
that 7.056 +/- 5 khz energy is going to at least be
diminished. 7.056 is S5. 7.051/7.061 are S3.
Anyway, I have no doubt the problem is my error. I
just want a little direction so that I can minimize
stress on the board.
Eric
KE6US
www.ke6us.com
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