Bill Dumke wrote....
So I next swept the input with the low
pass filter output of a DDS-60 fed through a 10 dB/step step attenuator
to set the level at the SoftRock-40 RF input....
End quote:
Hi Bill, thanks for the tip, I did as you described and had some
success but I can still only get rejection of the unwanted signals at
one point in the band. This is within +/- 2 kHz of the band centre. I
cleared the existing cal' data and injected a strong steady signal but
Rocky just seemed to ignore it until it was within a few kHz of the
band centre at which point it "latched" on to it and appeared to give
some rejection of the unwanted signal at that point. But try as I
might I have been unable to get rejection in any other part of the
spectrum. Rocky seems to be picking up a few more cal' points (besides
the one that gives some rejection) but for some reason none of the
additional cal' points seem to do any good. The best rejection I get
is about 15 to 20 dB.
Because of a posting on the KGKSDR mailing list I became aware of a
problem with the Soundblaster Live! series of cards and when a "fix"
was added to KGKSDR I found it solved all my I/Q problems. By setting
the "Delay right by 1 sample" option in KGKSDR I now get pretty good
rejection of unwanted signals using KGK's auto-cal. By performing a
manual calibration I do even better with about 50 dB over most of the
band. I think I can probably improve on that by doing another cal'
with more cal' points.
I do plan to upgrade my soundcard at a later date but the current PC
(Intel P4 1.4 GHZ, 384 meg RAM) is as hot as it will go so I plan to
upgrade the system first and probably update the soundcard at the same
time.
Thanks again for the feedback.
73's to all on the list.
Des (M0AYF)
--- In softrock40@..., Bill Dumke <billd@...> wrote:
Des,
Rocky will self calibrate to some degree if it sees strong signals
(>60dB on the left hand scale) across the band for a long time. Rovky
does not need to be tuned to the frequency. This is all done in the
background. I left mine running for about 3 months , but the image
rejection was only about 18 dB. So I next swept the input with the low
pass filter output of a DDS-60 fed through a 10 dB/step step attenuator
to set the level at the SoftRock-40 RF input. Then I got about 73 or
74 dB image rejection doing it with two different sound cards, an
Audigy
2 and a Delta 44.
Bill WB5TCO
