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Hi everyone,
Just thought I would add a commentary on this
topic, after an experience recently.
In the past I have had tuning errors which increase
in magnitude as the console displayed frequency increases. These were due
primarily to the DDS not being effectively driven/controlled via the parallel
port. Several reasons for this - poor electrical contacts on parallel pins,
signal drive fron the computer inadequate (mainly with noteboooks and laptops
using CMOS 3-3.5V levels rather than 5V), and problems with USB signal drive
when using the USB/parallel converter with some laptops. Usually all of these
problems have been cured temporarily by refitting connections and
powerdown/power up of the software and hardware. A more or less permanent cure
was obtained for use with notebooks and laptops by replacing the 74AC574 (IC1
& IC3 on PIO board) with 74ACT574, which are happy with a lower input
signal. All this time I had been using the Extigy soundcard, which at the time
seemed the best for notebook use. Computer details were Acer notebooks 1703
& 1714, former 2.6GHz P4, 1GHz RAM and middling graphics performance, the
latter 3.4GHz P4, 1GHz RAM and good graphics accelerator.
Seeking better receiver performance, I changed over
to the M-audio D-44 and a desktop machine- performance much the same as the
1714, but of course more poke on the parallel port. Tuning using this
combination was pretty secure, but not completely so, as after software changes
the SDR would on rare occasions start up a few kHz low, corrected by a
software/hardware re-start. I too use a whole range of known frequency standard
transmissions to check the SDR each time I switch on.
But I still preferred the notebooks for a quieter
and cooler life - so when the Firebox was accomodated in the SDR software I
reverted to the 1714 notebook to run the SDR-1000. Frustration again!! This time
the notebook was purged of all the old SDR related software and the Windows
registry cleaned up of Creative junk. All the installation went smoothly, but
the radio tuned 20kHz lower than the console displayed frequency throughout the
range.
No amout of uninstalling/reinstalling software
which could concievably affect the tuned frequency such as the database file,
the Cypress USB/parallel driver, and PowerSDR itself (1.4.5 Beta 3 & 7 in
use) made any difference. I checked the audio connections to ensure I could not
possibly be working with the image, and tried T-R-S jacks instead of T-S jacks -
all to no avail. I also swapped over to another USB/parallel converter cable -
again no joy.
In the end I loaded the console(1.4.5 Beta 7)
without the soundcard or radio powered up, and tuned the console to 11,025kHz.
This would force the DDS into an impossible position, since it would be trying
to produce a negative frequency! I then powered up the radio and soundcard, but
as expected no signals were received. After power down, and unloading the
console, I powered up the hardware and Firebox again, loaded and powered on the
console, and hey presto! - the console and the actual received frequency now
agreed.
I then tried the earlier 1.4.5 Beta 3, which had
also presented the same problem. It too was now giving the correct frequency
readout.
Since the only common thread in all this lot
appears to be the SDR hardware, it does appear possible for the oscillator/DDS
to get stuck in an off frequency mode, which requires a jolt such as I described
to get them back on frequency. However, I still have lurking suspicions about
the Windows registry.
I hope these ramblings are of use in helping
someone stuck with the same problem, but I would like comments from our hardware
and sortware gurus.
Peter G3LWT
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