??? Good deal, Tony, Bill. Well, Bill, I'm afraid I'll
just have to be one of those that reaps the benefits of the experimentation done
by others. I am new to most of this. I'll be the first to admit, I need to get
more involved in this kind of stuff so I'll learn more about it. I'm kind of
starting to head in that direction, though. I am trying to find some beginners
documentation on programing Pic Microcontrollers. Won't really know what to do
with it once I learn (applications), but I'll at least have a working knowledge
of what some of the guys are talking about when discussing Pics and firmware.
??? I am not sure if I asked the question the right way, and so
I am not sure if you guys are talking about the same signal that I am refering
to at 7.056. The 600Hz tone I hear is -75 dbm. Are we talking about the same
thing?
??? Also, I noticed that when I start Power SDR, the volume for
the soundcard is turned up to Maximum. Is there a way to have it initialize to
the previous setting on start up? I may be missing something
here.
?
?
Stan Rife
W5EWA
Houston, TX
K2 S/N
4216
?
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I believe the gunk you see near
the center frequency with or w/o hardware
attached is an artifact of the
signal processing architecture, and? noise
near DC on the audio input
(ie 60 hz hum,. etc)
How the software tuning works is by having a
software mixer fed with the
local oscillator to dowconvert the signal of
interest.? In the case of the
SoftRock, you have the hardware QSD
downconverting and quadrature sampling
a 48 khz swath of RF centered on
7.056 Mhz.? To listen to a signal at 7.060
MHz, the software mixer is
set to run with a local oscillator at 4 khz to
mix down the 7.056 Mhz
centered signal.? As you tune closer to the center
frequency, the
freq of the software local oscillator gets lower and
lower.? Very
near the center frequency, the local osc will be in the 100's
of hz,?
Mixed with 60 hz (and harmonics) hum this will tend to product
responses
at 60Hz (and harmonics)? +/- SoftwareLO? etc -- thereby getting
you the gunk around the center frequency.
I will have to admit that
my understanding of this phenomenon is not as
crisp as I would like it to
be. As Tony said in a prior post, there's some
hope that some of this can
be eliminated with DSP magic.? One approach
would be to record and
characterize the noise characteristics with no input
and then try and
remove them with DSP magic when fed with a real
signal.? One could do
this in either the time domain or frequency domain,
although I'm not sure
how one would maintain sync with the noise signal in
the time
domain.? My DSP skills are not strong enough (yet) to really know
how
feasible such an approach would be.
Another (potentially naive)
approach might be to try a different tuning
approach.? If one is
interested in a sig at 7.060, I'd think one could take
an FFT of the input
signal, zero the FFT bins outside the passband of
interest, shift the
spectrum over by 4 khz and then IFFT the result to
recover the signal of
interest.? I think this should work, but not having
tried it really
can't? say for sure.? Also not too clear in my mind if this
would be better than the software mixing approach currently used as you
get
closer to DC.
One thing to be aware of -- in a more complete
SDR with a frequency agile
downconverter, you typically don't tune in
software down near DC.? For
example, on the SDR 1000, the tuning is
always such that you're
approximately 11khz above DC for software
tuning.? The reason for this
being approximate is that the DDS tuning
is limited to tuning to freqs
where the DDS generates a minimum of
spurs.
I will be the first to admit I'm somewhat of a neophyte with
DSP.? SDR is
an area rich with opportunities for learning and
experimenting, and we've
now got hardware and software accessible to the
amateur community to
experiment with.? Hoping folks get in there and
do some experimenting and
learning.
Cheers,
Bill
(kd5tfd)
At 01:25 PM 9/18/2005, Stan Rife
wrote:
>??? Tony, one thing I noticed, when just playing
around with the SDR
> software, was the 600 Hz tone that comes from the
soundcard (?) at the
> 7.056 freq. after the Fixed HW oscilator is set
up per the instructions.
> This is, of course, without the SoftRock
hardware installed, as I do not
> even have my kit YET. (boo
hoo).
>???? Is this oscilator tone, at that freq.,
something that will always be
> there? I saw a procedure in the manual
that said something about going
> above 7.056, as much as the test
oscilator is below 7.056 and adjusting
> for a null. I am not quite
clear on all of this. I do have an XG-1, and I
> assume that is what
this is refering to.
>
>
>Stan
Rife
>W5EWA
>Houston, TX
>K2 S/N
4216
>
