"gareth" wrote in message
...
"Brian Howie" wrote in message
...
In message , gareth
writes
Presumably those SDR rigs which do not work on the IF but
directly from antennae must have, separately from the DSP processor,
some semblance of a DDS generator (but without the final DAC) to
act as the equivalent of the VFO, for I cannot perceive that a
fractional-Hz
tuning rate could be achieved with machine code running in the DSP
processor?
I'm not an expert ,but I think what you're asking is " how is the local
oscillator generated" in a direct conversion SDR and "what determines its
resolution"
There is an example here, :-
http://www.radioelementi.it/public/saqrx.pdf
The "c" source code is here,which I can just about understand ( My
software background is FORTRAN and Matlab) :-
https://sites.google.com/site/sm6lkm/saqrx/
Softies shouldn't have a problem with it although I was able to mess
about with it and recompile it successfully
In this case the spectrum is dc to 22050Hz in 512 steps. It's not the LO
precision ( it's floating point in this one) that limits it but the size
of the FFT , the sample rate and thus the record length, that sets the
minimum FFT bin width . This one tunes in lumps of about 43Hz
Thank-you Brian, but what you have URLed is already at baseband, being
VLF.
Multitudinous apologies, Brian, I tell a lie, so thank-you.
I've downloaded all (and even run the prog although nothing is attached to
the
input of the sound card) and will proceed to study the code, although,
obviously in this
case, at a max freq of 22kHz, creating a VFO is quite within the
capabilities of
PC-based machine code.
Also, a study of the code should answer a previous question of mine about
how to use
the sound card in applications such as this.
As I indicated elsewhere, the nub of my question was correct, there is a
VFO-equivalent
in the front end of wide-range DSP SDR rigs, that is separate from the
DSprocessor
(except in those cases where a DFFT is applied to the whole input range)
Thanks again for your effort.
Otherwise, for light relief and to ease the aggressive tensions that my
innocent enquiry
seems to have fomented, I append an article that I wrote nearly 20 years ago
....
-----ooooo-----
Alimentary Fourier Anal-ysis
----------------------------
(C) Copyright 1997 Gareth Alun Evans G4SDW
The Fourier Transform is rather like the Human Alimentary canal....
The Alimentary Canal takes food, and then transforms it into
something that is a function of the rear end, and hence may
be referred to as, "The Food-Rear Transform".
The Fourier Transform causes integration over a one second
period, and results in an infinite series of sines.
The Food-Rear Transform causes disintegration over a 12-hour
period, and results in an infinite series of signs...."Gents",
"Ladies", "Rest Room", etc. etc.
The Fourier Transform (Tukey's reasoning?) takes Time and
transforms it to a frequency spectrum.
The Food-Rear Transform takes Parsley, Sage and Thyme (Turkey's
seasoning?) and transforms it to a frequently active rectum.
"Laplace", literally, "The Place", is how you deal with
the transform.
"Walsh" is something you do to your hands afterwards.
"JPeg" is what you hang your jacket on to. It is a form of
compression such that you take up less space and spend less time
there (although the output may be more granular).
The Fast Fourier Transform must be done through an appropriate
window, otherwise an effect known as the "Picket Fence" problem,
also known as "Spectral Leakage", can occur.
Diarrhoea is known as the FAST Food-Rear Transform, and if done
through the wrong window can result in a problem (also known
as Rectal Leakage) all over the Picket Fence.
The Picket Fence problem is also known as, "Scalloping",
something which is normally done to pie crusts. In the old
days, pies used to be made with "Four And Twenty Blackbirds";
but nowadays, members of the Jay family are used. When you go to
someone's house for tea, you politely eat everything that is
put in front of you, but your reaction might be,
"Eat the Jay (Oh my God!) Tea???".
Similarly, the Fourier Transform is a great consumer
of e^(jwT)... "E to the j, omega, t".
The Fast Fourier Transform is famous for its Butterflies.
The Fast Food-Rear Transform is famous for its
Butterflies-in-the-stomach.
Dirichlet's are the Fourier Transform's conditions, without which
the result is divergent.
Diarrhoea ****s are the Food-Rear's conditions, with which the
result is VERY divergent.
Having experienced the Fast Fourier Transform, you are then faced
with the Inverse, which can be very daunting. It is a paper
exercise but it can be quite messy.
Having experienced the Fast Food-Rear Transform, you are then
faced with the "In Verde", literally "In The Grass", which can be
very daunting indeed. (The paper exercise can be quite messy.)