In rec.radio.amateur.equipment Jerry Stuckle wrote:
On 2/6/2016 1:07 PM, gareth wrote:
"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.


When moving from FORTRAN to C, the major difference (apart from the
nitty-gritty
of statement syntax) is that in FORTRAN, variables are always passed by
reference
(at least in FORTRAN '66 which I did 47 years ago) and in C you have the
choice of passing
by value or by reference.


Wrong on both counts. Fortran and C are both pass by value. Neither
defines pass by reference in their respective standards although some
recent Fortran compilers have an extension to pass by reference).

In C, if an arguement to a function is defined as a pointer, then the
local values are references to the storage locations of the original
arguments passed in and changes will change the original value. This
is called pass by reference.

In C, if an arguement to a function is not defined as a pointer,
then the local value is a copy of the original value which will not
be changed. This is called pass by value.



--
Jim Pennino