odd results working at 250ksp/s

Ralph A. Schmid, dk5ras ralph at schmid.xxx
Mon Feb 10 10:54:08 UTC 2014

Thanks a lot - very interesting!





From: Sylvain AZARIAN [mailto:sylvain.azarian at gmail.com] 
Sent: Sunday, February 09, 2014 8:55 PM
To: David Jacobowitz
Cc: jdow; Sylvain Munaut; osmocom-sdr at lists.osmocom.org; Ralph A. Schmid,
Subject: Re: odd results working at 250ksp/s




With a lot of delay and my apologizes, as promised a while ago, I finally
published some quick notes on the VOR signal today.

The complete signal analysis as been posted on my blog at this page :


I need to save the recorded WAV signals somewhere so people interested in
good SNR recordings would get it.

I decided to return close to the nearby VOR transmitter (Rambouillet, south
west Paris) and put the RTL SDR on my car rooftop and turn on the laptop to
capture some signals... 


Best regards

sylvain F4GKR


2013-10-22 17:07 GMT+02:00 David Jacobowitz <david.jacobowitz at gmail.com>:

I originally posted about 115.8 and 116.8 MHz, both square in the VOR band
of 108 to 117.95. I might have sent something else in a PM, but if so it was
a typo. :-) I am definitely only interested in the VOR band right now --
though, as you say, it is adjacent to commercial FM with its high power.


My application is simple in concept: A fully auomatic VOR-based positioning
system, a fallback from GPS. I want to scan the entire VOR band, looking for
signals in the standard VOR format that can be demodulated. I do the initial
scan with a fast sample rate and FFT, just looking for peaks. From those, I
examine the signals to see if it looks like a VOR signal. From that list, I
will "park" on each signal long enough (~30s) to decode the VOR's morse code
station ID. From that, I will have a short list of VORs that I can currently
receive. From those, if the geometry is appropriate (I know the VORs
positions from a database) I can calculate a position.


The software then just round-robin tunes the VORs in range and continually
tries to calculate positions. If too many drop out, it returns to the
initial scan mode.


Not being able to receive this VOR or that VOR is not generally a problem,
but obviously, the more the better. With extra VORs I have better options
for choosing the closest ones or the ones with the best geometry.



This is actually quite difficult to test, because VORs can generally only be
received line-of-sight -- which means in the air. I'm a private pilot but I
found that flying and noodling with a laptop is too much trouble. From my
office window, on a high floor in Oakland, CA, I can receive one VOR from
the Oakland airport. And I have now discovered a ridge near my home with a
scenic overlook from which I can receive two.


I've tested the software enough to know that the initial scan function seems
to work, and the morse decoding kind of works, but I am not confident I'll
get it to work very well. (One transmitter's dit looks a lot like another's
dah.) The nav signal decoding is simple. (A 30 Hz AM modulated tone is phase
compared with a 30 Hz tone FM modulated at 9960 Hz. The phase obtained is
the azimuth to the station.)



If I ever get this working, I looking forward to sharing it with the
community. In the process of building this, I created a simple SDR toolkit
of DSP functions. It's like Gnuradio in concept, but 16b fixed-point, and
has no external dependencies, and C89, so is easier to build on weird and
limited platforms. It also has perl bindings. Compared to GR, it looks like
the work of a rank amateur just learning DSP, but I do like the concept of
there being a GR-like library out there, lightweight and embedded-friendly.




Dave J


On Tue, Oct 22, 2013 at 3:06 AM, jdow <jdow at earthlink.net> wrote:

He sent me two frequencies in private email. These were in the FM
broadcast band (in the US). He probably needs to notch them out
in order to get adequate response.

As for wanting narrow bandwidth - I am not quite sure why he thinks
that is a benefit. I use a large FFT (about 10 Hz per bin) and use
the zoom control to see fine detail. (Different FFT settings suite
different uses. This one seems to be a good compromise with my two
needs. I'm too lazy to change it.)


On 2013/10/22 02:08, Sylvain Munaut wrote:

Effective filtering must occur between antenna and receiver. All the
problems that a saturated preamp and ADC cause can't be repaired by
software. Never.

But his original problem is not with saturated preamps or ADC ... it's
aliasing ... and that can be solved in the digital domain provided
fast enough ADC.





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