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 b= and right now -- though, as you say, it is adjacent to commercial FM with i= ts high power.

My application is simple in concept: A fully auomatic VOR-ba= sed 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 demodulate= d. 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 sign= al. From that list, I will "park" on each signal long enough (~30= s) 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 geometr= y is appropriate (I know the VORs positions from a database) I can calculat= e a position.

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

Not being able t= o receive this VOR or that VOR is not generally a problem, but obviously, t= he 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 wi= th 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 re= ceive two.

I've tested the software enough to know that the in= itial scan function seems to work, and the morse decoding kind of works, bu= t I am not confident I'll get it to work very well. (One transmitter= 9;s dit looks a lot like another's dah.) The nav signal decoding is sim= ple. (A 30 Hz AM modulated tone is phase compared with a 30 Hz tone FM modu= lated 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 proces= s 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 depen= dencies, 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 am= ateur just learning DSP, but I do like the concept of there being a GR-like= library out there, lightweight and embedded-friendly.

<= div class=3D"gmail_extra">Regards,

Dave J

On Tue, O= ct 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=92t be repaired by
software. Never.

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

Cheers,

=A0 =A0 Sylvain