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. Regards, Dave J On Tue, Oct 22, 2013 at 3:06 AM, jdow <[email protected]> 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. >> >> Cheers, >> >> Sylvain >> >> >>
