odd results working at 250ksp/s

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jdowjunkmail at earthlink.net jdowjunkmail at earthlink.net
Tue Oct 22 19:17:22 UTC 2013

Use a 2.4 ksps sample rate and receive both of them. In general you want
to stay away from the teat in the middle of the FFT, which is zero frequency
on the input. That is a noisy spot due to semi-conductor physics. You can
set then up with say a 250 kHz "IF" and a 750 kHz "IF" and in principle
receive both at the same time.

Of course you are going to graduate from this to receiving GPS, aren't you?

{^_-}   Joanne

On 2013/10/22 08:07, David Jacobowitz wrote:
> 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 <jdow at earthlink.net
> <mailto: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.
>         Cheers,
>              Sylvain

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