22 Oct
2013
22 Oct
'13
2:28 a.m.
Hi,
I've been working on a project the involves tuning VOR signals. They are
narrow band signals. > 25 kHz. For that, the wide bandwidth of the RTL
dongles isn't that helpful to me. (and actually a hindrance when two
signals are inside the sample range but widely different strengths)
Anyway, the lowest I can get my dongles (R820T) to go is 250ksps, which is
fine. I've been working at that frequency for awhile, but getting some
strange results. For example, if I tune in to 116.8 MHz, which should be
the OAK VOR, it works fine. But if I tune to 115.8 MHz, which should be the
SFO VOR I get ... the OAK VOR. That is, there is a perfect copy of the OAK
signal 1 MHz shifted down.
If I switch the sample rate to 1.024Msps or 900ksps, I don't get this
problem.
I don't understand what is happening here. 1 MHz is an even multiple of
250kHz, so maybe I'm getting an image of OAK overpowering the relatively
weaker SFO signal. But should there not be filters that manage this?
I guess I was expecting that if the device is set to 250ksps, then it would
"close down" filters appropriately to reject signals out of that band. But
maybe the filters don't work properly below a certain sample rate? Like the
rtl2832 can sample down to 250ksps, but the 820T tuner was not designed for
it?
Or perhaps I'm doing something wrong? Can I control the filters directly?
I'm glad I find this issue because I was going nuts thinking my software
had some mysterious bug. But I can reproduce this issue just with SDR# or
whatever. :-)
Regards.
Dave J
22 Oct
22 Oct
10:29 a.m.
Hi,
I don't understand what is happening here. 1 MHz
is an even multiple of
250kHz, so maybe I'm getting an image of OAK overpowering the relatively
weaker SFO signal. But should there not be filters that manage this?
Yes most likely an image.
I guess I was expecting that if the device is set to
250ksps, then it would
"close down" filters appropriately to reject signals out of that band. But
maybe the filters don't work properly below a certain sample rate? Like the
rtl2832 can sample down to 250ksps, but the 820T tuner was not designed for
it?
The filters you'd need wouldn't be in the R820T anyway (it has filters
but not that narrow).
IIRC, the rtl2832 always sample at a fixed high frequency but it has
an internal downconverter + FIR filter inside to convert the sample
rate down. But that FIR probably can't handle such a big reduction ...
Can I control the filters directly?
If you find out, let us know, but AFAIK all attempts so far didn't
succeed to get it more narrow than this.
Cheers,
Sylvain
10:44 a.m.
The filters you'd need wouldn't be in the
R820T anyway (it has filters but not
that narrow).
IIRC, the rtl2832 always sample at a fixed high frequency but it has an internal
downconverter + FIR filter inside to convert the sample rate down. But that
FIR probably can't handle such a big reduction ...
Still we must not forget that we're talking about some piece of silicon with nm
structures, and therefrom no extraordinarily sharp filtering can be expected.
When analog meets digital, it is important that the analog part protects the
ADC from being overwhelmed by strong signals that are not in the scope of
interest.
All those silicon-only broadband receiver concepts drastically fail when being
directly connected to a real good antenna in urban vicinity. They are good for
a rubber duck style antenna sitting on your table, but that's it.
There's a reason for all the solid silver and brass stuff in real world receivers :)
Cheers,
Sylvain
Ralph.
12:05 p.m.
Sample rate is 28.8 MHz. That is your basic bandwidth starting point. With
the E4000 delivering I and Q data for the sampler you have a starting
bandwidth of 28.8 MHz. The R820T converts to around 3 MHz I only. That
means the bandwidth is 0 to 14.4 MHz with the dongle slicing out up to
2.4 Msps of I/Q data. The R820T potentially has some interesting image
issues if it is really delivering 3 MHz to the RTL chip and not 7 MHz.
These are TV devices. That means any analog filtering they may or may not
have is 6 to 8 MHz wide. So up front it has that is the bandwidth to worry
about for strong signals.
The RTL chip delivers as little as 250 ksps. The word I've heard from
people who test the chip is that it's not very good for image (alias)
rejection much below 1 Msps. So I simply live with the 2.4 Msps and
learn to like it. SDRSharp more or less properly (more compromises than
some) decimates this down to something usable. But there's no way to
tell SDRSharp, "Really, Essdee, I'm only interested in this 200 kHz
so please only show that in the FFT window." It could be done. The code
would be only a little more complex. I don't worry about it. I use the
zoom slider and Bob's my uncle.
The fellow who posted the question indicates he's interested in VOR
reception. That's a 10 MHz band between 108 MHz and 118 MHz. I have
a hunch there is a small band near the FM broadcast band that's more
or less useless. So 4 dongles with appropriate tuning could get all
but about 400 kHz of that range all at once on a suitable computer.
5 could do it with considerable overlap. But one, really, could do
the job if it knew what frequencies to tune to. One of the scanner
tools would maybe make that a little easier.
He'll likely want to use some 1/4 wave coax notch's since he indicated
two frequencies that had huge signals were 1 MHz apart up around 106 MHz.
That would make a big difference for his weak signal work.
{^_^}
On 2013/10/22 01:44, Ralph A. Schmid, dk5ras wrote:
The filters
you'd need wouldn't be in the R820T anyway (it has filters but not
that narrow).
IIRC, the rtl2832 always sample at a fixed high frequency but it has an internal
downconverter + FIR filter inside to convert the sample rate down. But that
FIR probably can't handle such a big reduction ...
Still we must not forget that we're talking about some piece of silicon with nm
structures, and therefrom no extraordinarily sharp filtering can be expected.
When analog meets digital, it is important that the analog part protects the
ADC from being overwhelmed by strong signals that are not in the scope of
interest.
All those silicon-only broadband receiver concepts drastically fail when being
directly connected to a real good antenna in urban vicinity. They are good for
a rubber duck style antenna sitting on your table, but that's it.
There's a reason for all the solid silver and brass stuff in real world receivers :)
Cheers,
Sylvain
Ralph.
10:45 a.m.
Thank you. You and another forum member have helped me understand the
situation. I am beginning to see the attraction of SDR platforms that
integrate an FPGA immediately after the ADC. You can have wide analog
filtering going to an ADC running fast, and then apply digital filters in
the FPGA before downsampling to a lower rate for consumption by
what-have-you.
I can accomplish the same completely in software; I'll set the sample rate
high, then filter and decimate, probably in a few stages to keep the
filters reasonable. I tried it this evening with some raw IQ recordings I
made this afternoon and it works pretty well -- on a modern PC.
Regards,
Dave J
On Tue, Oct 22, 2013 at 1:29 AM, Sylvain Munaut <246tnt(a)gmail.com> wrote:
Hi,
I don't understand what is happening here. 1
MHz is an even multiple of
250kHz, so maybe I'm getting an image of OAK overpowering the relatively
weaker SFO signal. But should there not be filters that manage this?
Yes most likely an image.
I guess I was expecting that if the device is set
to 250ksps, then it
would
"close down" filters appropriately to
reject signals out of that band.
But
maybe the filters don't work properly below a
certain sample rate? Like
the
rtl2832 can sample down to 250ksps, but the 820T
tuner was not designed
for
it?
The filters you'd need wouldn't be in the R820T anyway (it has filters
but not that narrow).
IIRC, the rtl2832 always sample at a fixed high frequency but it has
an internal downconverter + FIR filter inside to convert the sample
rate down. But that FIR probably can't handle such a big reduction ...
Can I control the filters directly?
If you find out, let us know, but AFAIK all attempts so far didn't
succeed to get it more narrow than this.
Cheers,
Sylvain
11 a.m.
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.
Ralph.
From: osmocom-sdr-bounces(a)lists.osmocom.org
[mailto:osmocom-sdr-bounces@lists.osmocom.org] On Behalf Of David Jacobowitz
Sent: Tuesday, October 22, 2013 10:45 AM
To: Sylvain Munaut
Cc: osmocom-sdr(a)lists.osmocom.org
Subject: Re: odd results working at 250ksp/s
Thank you. You and another forum member have helped me understand the
situation. I am beginning to see the attraction of SDR platforms that
integrate an FPGA immediately after the ADC. You can have wide analog
filtering going to an ADC running fast, and then apply digital filters in
the FPGA before downsampling to a lower rate for consumption by
what-have-you.
I can accomplish the same completely in software; I'll set the sample rate
high, then filter and decimate, probably in a few stages to keep the filters
reasonable. I tried it this evening with some raw IQ recordings I made this
afternoon and it works pretty well -- on a modern PC.
Regards,
Dave J
11:08 a.m.
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
11:17 a.m.
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.
Yep - if this really is the problem. Aliasing-alike effects also often come from
nonlinearities in the overdriven input stages...BTST, GTTS :) The RTL2832 shows
this all the time, copies of strong signals show up in expected (from aliasing, proven by
simply doing the maths) and not-so-expected areas of the spectrum.
Cheers,
Sylvain
Ralph.
12:06 p.m.
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
5:07 p.m.
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.
9:17 p.m.
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(a)earthlink.net
<mailto:jdow@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
9 Feb
9 Feb
8:55 p.m.
Hi,
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 :
http://www.f4gkr.org/2014/02/in-depth-study-of-the-vor-signals/
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(a)gmail.com>om>:
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(a)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
>
>
>
10 Feb
10 Feb
11:54 a.m.
Thanks a lot - very interesting!
Ralph.
From: Sylvain AZARIAN [mailto:sylvain.azarian@gmail.com]
Sent: Sunday, February 09, 2014 8:55 PM
To: David Jacobowitz
Cc: jdow; Sylvain Munaut; osmocom-sdr(a)lists.osmocom.org; Ralph A. Schmid,
dk5ras
Subject: Re: odd results working at 250ksp/s
Hi,
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 :
http://www.f4gkr.org/2014/02/in-depth-study-of-the-vor-signals/
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(a)gmail.com>om>:
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(a)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
1:15 p.m.
I added this morning a post with links to the wav and mat files (matlab
files are centered on the VOR signal, and have limited sampling rate to 30
KHz)
feel free to ask further details if required
sylvain
2014-02-10 11:54 GMT+01:00 Ralph A. Schmid, dk5ras <ralph(a)schmid.xxx>xx>:
Thanks a lot - very interesting!
Ralph.
*From:* Sylvain AZARIAN [mailto:sylvain.azarian@gmail.com]
*Sent:* Sunday, February 09, 2014 8:55 PM
*To:* David Jacobowitz
*Cc:* jdow; Sylvain Munaut; osmocom-sdr(a)lists.osmocom.org; Ralph A.
Schmid, dk5ras
*Subject:* Re: odd results working at 250ksp/s
Hi,
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 :
http://www.f4gkr.org/2014/02/in-depth-study-of-the-vor-signals/
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(a)gmail.com>om>:
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(a)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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14 comments
6 participants
participants (6)
-
David Jacobowitz -
jdow -
jdowjunkmail＠earthlink.net -
Ralph A. Schmid, dk5ras -
Sylvain AZARIAN -
Sylvain Munaut