On 2014-07-31 11:05, [email protected] wrote:
Note that at least in the US the frequency accuracy of broadcast FM
stations is no better than the transmission mode needs. NFM weather
broadcasts are about as good as it gets for most things on VHF.
Cellphone signals may well provide the best calibration sources. The
towers have ppb oscillators in profusion and one presumes they use them.
OK. Good to know. I tested a few stations here (in Stockholm Sweden) and
they diff by like 2ppm so it's within acceptable range for many
applications. As compared to GSM cell frequencies, the broadcast
frequencies are well known which the GSM frequencies probably are not
for the average user.
I first tried to use the kalibrate-rtl utility that use GSM frequencies
but the DVB-T dongle was so off in frequency so that the scanner found
the wrong channels and I got completely wrong frequency correction
values. One way to do it is to first do a course calibration using
rtl_fm on a broadcast station and then use kalibrate-rtl with the course
frequency correction ("-e" switch).
The calibration should work on NFM as well. It's just that if you have
many closely spaced transmissions in a band there is no easy way to know
which is which and you need to use a small bandwidth to filter out one
station. If the frequency error is larger than the bandwidth you're out
of luck. By using broadcast stations, which are spaced far apart, the
frequency error can be quite large without causing any ambiguity. I
guess the same method could be used here as suggested above: course
calibration using broadcast FM and then fine tune it with a NBFM weather
station. Let me know how that work if you try it.
Regards,
Tobias
{^_^} Joanne/W6MKU
On 2014-07-31 01:23, Tobias wrote:
Hi!
I have just started using rtl-sdr in my application, SvxLink:
http://svxlink.org/. Thanks for your work on the RTL code. It opens
up the world
of wideband receivers to everyone. The SvxLink RTL code is in the
rtl-branch on
GitHub right now if anyone is interested:
https://github.com/sm0svx/svxlink/tree/rtl
I was missing some simple utility to calibrate the frequency error
for a dongle.
The simplest way I came up with was to modify the rtl_fm utility
since it is
very easy to find out the frequency error of an FM signal. To get a
measure of
the frequency error, all that have to be done in the FM demodulator is:
samp_rate * angle / (2*PI)
The angle is already compensated with PI in the current demodulator
so I only
had to do the rest in my added calibration code.
I first filed this patch as a pull request on GitHub
(https://github.com/steve-m/librtlsdr/pull/9) but then saw that
contributions
should be mailed to this mailing list.
The rtl_fm utility may now be used to calibrate the frequency error
of a DVB-T
dongle using the "-c" command line switch:
| [-c] Do frequency error calibration
Frequency error calibration will only work for FM.
Use a higher sample rate, like -s170k, to handle
large frequency errors. A strong and noise free
signal is needed for good results. For example, use a
broadcast FM station for calibration. Let the
calibration run for some minutes until the values
have stabilized. Rerun with -p option using the
suggested ppm_error. Try this on a couple of stations.
Repeat until the error is stable at about zero.
Example: rtl_fm -f99.3M -s170k -r48k -c - | aplay -r48k
-fS16_LE -
|
Regards,
Tobias
