[time-nuts] VLF time station, cheap sdrs

[Dana Whitlow]

Here is my capture of a few minutes of WWVB amplitude and phase from last
fall.
It's somewhat larger of a file than I had remembered, more like 3.5MB, and
it has
lots of fine detail.  So be prepared for it to take half a minute or so
(maybe more,
depending) before it starts to plot.

The data was recorded with an SDR-14 SDR, using a DXE broadband loop
antenna.  I had to wait until the wee hours to get a decent-looking signal
in
my high-line-noise area (Kerrville, TX).

The demodulation was done after the capture was complete, in 'C' SW that
I've written to make this kind of signal analysis fairly easy and regular.
Basically
I examine the spectrum of the raw signal using an FFT, and manually pick the
band limits within which I want to work.  Then I shift that band to roughly
zero-
center frequency, and take an IFFT to return to time domain.  At this point
the
frequency is invariably a bit off, and I have an interactive graphical
means of
correcting the frequency error pretty accurately.  Hence, my "Costas loop"
is
really a manual process to de-tilt the phase versus time plot.

Normally I quit there, but realized that I needed a better way to plot such
a
long record, so I developed a little program to plot the data in spiral
form to
an EasyCAD file, to which I manually added explanatory text.

Thanks to TVB for converting my oversize pdf plot to something reasonable.

Enjoy,

Dana


On Wed, Dec 5, 2018 at 4:29 PM Dana Whitlow <k8yumdoober at gmail.com> wrote:

> Here is my capture of a few minutes of WWVB amplitude and phase from last
> fall.
> It's somewhat larger of a file than I had remembered, more like 3.5MB, and
> it has
> lots of fine detail.  So be prepared for it to take half a minute or so
> (maybe more,
> depending) before it starts to plot.
>
> The data was recorded with an SDR-14 SDR, using a DXE broadband loop
> antenna.  I had to wait until the wee hours to get a decent-looking signal
> in
> my high-line-noise area (Kerrville, TX).
>
> The demodulation was done after the capture was complete, in 'C' SW that
> I've written to make this kind of signal analysis fairly easy and
> regular.  Basically
> I examine the spectrum of the raw signal using an FFT, and manually pick
> the
> band limits within which I want to work.  Then I shift that band to
> roughly zero-
> center frequency, and take an IFFT to return to time domain.  At this
> point the
> frequency is invariably a bit off, and I have an interactive graphical
> means of
> correcting the frequency error pretty accurately.  Hence, my "Costas loop"
> is
> really a manual process to de-tilt the phase versus time plot.
>
> Normally I quit there, but realized that I needed a better way to plot
> such a
> long record, so I developed a little program to plot the data in spiral
> form to
> an EasyCAD file, to which I manually added explanatory text.
>
> Enjoy,
>
> Dana
>
>
> On Wed, Dec 5, 2018 at 8:56 AM jimlux <jimlux at earthlink.net> wrote:
>
>> https://www.rtl-sdr.com/tag/vlf/
>>
>> a fair number of examples -
>>
>> As I speculated, the 8 bit RTL-SDR has a problem with strong broadcast
>> band AM, so you need some sort of LPF.
>>
>> There are some examples of "amplified whip into sound card into Rpi"
>> implementations.
>>
>> I didn't see anybody actually decoding or measuring the phase though -
>> lots of waterfall plots showing the existence of a signal.
>>
>>
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