[time-nuts] Synchronous LF Reception

[Murray Greenman]

Hi,
There was a discussion a week or so back about SoftPAL and techniques
for LF reception which would allow us to study the phase and signal
strength of VLF and LF transmissions. Effects to be studied include
detection of solar events and prediction of earthquakes.

At the time I forgot to mention Clicklock by Con ZL2AFP
(http://www.qsl.net/zl1bpu/SOFT/click.htm). There's also a simpler
program by Scott VE7TIL, and another by Peter G3PLX who invented the
idea, although I don't have a URL for either of those.

While nowhere near as sophisticated as SoftPAL, Con's Clicklock is FREE,
and will do the job required. It displays a pair of graphs showing
relative signal power and phase angle, as well as an integrator phasor
diagram. It will handle carrier phase integration times from 1s to 1000s
or more and with careful setup can be used up to 500kHz.

The Clicklock technique was developed by Peter G3PLX, and uses a GPS
1pps reference fed into the computer sound card. It uses this reference
to sample the phase of harmonics of the same 1pps pulse fed into the
receiver antenna. If the receiver is tuned perfectly to an integer 1Hz
step, the phase of the harmonics within the receiver bandwidth will be
much the same (not zero, as it depends on receiver delay, and group
delay in the receiver has some effect). If the receiver is mistuned,
each successive harmonic will have increasing or decreasing phase. The
software operates an NCO to reduce the incoming signal to zero Hz, and
this NCO is steered by analysis of these harmonics to remove errors due
to drift and offset in the receiver. The zero frequency I and Q
components are then integrated over time to recover the signal.

With a conventional ham receiver and LF antenna you can easily achieve
lock and use 100 sec or more integration times. This technique does of
course make the study of drying paint seem very speedy, but it is
effective and VERY sensitive. On the web page referred to above is an
example of WWVB (60kHz) recorded with 600 second integration time, and
you can clearly see the hourly phase advance. This was recorded at a
range of 10,000km. I have also copied signals from Europe in this way,
and Con has copied (at 500km range) my extremely weak 10uW EIRP test
transmissions on 181kHz which include intentional phase changes for
identification purposes.

73,
Murray ZL1BPU