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Dear Jim,
that is an excellent reception! But looking at the
screenshot immediately sent me fumbling for my eyeglasses ;-) The strong line
appears to be split up by a regular ~ 10 mHz modulation, in agreement with
the observed temperature cycle.
A possible remedy could lie in SpecLab's
MSK-based samplerate tracking function. Here I've been locking to
GBZ 19.58 kHz for a few weeks now, with next-to-perfect stability. You
may want to reduce the integration time (Ud. cycle, default 10s) to track
faster variations, but being close to the transmitter there should be plenty of
SNR available.
73, Markus (DF6NM)
----- Original Message -----
Sent: Sunday, October 03, 2010 12:42
AM
Subject: LF: Re: DK7FC's 7th VLF kite
experiment
Dear Stefan, LF
Group,
This is the screen shot of today's signal from DK7FC at my home
QTH (IO91vr)
from about 1020 until 1300 utc. The time markers are at 1
hour intervals.
The QRN level seemed to be quite low. The horizontal
streaks are local
mains-related QRM. My laptop decided to switch itself
off at about the time
Stefan's transmission started, but fortunately I was
present at the time and
only a few minutes of signal were lost. The SNR
reached about 15dB with 3
millihertz FFT resolution - I think if the
maximum ERP could be maintained,
copy of QRSS60 or QRSS30 signals would be
possible under these conditions.
A spectrogram with this resolution
begins to show the limitations of using
the laptop's internal sound card.
The start of the trace shows the warm-up
drift of the sound card, which
isn't too bad. But you can see that the
signal trace is spread over
several FFT bins, apparently due to some
short-term instability of the
sampling rate. I think this is due to the
cooling fan in the laptop
cycling on and off every minute or two, frequency
modulating the soundcard
clock by several millihertz due to the changing
temperature. I think
if this instability could be eliminated, the SNR would
be improved by some
dBs. Probably an external sound card would be a good
idea.
Thanks
to Stefan for another successful VLF test,
Cheers, Jim Moritz
73 de
M0BMU
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