LF,
After some time of maintenance, my LF grabbers
(http://www.iup.uni-heidelberg.de/schaefer_vlf/DK7FC_LF_Grabber.html and
http://www.iup.uni-heidelberg.de/schaefer_vlf/DK7FC_LF_DX_Grabber.html)
are now back on.
Due to the cold days the institute was heated and this caused strong
temperature changes (when opening the door and so on) and a visible
drift of the signals in QRSS/DFCW60/120, so i decided to build a PLL for
my 125 kHz IF. The current grabber RX is using a BF981, downconverting
by 125 kHz and running 7 SpecLab instances.
The 125 kHz signal is generated by a 8 MHz xtal and a 74HC4060, down
dividing to 250 kHz (for the PLL) and 125 kHz, for the IF. The 250 kHz
signal and 10 kHz signal is fed to a CMOS4070 XOR gatter that generates
a regulation voltage as a function of the duty cycle. The waveform is
smoothed by a RC circuit and fed to a 1N4007 (reverse) as the regulating
part of the VCXO. 10 kHz comes from a GPS module that is the reference
for all my grabbers.
The system is running since some hours and seems to work fine. I will
display the IF drift for some days. BTW i found that remote controlling
the PCs is causing some overflows in SpecLab (if many instances are
running). This causes a sudden drift / unlocking during the operation,
can still be seen on my LF DX grabber. Just a hint for the remote
controlling LF/VLF grabber runners :-)
73, Stefan/DK7FC
PS: Tnx to Markus/DF6NM for many suggestions and advice!
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