Thanks Andy, nice project.. maybe I should dig out the PIC
programmer again... !
One suggestion for VLFers: A PIC doesn't cost a fortune. Build two
of these generators, and clock them with the same source (GPSDO or
OCXO). For the FSK simply flip between both generators. This has the
*big* (cannot emphasize this enough..) advantage that if the signal
is to weak to be seen as DFCW etc, we can still dig it out through a
long FFT because each of the two carriers remains coherent, and can
be coherently averaged as long as necessary (much longer than a
single DFCW symbol). I noticed this on Stefan's signal today : With
the very slow settings, all you see is a rounded hump in the
spectrum because the signal isn't coherent. No chance to detect it
for "DX" receivers.
Of course flipping the (FSK-) switch may cause a heavy click then
(depending on the momentary phase shift between the two carriers)
but I think a robust PA will survive this.
Have a nice sunday,
Wolf .
Am 05.02.2011 19:57, schrieb Andy Talbot:
9kHz OPs may be interested in this DDS suitable for audio/
VLF using just a 16F628 PIC, , R-2R ladder and op-amp filter.
It can generate frequencies up to about 40kHz with typical DDS
high resolution / frequency steps, (higher if you overclock the
device) offers two-tone FSK operation by toggling a single
input line (ideal for DFCW), The two frequencies are stored in
non-volatile memory and are user-programmable from an RS232
interface using ASCII text based commands.
Andy
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