[Lowfer] WD2XES WSPR Again

[Bill de Carle]

At 05:29 AM 3/29/2008, Johan, SM6LKM wrote:
>John, Lowfers,
>
>beeing a QRPer at heart, I don't like the idea of using a big PC as 
>a QRPp beacon controller so I wrote a MEPT_JT beacon controller 
>program that runs on a PIC. UTC sync comes from a GPS receiver, with 
>NMEA output, connected to the PIC's serial port. The current version 
>has two simultaneous outputs: AD9850 DDS control and/or two pins for 
>VCXO control plus a TX keying pin. In my experimental setup, the 
>VCXO control pins go to a 'HC4052 analog switch which selects one of 
>three or four trim pot's. The output of the HC4052 goes to a varicap 
>diode which is loosely coupled to the xtal oscillator. The pot's are 
>set for 1,46484375 Hz spacing between FSK tones.
>
>Beacon message, DDS symbol frequencies and GPS baud rate can be 
>programmed via the serial port (just download a small text file from 
>a terminal program).
>
>My PIC beacon is now running on the lab bench but there are still a 
>few wrinkles (bugs) to iron out in the message download code. If 
>there is interest, I'll be happy finish the program ASAP and share the code.

Great idea, Johan.  Using the varactor to pull the xtal frequency 
seems like a long way to go.  Why not just change the phase increment in the
AD9850?  I looked at the data sheet - it says you can load in a new 
40-bit control + phase increment value at a leisurely pace then at 
just the right
instant (using the 12000/8192 Hz edge you must have already anyway) - 
make the FQ_UD pulse the AD9850 needs to load the serial shift register into
the adder so its output frequency will change as of the next 
reference clock pulse.  With keying at only 1.46 baud there is plenty 
of time to
reload the 40-bit shift register from the PIC between message bits 
(the PIC can anticipate what the next output frequency will have to be), and
a 32-bit phase increment register would give more than adequate 
frequency resolution, I'd think better and more stable than the 
varactor arrangement.
I haven't used an AD9850 but I've done the same kind of thing with a 
Harris HSP45102 DDS chip which has two phase increment registers and you can
instantly switch between them to make a FSK signal.  If I understand 
the AD9850 datasheet correctly there is no reason you couldn't do the 
same thing
with the single increment register (plus shift register for 
buffering) on that chip.  You would have to reload the frequency 
increment (probably for
every message bit) but at this low keying rate that should be no 
problem.  Perhaps I've missed a hitch somewhere that would prevent 
you from doing it
that way?
Bill