[Lowfer] WD2XES WSPR Again

[Johan H. Bodin]

Hi Bill,

I was a bit unclear, sorry... The PIC program can control both a '9850 
DDS *and* a varicap tuned VCXO, whichever you choose to use. According 
to Joe, K1JT, small errors in FSK shift accuracy (say 0.1Hz) will not 
degrade decoding performance very much so the hardware VCXO approach 
seems to be a viable idea.

The DDS is controlled exactly as you wrote. The phase increment word is 
downloaded to the DDS chip in advance and the FQ_UD pin is strobed by a 
timer interrupt routine exactly at the "12000/8192 baud tick".

73
Johan SM6LKM


Bill de Carle skrev:
> 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
> 
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