Re: LF: Simple PIC based DDS for VLF

[Andy Talbot <andy.g4jnt@gmail.com>]

That's a very good point - having suffered this FSK-losing-coherence problem once before.  

It could all be done inside the PIC, maintaining two accumulators and then using one or the other to address the sine lookup table.   As you say, there will be a discontinuity, but hopefully not a big problem.

 

The interrupt routine is at maximum capacity, so the sampling rate will have to come down a bit to allow nearly double the instructions, but it should be possible - I'll think about it :-)

 

And something a DDS chip like the AD985x can't do !

 

Andy

www.g4jnt.com

 

On 6 February 2011 11:47, wolf_dl4yhf <dl4yhf@freenet.de> wrote:

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.

 

See www.g4jnt.com/PIC_DDS.zip for full details.

 

Andy

www.g4jnt.com