Here's a DDS program for Linux PC and Raspberry Pi,

 http://abelian.org/ebnaut/ebsynth.shtml

Intended for sending EbNaut but can be used as a standalone
signal generator.  I've been testing it in the near field at
VLF and it works well, reliable sending EbNaut down to 50mS
symbols.

I wonder if it will work at LF using the 3rd harmonic?  I haven't
tried that yet.

The program uses the Goertzel algorithm to track the phase and
frequency of the reference signal and outputs a sine wave which
is in some fixed frequency ratio to the reference.  The output
is phase and frequency locked.   The phase jitter is small and
difficult to measure when the program is running on a 192k/sec
PCI soundcard.

It hasn't had a huge amount of testing so consider it a 'beta'
but it seems to work well so I thought I'd share it.

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Paul Nicholson
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