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Date: Sun, 6 Feb 2011 12:05:33 +0000
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From: Andy Talbot <andy.g4jnt@gmail.com>
To: rsgb_lf_group@blacksheep.org
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Subject: Re: LF: Simple PIC based DDS for VLF
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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
>
>
>
>
>

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<div>That&#39;s a very good point -=A0having suffered this FSK-losing-cohe=
rence problem once before.=A0=A0 </div>
<div>It could all be done inside the PIC, maintaining two accumulators and=
 then using one or the other to address the sine lookup table.=A0=A0 As yo=
u say, there will be a discontinuity, but hopefully not a big=A0problem.=
 </div>

<div>=A0</div>
<div>The interrupt routine is at maximum capacity, so the sampling rate wi=
ll have to come down a bit to allow nearly double the instructions, but it=
 should be possible - I&#39;ll think about it :-)</div>
<div>=A0</div>
<div>And something a DDS chip like the AD985x can&#39;t do !</div>
<div>=A0</div>
<div>Andy</div>
<div><a href=3D"http://www.g4jnt.com/">www.g4jnt.com</a></div>
<div><br><br>=A0</div>
<div class=3D"gmail_quote">On 6 February 2011 11:47, wolf_dl4yhf <span dir=
=3D"ltr">&lt;<a href=3D"mailto:dl4yhf@freenet.de">dl4yhf@freenet.de</a>&gt=
;</span> wrote:<br>
<blockquote style=3D"BORDER-LEFT: #ccc 1px solid; MARGIN: 0px 0px 0px 0.8e=
x; PADDING-LEFT: 1ex" class=3D"gmail_quote">
<div bgcolor=3D"#ffffff" text=3D"#000000">Thanks Andy, nice project.. mayb=
e I should dig out the PIC programmer again... !<br><br>One suggestion for=
 VLFers: A PIC doesn&#39;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 carrie=
rs remains coherent, and can be coherently averaged as long as necessary=
 (much longer than a single DFCW symbol). I noticed this on Stefan&#39;s=
 signal today : With the very slow settings, all you see is a rounded hump=
 in the spectrum because the signal isn&#39;t coherent. No chance to detec=
t it for &quot;DX&quot; receivers.<br>
<br>Of course flipping the (FSK-) switch may cause a heavy click then (dep=
ending on the momentary phase shift between the two carriers) but I think=
 a robust PA will survive this.<br><br>Have a nice sunday,<br>=A0=A0 Wolf=
 .<br>
<br>Am 05.02.2011 19:57, schrieb Andy Talbot:=20
<blockquote type=3D"cite">
<div>9kHz OPs may be interested in this DDS suitable for audio/ VLF using=
 just a 16F628 PIC, , R-2R ladder and op-amp filter.=A0=A0 It can generate=
 frequencies up to about 40kHz with typical DDS high resolution / frequenc=
y steps, (higher if you overclock the device) =A0offers two-tone FSK opera=
tion by toggling a single input line=A0(ideal for DFCW),=A0 The two=A0freq=
uencies are stored in non-volatile memory and are=A0user-programmable=A0fr=
om=A0an RS232 interface=A0using ASCII text based commands.</div>

<div>=A0</div>
<div>See <a href=3D"http://www.g4jnt.com/PIC_DDS.zip" target=3D"_blank">ww=
w.g4jnt.com/PIC_DDS.zip</a> for=A0full details.</div>
<div>=A0</div>
<div>Andy</div>
<div><a href=3D"http://www.g4jnt.com/" target=3D"_blank">www.g4jnt.com</a>=
</div>
<div>=A0</div>
<div>=A0</div></blockquote><br></div></blockquote></div><br>

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