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From: "Talbot Andrew" <ACTALBOT@dera.gov.uk>
To: rsgb_lf_group@blacksheep.org
Subject: LF: RE PICs and DDS etc.
Date: Tue, 20 Mar 2001 16:07:36 -0000
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NO Think again, the high order bit of a DDS output is very definitely
not a squared up version of the fundamental !  Used on its own it would
have a spectrum not totally unlike noise.

Before the DDS output anti-allias filter, for a given output frequency
all terms of   Fclock +/- Fout are present, as sampled sinewaves with a
SIN(X) / X amplitude roll-off.   Also present is what effectively is
quantising noise (Nq) .   The level of this  Nq is given approximately
by  6.N dBc, so for a 12 Bit D/A in the DDS (as in the AD9850),   Nq can
only be 72 dB down at most.   In fact the manufacturers clain about
65dBc to play safe.   Just using the MSB gives 1 bit quantisation so
spurious products will be at 0 - 6dBc.    To generate a square wave, you
have to generate a proper sinewaveform, low pass filter and feed back
into a high speed comparator.  The AD9850 has such a comparator built in
and all you have to supply is a Clock, command word and output filter.

I once looked at using a PIC plus external D/A for a low speed DDS, and
for audio freqs it could be done, but again with spurious limitiations.
Assuming 8 bit arithmetic - any more would slow it down considerably and
you do need a sine look up table with this resolution as well - the
output could only manage around -40dB spurious and determining timing
would be a horrendous programming job.  A 32 bit accumulator with the
attendant maths all within a critical timing loop.

For MFSK, why not just use a DDS chip / module and send it commands to
set the frequency.   Some of the the now obsolete DDS chips such as
AD9831 and AD9832 can cost not much more than a PIC .

Andy  G4JNT


> -----Original Message-----
> From: Alberto di Bene [mailto:dibene@usa.net]
> Sent: 2001-03-20 15:11
> To: rsgb_lf_group@blacksheep.org
> Subject: LF: Re: WOLF - am I missing something ?
> 
> 
> >  Then I abandonned software
> > timing by using an external interface (a PIC) on the serial port and
> > ended up with something very similar to the VE2IQ I/F.
> >
> 
> A few days ago I was thinking on how to generate a programmable
> frequency for an MFSK scheme (I did post a message on this reflector),
> and the DDS was one the possibilities. Thinking again at it, 
> after all we just
> need the high order bit of the phase accumulator, as the TX 
> is driven with a
> square wave. This makes the DAC completely unnecessary.
> Would it be possible then to implement a 32-bit phase accumulator with
> a PIC, to generate a frequency on the 137 kHz band ?
> My knowledge of PICs is quite limited, any PIC guru with an answer ?
> If positive, then it would be possible to make a really 
> inexpensive outboard
> generator for such an MFSK modulation scheme.
> 
> 73  Alberto   I2PHD
> 
> 
> 


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