[Fwd: LF: DDS Sources in receivers]

Andre' Kesteloot akestelo@bellatlantic.net
Mon, 17 Jul 2000 08:20:11 -0400


Talbot Andrew wrote:

> Unfortunately, the output of the numerical controlled oscillator has to
> be turned into a real signal, and as it represents a quantised series of
> angles, a sine ROM must be used to generate a sinewave.  Any other
> mapping would generate an unpredictable set of non harmonically related
> spurii.  A DDS output unfiltered is not a square wave.  The quality /
> resolution in this ROM sets the spurious output level and for the AD9850
> this is sufficient to meet the specified 60dBc output purity spec.
> To generate a squarewave, you need to take the filterd sine output and
> apply it to a comparator.  The AD9850 chip includes such a comparator,
> capable operating up to beyond 40MHz
>
> The 9850 PCBs are now being supplied by HF Instruments 8.00 for the
> PCB, 40 for one with the DDS chip pre-mounted with only a few other
> components to install.  Phone Chris Tew or Paul Phillips  on  01420
> 590000  (+44 1420 590000) to order.  Designs and software using this
> module will follow from time to time.
>
> I am about to make a start on a custom LF receiver based on a 455kHz IF
> and surplus 300Hz mechanical filter, all locked to a master reference
> oscillator.  A DDS will be used as the Local Oscillator (properly
> filtered of course !) and possibly another one for the BFO although this
> is not so essential if a constant output tone of 1kHz is deemed
> satisfactory
>
> Andy  G4JNT
>
> > ----------
> > From:         Rik Strobbe[SMTP:rik.strobbe@fys.kuleuven.ac.be]
> > Reply To:     rsgb_lf_group@blacksheep.org
> > Sent:         2000-07-17 10:47
> > To:   rsgb_lf_group@blacksheep.org
> > Subject:      Re: LF: DDS Sources in receivers
> >
> > At 21:44 14/07/00 +0200, DJ5HG wrote:
> > >...
> > >In my previous mail I pointed out, that the spurs generated by a DDS
> > >are due to an imprecise implementation of the sine function. This
> > >imprecision is a result of a compromise between the diverging goals,
> > >speed and precision.
> > >...
> >
> > I followed the discussion about DDS signal sources in receivers 'with
> > one
> > eye', so maybe I missed something.
> > But is it nessecary to create a sine wave ?
> > At LF & VLF analog switches can be used to make 'state of the art'
> > mixers.
> > Analog switches with an on-resistance as low as a few Ohm and a
> > switching
> > time as fast as a few nannoseconds are cheap an easily available.
> > These analog switches have to be driven by a square wave, maybe this
> > can
> > simplify the DDS design.
> >
> > 73, Rik  ON7YD
> >
> >