[Fwd: LF: DDS Sources in receivers]
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
> Andy G4JNT
> > ----------
> > From: Rik Strobbe[SMTP:email@example.com]
> > Reply To: firstname.lastname@example.org
> > Sent: 2000-07-17 10:47
> > To: email@example.com
> > 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