How fast do you think a PC, let's say a huge gaming machine built for
raw speed with money not an issue could cycle through a sine table
undisturbed (not possible with Windows, or Linux) , a large enough
table so spurs are minimize?
You are going to find out really quick why they have hardware in a
DDS go and do the dirty work, even raw hardware is not fast enough
and accurate enough, that is the main problems with DDS. The DAC are
not accurate enough when they have to rip through a sine wave at 1
GHz they have glitches, feed though and non-linear steps, all which
creates phase noise and spurs.
This technology keeps getting better and better, the present AD99XX
families are way better than the AD98XX families but are expensive,
and power hungry, give it another five years and DDS technology will
be hard to beat, but we are not there yet. They are way better than
the DDS-30's and DDS60 boards we commonly use but by the time you are
through you spent $200 on parts for the a fairly clean DDS.
That is why I'm leaning towards building a Vackard oscillator with a
PTO system for tuning and a PLL based H&P to lock it on
frequency. With a digital phase comparator the reference and the
Vackard will be phase locked. The Vackard oscillator is extremely
clean. But even this scheme will be costly I will need a accurate
temperature stable, low phase noise reference oscillator. I bought
one at Dayton for $35 a bargain but I can kick myself for not buying
more than one.
At 04:10 PM 6/13/2006, you wrote:
>DDS
>
> I was reading the article from the ARRL Handbook again on DDS using
>the hardware method. A mostly software method however sounds better.
>Thats herein below.
>
> Since the idea is to use a digital to analog convertor for this
>approach. The idea of a all hardware approach in a radio is the draw
>back with this. Too much hardware or parts, etc: as well as
>technolical difficulties.
>
> It appears that the better way to go with DDS is to use software in a
>PC to drive one or two D/A convertors. Where the D/A chips are
>external to the PC and in the radio. Simply interfaced to the PC in
>the back. Complexity then is a matter of the software and not so much
>the hardware in this approach. Still filtering of the output and the
>use of traps are required. Overall parts are minimized and this then
>is the SDR approach since here I am talking of mostly using software in
>place of hardware. {Feasibily, well, someone here will make comments
>on the feasibility and we will better know!}
>
> The D/A chips then need to be high speed CMOS devices if there is to
>be some use in the RF regions. If however there are only widely
>available devices that will work resonably well in the kHz regions at
>best then multiplication of the RF upwards in frequency will be
>required after the D/A section. Which will widen phase noise and
>jitter with multiplication.
>
> I am however sure someone is expert enough on D/A specs to let us
>know what RF region devices are available for use. Being there are
>already many instances of mostly hardware types of DDS systems. And so
>there should be some good parts available. Mostly hardware also means
>that these systems may include their own onboard micro processor such
>as in the Intel 8000 series or other micro processors that might be
>found in radios. Since these ideas are bit driven schemes.
>
> Here the micro processor is the PC.
>
>ka9rza
>
>
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>Yahoo! Groups Links
>
>
>
>
Cecil Bayona
KD5NWA
www.qrpradio.com
"Windows, the most successful software virus ever" Don Seglio Batuna
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