Re: [soft_radio] Re: DDS + PLL to minimize spurs?

Wed, 25 Jan 2006 17:29:57 -0800

The problem is when the DDS is at the output. Say your dds needs to output 40 mhz. This means you get 10 D/A samples to create a sine wave...pretty course. Now say you want to make 80 MHz then you only get 5 D/A samples.  (DDS clock is 400 MHz)
The best source will be when there the maximum samples with a divider after it. I would rearrange the LO myself to have the dds feed the phase detector and put the pll at the output. Say you have a 400 to 800 mhz VCO followed by a divide by 10 to make 40 to 80 MHz. for the radio. Then further divied it down to 400 to 800 KHz feeding the phase detector. Not take the DDS and make say 4 to 8 MHz followed by a divide by 10
or 400 to 800 khz to the phase detector. Then you just need a 400 mhz source for the DDS clock. I would use a saw oscillator for that. I'm told that is the cleanest configuration these days. There was an article coveringthis a couple years ago in QEX. Frank wa1gfz 

dcappels <[EMAIL PROTECTED]> wrote:
--- In [email protected], "Chris Hirt" <[EMAIL PROTECTED]> wrote:
>
> Yaesu goes with FTDX9000 another way:
>             
> OCXO--PLL--VCO(400MHz)--DDS(AD99xx ?)-->1.LO
> >        |_____|
I'm probaly going to catch heck from somebody for saying this, but I
think that if you objective is a clean sine wave, you should start
with a clean sine wave, not something made of a bunch of harmonics
that you have to hammer and filter into a clean sine wave.

Flack vest is now on. Fire at will :-)


Dick

> 
> Whats Your opinion ?
> 
> 73
> Chris
>
>
>
> --- In [email protected], "Chris Hirt" <[EMAIL PROTECTED]> wrote:
> >
> > Yaesu goes with FTDX9000 another way:
> >            
> > OCXO--PLL--VCO(400MHz)--DDS(AD99xx ?)-->1.LO
> >        |_____|
> >
> > Whats Your opinon ?
> >
> > 73
> > Chris
> >
>








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