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30703Re: [softrock40] DDS-34 / Re: Kits and a Website

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  • k5nwa
    Feb 2, 2009
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      At 05:26 PM 2/2/2009, you wrote:
      >Well, seems to me that it is also a DDS, perhaps it is one with much
      >cleaner output, and maybe they could use it in the high end top
      >dollar transceivers....

      Look at the block diagram of a DDS and then look at the block diagram
      and you will see that they are not the same.

      DDS ie; AD98XX a hybrid solution to generating a stable clock

      * High frequency Reference clock, the output in order to be
      reasonably clean can't approach the clock source due to Nyqist
      issues, output typically limited to 1/3 reference clock.
      * programmable divider whose output triggers a fractional addition
      * fractional adder every clock from the divide adds the fraction
      that controls the frequency to a large accumulator
      * The upper bits of the accumulator are feed through a sine
      lookup table into a D/A converter
      * The D/A output is filtered and is used as the output

      Pros:
      Very fine frequency control
      Offset register could be added for phase control

      Cons:
      Output of D/A subject to A/D common flaws such as uneven steps, or
      feed through of bits causing errors in the output cause phase noise.
      Noise from digital section can couple into the analog section for
      increased phase noise.
      Analog output must be turned back into a digital signal which we need
      in the clock chain, this adds further to the noise.


      Variable XO ie; XI570 a digital solution to generating a stable clock

      * Built in crystal reference clock
      * PLL multiplier with variable rate divide in the feedback to the
      digital phase comparator, operates in the 2GHz 5 GHz range
      * divider on the PLL out out
      * Output is digital.

      Pros:
      Output can be higher in frequency than the reference clock, as high as 1.7GHz
      No Nyqist limits since we are dealing with digital circuits.
      Entire chain is Digital for fewer noise issues.
      No analog D/A with it's problems for cleaner output and way less phase noise
      Output is digital, less phase noise since this what we wanted in the
      first place to drive the I/Q clock generators

      Cons:
      Changing the divider on the PLL cause a small delay before the
      frequency stabilizes, no problem if all you are changing is the output divider.

      They each have their unique weaknesses but overall I personally
      prefer the SI570, there are cases though were it will fail for
      example if you want to change the frequency repeatedly specially over
      a range bigger than 3500ppm



      Cecil
      k5nwa
      www.softrockradio.org www.qrpradio.com
      < http://parts.softrockradio.org/ >

      Never take life seriously. Nobody gets out alive anyway.
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