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Fw: [softrock40] Re: DDS spurs

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  • windy10605@juno.com
    Like Allen says, In practice, pcb layout, with adequate grounding and active component decoupling is extremely important, so keep this in mind if you wish to
    Message 1 of 1 , Jan 22, 2006
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      Like Allen says, "In practice, pcb layout, with adequate grounding and active component decoupling is extremely important, so keep this in mind if you wish to do some experimenting".
      What I've learned so far on making these prototype boards:
       
      1) Use short wiring paths everywhere (that's why it pays to just make a little
         prototype imbedded board for these designs vs leads all over the place on a
         breadboard at 100Mhz+
      2) Look at the GROUND paths (these are all 2 sided boards) and make sure they 
         are not blocked by clearances and wiring. This is VERY IMPORTANT, not only for
         minimizing ground loops. Minimize wiring on the ground plane(locating some
         active components there is good, also ALL the decoupler caps)
      3) Locate all the regulators on the bottom for closer proximity to the heat sinking
         ground plane.
      4) Always use "fill area" on the ground plane. Not necessarily on the top. Remove
         resulting "slivers" ....they will just come off and short something.
      5) Put a decoupler cap at EACH voltage pin, on the bottom, AT THE PIN ....no "wiring"
         from the pad to the via.
      6) Punch two vias (one on each side of the lead pad) to the ground plane at each
         ground pin ....again no "wiring" between the pad and the via.
      7) Look at the wiring up close and then from far away (can't see the forrest for the
         trees situations)....caught a few wiring simplifications like that. Check and re-
         check, then wait a day and check again ....then get some boards.
      8) Print out a 1:1 copy and try placing all the components. This will show you if
         spacing is too small or if you selected the wrong 28 pin SSOP footprint (#!%%##!!
         caught me on Pass #2).
       
      73 Kees K5BCQ
       
       
       
      ----- Forwarded Message -----
      From: "allen_fulmer" <afulmer@...>
      Date: Sun, 22 Jan 2006 16:36:45 -0000
      Subject: [softrock40] Re: DDS spurs
       
      In an earlier post I used an equation for predicting the effect of a
      divider on spurs, namely 20 Log multiplied by the ratio of the divider.

      I did not anticipate anyone having trouble interpreting or using this
      to predict the effect on spurs  but obviously  a couple of members have.

      For example if a simple divide by two is used, Log 2.0 =  0.30103, and
      20 log 2.0 = 6.0206, so one would expect a spur reduction of
      approximately 6 db.    (I assume the familiar factor of  20 arises
      from the consideration that we are concerned with the power  contained
      in the spur.)
      For each following divide by two the the equation predicts the spur is
      reduced by an additional 6 db., so that a divide by four results in a
      12db reduction, a divide by eight  an 18db reduction, and so on.  I
      decade or divide by 10 counter results in a 20 db reduction.  (log
      10.0 = 1, 20 x 1 = 20)

      There are other important considerations, both in theory and in
      practice regarding spur reduction and noise reduction in general, but
      it is my understanding that this is probably not the proper forum in
      which to address these.   In practice, pcb layout, with adequate
      grounding and active component decoupling is extremely important, so
      keep this in mind if you wish to do some experimenting.

      Hope I have not insulted anyone.

      Allen, K4KDG


      >
      >
      > At 11:22 PM 1/21/2006, you wrote:
      > >This topic is veering off topic, but I think enough SoftRock users
      plan to
      > >adapt a DDS to their SR, to make this at least educational. 
      8-)
      > >
      > >Can someone explain the counter/divide process and
      its effect on spurs?
      > >
      > >As I read the specs for DDSes,
      there normally is one that describes a
      > >"Spurious Free Dynamic Range"
      with values in excess of 40 dBc and
      at times
      > >significantly more
      than that.
      > >
      > >Based on pictures/graphs presented, the
      desired signal is
      accompanied by
      > >bogus signals created as a
      byproduct of the D to A process within
      the DDS.
      > >These bogus
      signals, aka "spurs" are stable in frequency and
      amplitude, and
      > >are repeatable.  By repeatable I mean that if you encounter a
      spur
      and tune
      > >the DDS to a different frequency and return to the
      original
      setting, the
      > >spur likewise returns to its original
      frequency and amplitude.
      > >
      > >Someone that understands the
      math and formulas should be able to
      give exact
      > >values, but to the
      untrained eye, the magnitude of the spur is
      quite small
      > >as
      compared to the desired signal.
      > >
      > >I'm assuming both the
      desired output of a DDS and any spurs are
      sine waves
      > >or behave
      like sine waves.  Is this wrong?
      > >
      > >Also the DDS
      output has gone thru several tuned circuits and that
      things
      > >like
      the second and higher harmonics are way, way down in amplitude
      and that
      > >the signal looks like a sine wave with minimal or
      no
      discontinuities.  Is
      > >this a bad assumption?
      > >
      > >My assumption is that for a greater majority of the cycle
      the
      desired signal
      > >is so much stronger than the spur, that the
      spur has no
      impact/effect on the
      > >counting/division
      process.
      > >
      > >Are the counters and/or dividers we are apt to
      use sensitive enough to
      > >transition/detect signals 40 dB down? 
      60 dB down?
      > >
      > >Does a signal 40 dB down mixed with the
      desired have any effect
      except when
      > >the desired passes thru
      zero?
      > >
      > >If the spur only has an effect near zero
      crossing, then might a Schmitt
      > >trigger be a possible
      solution?
      > >
      > >Joe, K9HDE
      > >
      > >PS 
      Do all spurs generated as a byproduct of data truncation within
      a DDS
      > >behave the same way, that is, are all spurs frequency and
      amplitude
      stable?
      > >
      > >
      > >
      > > >
      -----Original Message-----
      > > > From:
      softrock40@yahoogroups.com
      > > > [mailto:softrock40@yahoogroups.com]
      On Behalf Of jabauzit
      > > > Sent: Saturday, January 21, 2006 8:27
      PM
      > > > To: softrock40@yahoogroups.com
      > > > Subject:
      Re: Fw: [softrock40] DDS spurs, was Got my
      > > > 5.0....Now what do
      I need?
      > > >
      > > >
      > > > --- In
      softrock40@yahoogroups.com, "allen_fulmer"
      > > >
      <afulmer@t...> wrote:
      > > > >
      > > > > The
      following is an excerpt from "Replacing or Integrating
      > > > PLL's
      with
      > > > > DDS solutions" by Rick Cushing, Applications
      Engineer, Analog
      > > > Devices,
      > > > > Inc.
      > > > >
      > > > > "Another strategy to control spurs,.....,
      is to place the
      known spur
      > > > > source ahead of a
      divide-by-N stage to reduce spurs by 20 Log
      > > >
      (Fout/Fin)."
      > > > >
      > > >
      > > > Which
      means that the DDS runs at N times the frequency of
      > > > interest.
      It
      > > > also means that the spurs are worse.
      > > >
      Would the result be better than running the DDS at the fundamental?
      > > >
      > > > Jean-Claude PJ2BVU
      > > >
      > >
      > >
      > >
      > >
      > >Yahoo! Groups Links
      > >
      > >
      > >
      > >
      >
      >
      > Cecil
      Bayona
      > KD5NWA
      > www.qrpradio.com
      >
      > "I fail to see
      why doing the same thing over and over and getting the
      > same results
      every time is insanity: I've almost proved it isn't;
      > only a few more
      tests now and I'm sure results will differ this time
      ... "
      >
      >
      >
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