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Re: [softrock40] Re: DDS-60 and Softrock V5 - Spurs

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  • Robert McGwier
    Given the structure of the SR offerings, and the DDS-60, you can find good frequencies for the DDS-60. I cannot remember all of the details on even the
    Message 1 of 4 , Feb 28, 2006
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      Given the structure of the SR offerings, and the DDS-60, you can find
      "good frequencies" for the DDS-60. I cannot remember all of the details
      on even the DDS-60 I own for my SR5's! so I cannot remember the size of
      the phase word or the accumulator. It is a straightforward calculation

      Take the oscillator input frequency and divide it by 2^(number of bits
      in the phase word for the DDS) to an idea of the order of magnitude of
      the distance between "good frequencies"


      Say the oscillator for the DDS is 100 MHz and its phase word is 12
      bits. The frequencies are spaced

      100,000,000/(2^12) = 244 kHz!!

      So 122 KHz + N* 244 kHz will get the "best frequencies". There is a
      more complex calculation that gives you the "next best" which can be
      derived from the paper cited. Also, the phase truncation spurs are not
      the only source of spurs from a DDS. The devices have DAC's in them and
      associated glue, etc. All of these are sources of nonlinear transfer
      functions. Each of these nonlinearities (like a doubler or a tripler or
      a mixer) will produce spurs as well. This thing is not supposed to
      perform like the SDR-1000 at $1500 or the IC7800 at $13000. What can
      you expect from a $29 receiver and a $30 VFO? It already outperforms
      the very best receiver I ever owned from 1964 until 2001. ;-).


      Bob
      N4HY


      Bill Dumke wrote:
      > Is it possible to miss the spurs by only using 25 kHz or 50 kHz steps
      > between DDS-60 frequencies? That would be good enough with a lot of
      > overlap for 48 kHz and 96 kHz sampling.
      >
      > Bill WB5TCO
      >
      > mikezingman wrote:
      >
      >
      >> Dick,
      >>
      >> I measure 0.8v P-P on my 20 meter BPF board, so I suspect that you
      >> should be able to drive it with the DDS-60. I am in the process of
      >> doing exactly the same thing as soon as I build up a new BPF minus the
      >> OSC.
      >>
      >> As far as the spurs, quite a lot has been done based on the
      >> information in the Analog Devices white papers on the subject and then
      >> translated into code by Bob, N4HY. Refer to section 4 in this link:
      >> <
      >> http://www.analog.com/UploadedFiles/Tutorials/450968421DDS_Tutorial_rev12-2-99.pdf
      >>
      >> Mike N4IRR
      >>
      >>
      >> --- In softrock40@yahoogroups.com, "n3hkn" <n3hkn@...> wrote:
      >>
      >>> I also posted on QRP-L...
      >>>
      >>> I have my PIC-EL/DDS-60 as a VFO for my 20 meter V5 QSD. Works well
      >>> except for the spurs. I run the V5 from a small battery to eliminate
      >>> any pasty path that way. However, on some frequencies no spurs in a
      >>> 48khz bandpass while on others total spurs end-to-end. Mostly I can
      >>> position a station between spurs but this is a bit awkward.
      >>>
      >>> So, I know that there has been discussion on the spurs issues. Has
      >>> anyone come up with some solutions that reduced the number/amplitude
      >>> of the spurs? As an example, does putting either the PIC-EL or the
      >>> Softrock V5 in a case do any good?? DDS-60 connected to Pin-4 of the
      >>> header via 6 inches of RG-174. No components on the OSC/BPF board are
      >>> connected to Pin-4. Ground to Pin-7.
      >>>
      >>> One other issue is that the max I can get from the DDS-60 is 2V P-P.
      >>> Is this enough?
      >>>
      >>> Thanks-Dick Boley N3HKN
      >>>
      >>>
      >>
      >>
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      --
      AMSAT VP Engineering. Member: ARRL, AMSAT-DL, TAPR, Packrats,
      NJQRP/AMQRP, QRP ARCI, QCWA, FRC. ARRL SDR Wrk Grp Chairman
      Laziness is the number one inspiration for ingenuity. Guilty as charged!
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