[time-nuts] GPSDO project

Mike Monett xde-l2g3 at myamail.com
Sun Jun 28 22:52:57 UTC 2009

  > GPSDO project
  > Brad Dye brad at braddye.com
  > Sun Jun 28 19:33:14 UTC 2009

  > I have  just purchased a Thunderbolt GPS receiver. I  hope  to use
  > this to  discipline an oscillator on 66.66666 MHz so I can  use it
  > as a LO on my Software Defined Receiver (SDR-IQ).

  > Building such an oscillator is a little over my head so  I thought
  > I would  ask  the  group if anyone knows  where  I  could  buy the
  > missing link  of this project. I have put a drawing of  my project
  > on the  web  showing the specs of what I need.  The  oscillator is
  > shown in the blue box.

  > Any help or suggestions would be greatly appreciated.

  > http://www.braddye.com/gps_do.html

  > 73s

  > Brad Dye K9IQY
  > ex KN9IQY, KN4BK, KM5NK, WB4JCF, ZP5TQ, WA4VXU, WA9RVL, /TI2, /9Y4, /
  > 6Y5, /KP4

  As already  mentioned,  a  DDS   produces  unwanted  spurs.  You can
  elminate them by using it backwards.

  1. get a 66.6666MHz VCXO with good phase noise
  2. drive the SDR as normally done
  3. also drive a suitable DDS chip
  4. set the DDS to produce 10MHz output with 66.6666MHz reference
  5. feed the TBolt and DDS outputs to a 10MHz pfd
  6. use loop filter to correct 66.6666MHz VCXO
  7. you are now locked to GPS without the spurs from the DDS

  Simplified Diagram (view in fixed font)

  TBolt 10MHz -------------------------------> PLL Ref
                                                        Loop Filt ---+
  66.6666MHz  ----->  DDS -----> 10MHz ------> PLL Clk               |
    OCXO I/P  <------------------------------------------------------+

  Trim the frequency by adjusting the DDS as desired, perhaps  to give
  a slight offset to the receive frequency.


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