[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.
Mike
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