[time-nuts] A Highly-Accurate and Stable SDR Using GPS-DO and DFS

Brad Dye brad at braddye.com
Fri Jul 23 20:37:45 EDT 2010


My latest project is a general coverage radio receiver (SDR-IQ), tuning from very high audio frequencies (like bats) to 30 MHz in one-Hertz steps -- locked to the GPS constellation. The "dial" is a 24" color monitor connected to a laptop showing 190 KHz window of the band like a spectrum analyzer. 

I have been involved with many kinds of radio receivers for over fifty years -- amateur, military, and commercial. This one is a dream come true. When I was a young ham, I dreamed of the day when I might have a receiver that would read out to one kilocycle (before we used the term Hertz). Now I have assembled one that reads out to one Hertz and is accurate to a few millihertz -- and thanks to GPS -- it will maintain this accuracy as long as the GPS satellites keep working. 

I well remember the early radios that I used. I had to be very careful to not bump the table where the radio was or it would jump completely off frequency. Whatever frequency it indicated was only approximate. It made me nervous to operate near a band edge. 

Dave Powis, G4HUP designed and built a DFS for me. I really appreciate his help on this project, especially since several hams told me it couldn't or shouldn't be done. Some said that the frequency read-out on the SDR-IQ would not be accurate because the time-base in the computer's sound card would be a variable factor, but this is not true. The frequency accuracy and stability of this radio ONLY depends on the 10 MHz oscillator in the Trimble Thunderbolt and that oscillator is locked to the GPS constellation. 

Dave and I worked together on this project for about one year. The DFS box travelled across the Atlantic Ocean five times before we finished.

Dave has written an article "Precise Frequency Locking for the RFSpace SDR radios" and I have a web page showing how I connected all of this equipment together. Here are the links:

http://www.braddye.com/g4hup_dfs.html
http://www.braddye.com/gps_do.html

73s

Brad Dye, K9IQY
ex KN9IQY, KN4BK, KM5NK, WB4JCF, ZP5TQ, WA4VXU, WA9RVL, HH2FJ /TI2, /9Y4, /6Y5, /KP4
53 years as a FCC licensed amateur radio operator
37 years as a FCC licensed first class commercial radio operator





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