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

J. Forster jfor at quik.com
Fri Jul 23 23:05:21 EDT 2010

If you want an HF radio that reads to a Hz, ready made, get a used HP
(Agilent) 3486 Selective Level Meter. It covers essentially DC to either
20 or 30 MHz. It has a good internal ovenized oscillator, and can be
locked to an external standard.

A virtually unique feature is that it will lock onto a carrier and read
that carrier frequency to a Hz or better.

They are often abailable for a few hundred. The 3486A & B are similar.




> I have been involved with many kinds of radio receivers for over fifty
> years -- amateur, military, and commercial. This modified SDR-IQ 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 66.66666 MHz 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
> /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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