Thread: Homebrew GPSDO
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Old 21st Feb 2021, 11:50 am   #1
mole42uk's Avatar
Join Date: Aug 2010
Location: Bristol, UK
Posts: 2,142
Default Homebrew GPSDO

Here's some pictures of my GPSDO. I used the design form James Miller G3RUH ( and modified it to use the Jupiter GPS receiver and Bliley OCXO that I had. One extra modification (not pictured) is to add a 50Ω 1MHz output on the rear panel to lock my Marconi 2955 radio test set to an external reference.

Generally speaking, the GPSDO works as intended, the Marconi 2955 measures the output at exactly 10MHz and the GPS receiver responds well to NMEA commands and provides reassuring output messages that the receiver is tracking 10 satellites. Next I will borrow another GPSDO to attempt triple-redundancy - the OCXO is quoted at 10.000000MHz, the Marconi 2955 measures the output to 10.0000MHz so if I use another 10MHz reference and lissajous the two outputs I should have confidence in my new reference! I intend to make a polystyrene box for the OCXO to help it remain at a stable temperature, but the oscillator will be running 24x7 as soon as I''ve proved the whole.

In the images below, the front panel "Locked (1 pps)" lamp is simply a 'heartbeat' - if the system is running correctly that lamp flashes at 1Hz. The front panel 10MHz output socket is a buffered 50Ω feed to any equipment that needs it. The Jupiter GPS receiver is ex-military equipment and is the TU30-D400 version which uses 3.3V power but outputs directly to the 5V TTL on the Veroboard. The OCXO is another ex-military pull that has a pretty good specification, and the manufacturers in the USA still answer email enquiries about it!
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recursive loop: see recursive loop
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