A GPS disciplined 10 MHz oscillator and distribution amplifier project using a Sparkfun Pro mini microprocessor and an Isotemp 134–10 OCXO. (Click on images for larger version)
After working on the Hewlett Packard 5300B Measuring System, I decided that I needed a precision 10 MHz reference for my counters and other testing purposes. I had recently seen a blog post by K6JCA on his Arduino-based GPS Disciplined Oscillator, and wanted to build something similar.
I had also watched a YouTube video where Gerry Sweeney uses an old Extron video distribution buffer for his 10 MHz project.
So I decided to build a mashup of the two projects with a few upgrades and changes to suit my needs.
For the 10 MHz oscillator, I found an Isotemp Research Inc. model OCXO 134–10 ovenized crystal oscillator. The price was right and it had decent specifications.
After 30 days < ±1x10‑9
After 90 days < ±5x10-10
Short term < 1x10-10/second root Allan variance
@ 10 Hz < ‑105 dBc
@ 100 Hz < ‑125 dBc
@ 1 kHz < ‑140 dBc
I also liked the way the 6 connections were arranged inline on the bottom of the metal case. The inline pins made it easy to design a small purpose built circuit board for that module.
Before long I had a prototype circuit board from OSH Park, and was ready to do some testing.
The board uses a 12 to 13 volt input and regulates that with some Recom switching regulators for the 5 volt and 3.3 volt supplies, and a MAX864 Dual-Output charge pump for the +/- 10 volt supplies.
I also changed the microprocessor to a Sparkfun Arduino Pro Mini, mainly because I had several of them in stock. Another change from the K6JCA system was a 0 to 8.192 trim output for the Isotemp OCXO trim input instead of the ‑5 to +5 volt of the original design.
Because I only needed a 4 volt output from the 16-bit DAC, I opted for a 4.096 volt reference instead of the original 5.000 volt reference which provided smaller trim steps. To get the 8 volt trim output from the DAC I used an INA105KU Precision unity gain differential amplifier, in a precision (G = 2) configuration.
I also used a different GPS module which required some software changes, but I had them in stock and in previous testing had good 1PPS timing accuracy. The module is a SkyTraq S1315F which is a 65 channel Venus‑6 engine. I paired it up with a Motorola 2000 25dB GPS timing antenna that I already had.
After getting everything working on the bench, it was time to modify the Extron distribution board with 50 ohm terminator resistors and install all the parts in the case.
The Extron enclosure is made from aluminum and is very sturdy, with lots of empty space inside which made it perfect for this project. The 10 MHz OCXO is wrapped with a thin layer of insulation to reduce power usage. The Mean Well 12 volt power supply has an output trim adjustment, which I set to 13 volts DC.
The Extron distribution board was modified to remove the input BNC’s from the two rows that I used. I also replaced the surface mount 75 ohm resistors on all used channels with 49.9 ohm thick film 1206 resistors. I also removed the Extron V channel distribution board which made space to mount my GPS receiver module in the bottom center of the rear panel.
I designed a new front panel for the I2C RGB LCD display and had it fabricated by Front Panel Express.
The I2C RGB LCD interface I built includes 6 programmable I/O pins that are also on the I2C bus. that I used for the backlight off switch and OCXO Oven status indicator.
I am using the RGB backlight color to indicate the operational status and accuracy of the unit.
I added 50 ohm BNC terminators on all unused operational BNC channels.
I installed plastic hole covers on all removed BNC connector locations, and added a fuse holder for the AC power in.
Even though this was a very fun project to build and kept me busy for many hours. It ended up costing several times more than what a GPSDO currently can be purchased for.
It is still not perfect and I would like to complete at least one more revision of the main board, but cannot justify the additional expense. It currently works good for my intended use though.