GPS Disciplined 10 Mhz Oscillator

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)

GPS Disiplined 10 MHz Master Oscillator & Distribution Amplifier front panel
GPS Disciplined 10 MHz Master Oscillator & Distribution Amplifier front panel

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.

GPSDO 10MHz Board with Isotemp 10 MHz OCXO
GPSDO 10MHz Board with Isotemp 10 MHz OCXO

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.
Daily Aging
After 30 days < ±1x10‑9
After 90 days < ±5x10-10
Short term      < 1x10-10/sec­ond root Allan variance
Phase noise
@ 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.

GPSDO 10MHz Rev 1.0 bare circuit board top
GPSDO 10MHz Rev 1.0 bare cir­cuit board top
GPSDO 10MHz Rev 1.1 bare circuit board bottom
GPSDO 10MHz Rev 1.1 bare cir­cuit board bottom

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.

GPSDO 10MHz S1315F GPS module board top
GPSDO 10MHz S1315F GPS mod­ule board top

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.

GPSDO 10MHz Test assembly with a GNSS2, GPS from MikroElectronika
GPSDO 10MHz Test assem­bly with a GNSS2, GPS from MikroElectronika

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.

GPSDO 10MHz OCXO, main board, 12 volt power supply, Extron power supply
OCXO, main board, GPS mod­ule, 12 volt pow­er sup­ply, Extron pow­er sup­ply out­side front

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.

GPSDO 10MHz Inside display, front panel switches and indicators
GPSDO 10MHz Inside I2C RGB LCD dis­play, front pan­el switch­es and indicators

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.

GPSDO 10MHz Extron back panel with GPS antenna connection
GPSDO 10MHz Extron back pan­el with GPS anten­na connection

I installed plas­tic hole cov­ers on all removed BNC con­nec­tor loca­tions, and added a fuse hold­er for the AC pow­er in.

GPSDO10-V3 board layout Eagle CAD V8.5 Top
GPSDO10-V3 board lay­out Eagle CAD V8.5 Top

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.

Eagle 8.5 main board files Rev. 3 ZIP
Eagle 8.5 GPS mod­ule files Rev. 1.3 ZIP
Arduino 1.8.2 OrigS1315GPSDO.INO ZIP
Eagle 7.7 S1315F foot­print library ZIP

6 Replies to “GPS Disciplined 10 Mhz Oscillator”

  1. Would you be inter­est­ed in shar­ing the S1315F Eagle library data? I am build­ing one of these myself, but have a small stack of these GPS units. (Sparkfun junkbox day for the win!) I can make a foot­print but if you’ve already done it, and care to share it, it would be most appreciated. 

    Jake KI4YAN

  2. Hi Barbouri,
    Do you have Sketch of this project to be used with Ublox Neo series of GPS mod­ules , which are much more abun­dant­ly avail­able com­pared to SkyTraq S1315F GPS module.

    1. John,
      The orig­i­nal sketch from K6JCA used a Quectel L76 GPS receiver.
      You should be able to com­pare the two and see what needs to be mapped to uti­lize the Ublox receiver.

      Greg (Barbouri)

    1. Hi Don,
      I don’t use any of the Gerber files when order­ing the boards, just the .brd file.
      The ger­ber’s can be extract­ed from the files in Eagle CAD, or can be direct­ly ordered from OSH Park

      Greg (Barbouri)

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