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

2 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

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