In 2009 a seller from China appeared on ebay.com and offered a large stock of used, about 10 years old “Efratom LPRO-101 Rubidium time base” for a very reasonable price of about 40 US$. In a laboratory you should have an exact time base and I didn’t own one before, so I ordered it. Once arrived I started to create some electronics around the LPRO-101 module with the aim to build a high quality device that feeds other laboratory appliances with an exact clock.
This project uses a 10MHz rubidium atomic clock as the time base. Several 10MHz reference outputs are provided at different levels for my other workbench equipment. An integrated Cypress CY22150 CyberClock PLL uses this reference to make 6 PLL outputs with 2.5V and 3.3V level available. The PLL can be programmed with the integrated microcontroller Atmel ATMEGA644.
An USB-port is present to program the PLL alternative with the software from the project “CyberClock CY22150 with USB and control software”. For the USB to I2C interface I used also here the “I2C-Tiny-USB” adapter, developed by Till Harbaum with drivers available for Linux, Windows and MacOS. At the I2C-Tiny-USB homepage you will find the firmware for the ATTINY45 controller.
The device contains a keypad with four hotkeys and a rotary encoder for menu driven user input. A LCD-display shows the actual output frequencies and device status. Case and heatsink temperatures of the LPRO-101 are monitored with 1-Wire temperature sensors. Three LEDs displays the state of the device and signals errors and warnings.
All device states are shadowed in an FRAM RAMTRON FM24CL04. The last state is automatically restored at power up. A maximum of 8 different setups can be saved and recalled.
technical data
outputs
- 1x 10MHz reference out, 1.56Vpp, sinus
- 3x 10MHz reference out, 3.11Vpp, isolated, sinus, Vout adjustable
- 2x 10MHz reference out, 5V TTL
- 2x 10MHz reference out, 3.3V TTL
- 4x CY22150 PLL out, LCLK1..4, 2.5V
- 2x CY22150 PLL out, CLK5..6, 3.3V
timebase
- Efratom LPRO-101 Rubidium frequency standard
power supply
- 24V, 2.5A DC
The firmware for the integrated microcontroller ATMEGA644 is available for avr-gcc. It is written in C and developed with “AVR Studio 4.15”, an IDE for 8-bit AVR-controller by Atmel. The source code is provided under the GNU General Public License (GPL) and can be downloaded here.
Maybe the code has to be updated for the recent version “Atmel Studio 7”. But the old IDEs are still available here.
project goals
- high quality components for best performance
- very low distortion
- low jitter
- phase correct outputs of same type
- device state saved in FRAM and restored at power up
- non volatile memory for 8 setups
- USB interface to program the PLL from a PC
- multi platform graphical user interface
The schematics is available for download here.
Rubidium-ClockDistribution_schematics.pdf
A verified 2-layer printed circuit board layout is available too.
The design in Protel 98 format with Gerber Plots can be downloaded here.