EFRATOM LPRO Interface

Introduction

Perhaps one of my more silly Ebay purchases of late. I wasn't sure what I wanted to do with a Rubidium frequency standard but it looked like an interesting gadget so I stumped up the £50 or so and awaited delivery. From the LPRO user manual, The LPRO makes use of the atomic resonance property of rubidium (87Rb) to control the frequency of an unheated quartz crystal oscillator via a frequency-locked loop. In other words, this gadget uses the radioactive breakdown of rubidium87 to generate a 10MHz sine wave. Once the device has been switched on and warmed up, the accuracy of the output waveform is nominally ±5 x 10-8 Hz or 5Hz in 1GHz. That's quite accurate :-)

What does a computer fellow do with a 10MHz sine-wave?

In recent times, there seems to have become some competition in certain geek circles to see who can build the most accurate clock - somewhat akin to the schoolyard conversations of the 1980s:

"My watch is set by the talking clock."
"Well, mine is set by Big Ben."
"Shame... Mine is set by the Rugby signal."
etc. etc.

In order to have a clock to compete with my equally childish friends, the plan is to build an interface board for the LPRO to convert the 10MHz to 1Hz, or 1 pulse per second (PPS). This PPS signal can be used by the Linux NTP Daemon to discipline a time source, such as the computer's internal clock (or the time signal from GPS) to produce a nanosecond-accurate clock.

Hardware

For clarity, the interface is divided into two logical sections (both built on the same board) - one handling the power to and diagnostics from the LPRO unit and one handling the conversion of the 10MHz signal to PPS.

LPRO Unit Pinouts

• Ext C-Field is for calibration to an external, more accurate time standard and is not used by this interface.

Power & Diagnostics

As well as taking power, ground and signal connections, the LPRO unit provides several status outputs, which can be monitored for debugging purposes. Opamps in this section of the interface compare the diagnostic outputs of the LPRO to fixed voltages from potentiometers. BITE or "Built In Test Equipment" provides information as to whether the crystal oscillator is locked to the atomic transition. Once the BITE output is low, (less than 4.2V) lock has been acquired and accuracy should be within ±5 x 10-8. The BITE LED on this interface will light when lock is acquired. Lamp Voltage is monitored for inherent degredation of the photodiode. If Lamp Voltage drops below 3VDC, the LAMP V LED will light, indicating that maintainence is required. Xtal V Mon is used to indicate if the crystal is drifting out of the available trim range once the unit has warmed up. If this output falls outside the range of 0.55 to 12.6VDC, the XTAL V MON LED will light, indicating that service is required.

Signal Processing

74HC390 "Dual Decade Ripple Counter" chips each contain two "divide-by-two" and two "divide-by-five" sections. Using four such devices, it is possible to "divide-by-one-million", producing a 1Hz TTL signal. A MAX232 chip is used to convert the TTL signal to RS232 level for input to the serial port of a PC. A 7805 voltage regulator is included so that a single 24VDC supply can be used to provide power to both sections of the interface and also to the LPRO unit. The novely component in the circuit is the LED, which indicates the PPS. An oscilloscope should also be used to verify signal correctness!