The LPRO-101 DIY Rubidium Clock Generator

The DIY LPRO-101 ruibidioum clock generator. This box together with some additional electronics on a separate PCB will transform this LPRO unit into stand alone equipment. I’ve always wanted a good and affordable frequency standard for my home lab. Searching the eBay one might find quite a lot of solutions. The good thing is that nowadays we have plenty of options.

Second hand market is now flooded with old decommissioned rubidium standards pulled out of scrapped telecommunication equipment. There are a lot of chooses like the FE-5680A-10MHz or the FE-5650A. Some of those even come with programmable option (Option 58) that allows the user to program his own frequency.

Recently I stumbled across low priced decommissioned LPRO-101’s from China. The seller offered a good price and a free calibration(adjustment). The LRO-101 is a well known choice among the DIY community. It is a well documented piece of equipment and a good choice for the DIY enthusiast on a budget. You can find a comprehensive repair guide here and the datasheet is located here.

The LPRO-101 module pinout

Let’s see what you get with the LRPO-101. Here’s the pinout of the box:

The module provides a 10MHz sine wave output at around 0.5mV RMS into 50 Ohm load. You need a 24V DC power supply capable of delivering about 1.5A of current. The current draw is higher upon start-up and after around 4-5 minutes it will drop to around 0.5A. A good low-noise power supply is recommended here. DC-DC power supplies are generally noisy and might not be the best choice for this project. Pin 6 provides the built in test equipment (BITE) signal. This pin holds logic 1 until “cavity lock” status is obtained. Once the LPRO-101 is stable and the frequency locks, pin 6 goes low.

An interesting function is provided by pin 5 and pin 9. These will allow for some voltage monitoring of the LPRO-101 oscillator. For my project I’ve decided to implement a simple voltage monitoring of the lamp voltage. A good unit will have this voltage above 4.5V usually. The voltage rises upon power-up and after a short time it will hold around its nominal value.

The schematics overview

For my project I needed to have the following functions – voltage measurements for Vmon, temperature measurement (LM35), BITE monitoring, DC fan controls ( active cooling inside the enclosure), LCD display for ease of use, two outputs (sine and TTL).

Sine to TTL

For sine to TTL conversion I use the recommended schematic from LPRO-101’s datasheet:

Temperature measurement

The LM35 sensor senses the temperature and sends its output to a G=5 amplifier. The resulting output is now 50mV/°C. There is a RC filter before the op-amp for better noise reduction. I’ve found out that in my situation this filter is unnecessary. The result of the measurement is stable and there is no noise. Some 3M thermal adhesive holds the LM35 glued to the LPRO-101’s surface. Here is a photo of the sensor:

And here’s the schematic diagram:

50mV/°C allows for a better ADC resolution. With a 5V voltage reference and a 10bit ADC the maximum temperature read is 100°C. This is more than enough for this project. The following equation shows the calculation of the temperature:

LPRO-101 voltage measurement

This brings us to the voltage measurement for the Vomn output. The voltage on this pin is 3-8 volts. I assume 10V is the maximum input. Usually most of the modern microcontrollers work at 5V. A voltage divider takes care of this situation. Excessive voltage might damage the ADC input of out micro. A voltage follower buffers the output of the voltage divider and serves as input circuit to the ADC. A 5.1V zener diode serves as input protection for the ADC.

Elements R9, R6, R5 make the voltage divider. U5 is the voltage follower which buffers the divider’s output to the ADC input. R5 serves as a calibration adjuster. Use an external good quality DMM to calibrate the reading of the voltmeter.

The complete schematic

Finally we arrive at the complete schematic of this project. You can download a PDF of the LPRO-101 support PCB schematic here.

LPRO-101 rubidium standard gallery

This leads us to the gallery of the complete project. Here are some photos:

Software for the LPRO-101 support PCB

This project makes use of the ATMEGA148 micro. The internal 8MHz oscillator provides the clock so there is no need for external source. Please make sure you configure the fuse bits correctly. Use this calculator to configure the fuse bits. Hex file is located here. The micro outputs use info via RS-232. Configuration – baud 28800, Parity none, Data bits 8, Stop bits 1, Handshake none.