Getting Exactly in Step – Adding a Rubidium Frequency Reference

Based on very helpful information from a forum, the Knowledge Base Article at Flex-Radio’s website and very helpful direct correspondence from Brian Lloyd WB6RQN/J79BP I’ve taken the plunge and have sourced a surplus Datum LPRO Rubidium Frequency Reference from a regular supplier of salvaged and tested LPRO units.

From the Flex-Radio Article (at ):

The Model LPRO-101 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 (FLL). All commercial rubidium frequency standards operate by disciplining a crystal oscillator to the hyperfine transition at 6.834,682,612 GHz in rubidium. The amount of light from a rubidium discharge lamp that reaches a photo detector through a resonance cell will drop by about 0.1 % when the rubidium vapor in the resonance cell is exposed to microwave power near the transition frequency. The crystal oscillator is stabilized to the rubidium transition by detecting the light dip while sweeping an RF frequency synthesizer (referenced to the crystal) through the transition frequency.

The Efratom LPRO-101 is a Rb oscillator that can used to discipline the FLEX-5000 and FLEX-1500……

I’ve the appropriate header to use as a plug, a BNC Terminated Cable to carry the signal to the Flex-5000A, a suitable power supply and a very different disconnect plug (as I do not have 20V DC in use for anything else). I thought I had appropriate heatsinks in my parts box, but they must have been used for something else. So I will need to fabricate a heat dissipation plate before putting the Rb Frequency Reference in continuous use.

As a “step-two” the circuit to indicate the status of the unit “lock” will be added.

Here are some references on this:

I need to tip my hat to the folks at AR15.Com’s Amateur Radio Forum who pointed me to Time-Nuts at which proved to be phenomenally interesting and helpful.

Photos and use-report to follow once I have everything running to satisfaction.



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2 thoughts on “Getting Exactly in Step – Adding a Rubidium Frequency Reference

  1. Allan Kaplan W1AEL says:

    As a retired electronics engineer, I DO understand the aesthetic appeal of having the station equipment locked to a very accurate frequency standard. You will know your frequencies to the Hertz, or fraction thereof. Heck, I’d probably go that way if my toy-money budget had fewer demands on it. My bottom line is that while knowing frequency and time to laboratory standards of accuracy and stability feels good, there are NO PRACTICAL BENEFITS for hams, as compared to using fairly high-end equipment with frequency standards that provide accuracy and stability from 3 down to 0.5 parts per million. This probably sounds like “sour grapes” but it is meant to be practical.
    73 de W1AEL

    • k9zw says:


      You’re right that an HF station hardly ever “needs” the accuracy of a Disciplined Frequency Reference.

      It could be argued that even a TCXO is over kill.

      But a Rb Source is fun, and provides the in-shack reference to more effectively use VHF/UHF Transverters in the shack.

      All best and 73


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