LPRO-101 Rubidium (Rb) Disciplined Crystal Oscillator

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LPRO-101 Rubidium (Rb) Disciplined Crystal Oscillator

LPRO-101 Rubidium (Rb) Disciplined Crystal Oscillator

  System Dependencies
  Minimum PowerSDR Version:     N/A  
  Applicable Hardware:     FLEX-5000 & FLEX-1500  

Content provided by: Brian, WB6RQN/J79BP

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. Efratom used to be part of the Ball Corporation but is now part of Datum.  The LPRO-101 Rb reference is evidently very widely used in telephony applications; probably as a frequency reference for cell towers.  The telephone companies maintain very high availability by changing out components before they fail.  This means that life-limited parts like the LPRO-101 are swapped out well before they reach their end-of-life and are now becoming widely available on the used equipment market.  The going price for an LPRO-101 that has been removed from service seems to be in the $65-$100 range on eBay.

The LPRO-101 is very simple to use.  It has a 10-pin connector with the pins on 0.1" spacing, just like standard IDC headers so procuring a connector is simple.  The LPRO-101 only needs a source of power to operate.  Power range is 19V-32V at 1.7A peak. At start up the current is 1.7A as it is warming up the Rb lamp and the oven for the OCXO.  Once they reach operating temperature the current drain drops to about 0.5A.  The internal power supply is linear so running the LPRO-101 at a lower voltage reduces the total power dissipation.

The LPRO-101 is not large enough to dissipate all the power.  It needs to be mounted to a heat sink or have forced-air cooling.  One of the reasons for running it at a lower voltage instead of the "nominal 24V input" recommended by Efratom/Datum is that it reduces the heat dissipation.

There is a 10MHz output at +7dBm.  The output is cap-coupled.  The shield may be tied to case ground but there is also a DC-isolated RF ground-return if you are concerned about ground loops.

There are a couple of outputs for status.  There is a BITE "fault" output that is active-high.  When that is high it means that the LPRO-101 is not suitable as a reference.  When that goes low it means that output is locked to the Rb reference and the unit is meeting accuracy specs.   There are several simple circuits available on the Internet for building an LED locked/unlocked interface.  Here is once reference from KA7OEI: A portable 10 MHz Rubidium Frequency Standard.

There are also outputs to monitor Rb lamp voltage (an indicator of remaining life) and VCO voltage for the OCXO, which tends to give crystal aging information.

There is an input for adjusting the frequency slightly.  The correction voltage is 0V to +5V with the nominal center frequency happening at 2.5VDC.  This actually varies a magnetic bias on the Rb reference cell to slightly change the frequency.  This is how you control the LPRO-101 if you want to phase-lock it to GPS or another higher-accuracy source.

But even without the addition of a GPS discipline circuit, the LPRO-101 is a very accurate reference oscillator.  If nothing is connected to the external control voltage input there is an internal ten-turn pot that lets you tweak the LPRO-101 onto frequency.  The range of adjustment is a very small fraction of a Hz so you need a *really* accurate reference in order to make the adjustment.   It requires a known frequency source that has a greater presicion than the Rb standard, so it is probably best not to try and calibrate the LPRO-101 unless you have the proper equipment.

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Last Modified:Tuesday, March 30, 2010
Last Modified By: Administrator
Type: INFO
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