10 MHz Rubidium Frequency Reference |
Overview
Low-cost 10 MHz rubidium frequency references ("clocks") are starting to hit the amateur radio surplus market for under $100. Most of these rubidium clocks where originally used as the backup frequency standard in cellular phone and pager base stations. Because the rubidium "clock" inside the frequency reference itself has about a 10 year lifespan, they are constantly being swapped out as part of a preventive maintenance move. As long as you don't keep these surplus rubidium clocks continuously powered, they should still last for many more years..
The 10 MHz rubidium frequency reference discussed here is the common Efratom (DATUM) LPRO-101. This frequency reference is based on the atomic resonance of a rubidium gas discharge lamp exposed to a RF signal near its transistion frequency of 6,834,682,612 Hz. There is a "dip" in the discharge lamp's output when the extact atomic resonant frequency is found. This frequency is then used as a phase-lock source which is eventually divided down to just about exactly 10 MHz. The datasheet for the LPRO-101 dives into the technical operation in much more detail, so you should read that before starting this project.
There is really no long warmup period for the LPRO-101. After about 3 to 5 minutes from applying +20 VDC (1.7A) to the LPRO-101, the unit will come into "lock." After coming to lock, the current draw will then drop to around 500 mA. At this point, the 10 MHz output signal already about 1,000 times more accurate than zero-beating a regular clock oscillator to WWV! After about a 15 minute warm-up period, the unit will be operating at its most accurate frequency, within a few millihertz of exactly 10 MHz
The LPRO-101 does get fairly warm and SHOULD NOT be run without proper heatsinking. One trick which can be used to reduce the overall heating of the unit is to run it at only +19 or +20 VDC, instead of at +24 VDC.
The LPRO-101 has a Built-In Test Equipment (BITE) signal (pin 6) which can be used to determine if the unit is locked. When this pin is "high," the unit is unlocked and the output frequency will not be accurate. When the BITE signal goes low, the unit is locked and should be on frequency. A simple 74HCT04 hex inverter and two LEDs will be used to monitor the status of the BITE signal. A RED LED will light when the unit is unlocked, and a GREEN LED will light when the unit comes into lock.
One of the signal pins is for monitoring the rubidium lamp voltage (pin 5) which can be used determine the remaining life of the unit. This pin should measure above 3 volts in a "good" unit.
There is also an "External C-Field" tuning line (pin 7) which can be used to tweak the final output frequency a few millihertz up-or-down. This makes it possible to sync the LPRO-101 with an even more accurate clock source, like from a cesium or GPS-based reference source. Bertrand Zauhar (VE2ZAZ) has an article entitled "A Simplified GPS-Derived Frequency Standard" in the Sep./Oct. 2006 issue of QEX which should be compatible with the LPRO-101.
The 10 MHz output from the Efratom LPRO-101 is around +7 dBm, with a standard impedance of 50 ohms. It's safe to assume that the unit could drive a number of external devices, but just to be safe we'll be adding a simple buffer amplifier. This should also help to protect the Efratom LPRO-101 from any external mismatches. Depending on your load requirement, you may want to avoid "squaring up" the 10 MHz sine wave output signal to reduce the number of spurs and harmonics generated.
Pictures & Construction Notes

Overview of the label on a surlus Efratom LPRO-101 10 MHz rubidium frequency reference.
Pin # Description 1 10 MHz RF Output (Sine wave, 50 ohms, +7 dBm) 2 Chassis Ground 3 DC Isolated RF Ground (Isolated ground in case of ground loops) 4 Chassis Ground 5 Lamp Voltage (Above 3 volts = unit is good) 6 BITE (HIGH = unlock, LOW = lock) 7 External C-field (Remote frequency control) 8 +24V Ground 9 Crystal Voltage Monitor (Voltage monitor of the internal sweep oscillator) 10 +24V

Mounting the Efratom LPRO-101 10 MHz rubidium frequency reference in an old printer switch case.
The bottom of the printer switch case was sanded down to the bare metal and coated with heatsink compound before mounting the LPRO-101.
The front-panel has banana jacks for the +24 VDC input and some LEDs for the lock status indicators.

Three circuit boards were added for controlling the Efratom LPRO-101 10 MHz rubidium frequency reference.
One is a voltage regulator board based around a Micrel MIC29302, a 78M12, and a 78L05. This takes the incoming +24 VDC and converts it down to +20 VDC, +12 VDC, and +5 VDC, respectively
The middle circuit board contains an optional buffer based around an Avantek GPD-462 amplifier. This buffer probably isn't needed, but it will help to isolate the LPRO-101 from any external devices and will be capable of driving a longer run of coax.
The bottom circuit board contains a 74HCT04 hex inverter to monitor the LPRO-101's BITE signal status. This is used to control the respective LEDs on the front panel.

Alternate view.
The (buffered) 10 MHz output is via a panel-mounted BNC jack.
A panel-mounted RCA jack connects to pin 7 on the LPRO-101, for an optional external tuning circuit

Testing the Efratom LPRO-101 on a low-cost Fluke frequency counter.
It's safe to assume the LPRO-101 is "on" frequency and any observed frequency errors are from the reference clock timebase within the frequency counter itself.
Because the rubidium gas within the LPRO-101's lamp is "consumed" when the unit is operating, there will be a finite lifespan for these units. Use them sporadically, like only as an external reference source for test equipment.
For longer-life operations, stick will quality oven- or temperature-compensated crystal oscillator and use the rubidium reference source to "zero-beat" tune the final output frequency.

Completed Efratom 10 MHz rubidium frequency reference.
Use a Mini-Circuits ZFSC-2-1 (or similar) two-way splitter if you need to drive multiple loads.
This helps to maintain the proper 50 ohm impedance and isolation between all the ports.


