What are we talking about? Today I’ll talk about this magnificent little device found on eBay : the Efratom M-100.First of all, what is a Rubidium Frequency Standard?
It’s an atomic clock.Just in the 80-90 they were such an expansive units! The working mechanism is fascinating. It uses the hyperfine transition of electrons in rubidium-87 atoms to control an output frequency of (almost) perfect 10 Mhz. Of course it’s less accurate than a cesium one and they are normally used in TV station, cell phones stations, they are present inside the GPS satellites too. Efratom made two different variants of this model : the FRK-L , the commercial one and the M-100 , the military one. I’ve found this model on internet but you can easly find cheaper frequency standards like the FE-5680A that is widely used in large gsm repeaters to synchronize their frequencies to an almost perfect frequency standard.( +/-5 x 10 ^-11 ) In short, how it works?
“A 10 MHz quartz crystal oscillator is “disciplined” to the rubidium hyper-fine transition of 6 834 682 610.904 324 Hz. 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 synthesizer (referenced to the crystal) through the transition frequency. When lock is achieved, the crystal oscillator’s frequency is exactly 10 MHz. ” .So first since i have to test this unit, I’ve found on internet a good aluminium heat-sink. Yes because the flat aluminium size with rubidium will arrive at almost 40-50 ° C temperature and on manual it says not to exceed the operating limit of 68 ° C. Fortunately, with less than 10 euro I solved it with a perfect-size aluminium heat sink!
The M-100 needs a current from 22.5 to 32 VDC (I’ve used 26v 3 A with my bench power supply ) and normally less than 10 minutes to reach the atomic “Lock” of the 10 MHz perfect sine wave. It has a Winchester connector in the front with some pins . These are most important connectors:
There are two little screw in the front panel . The lower one on the right of J2 connector is the frequency Adjustment screw, the other one on the top left is the gigatrim centering on the range of the oscillator coupling. Unfortunately for now I’ve just tested it with my oscilloscope that is telling me is on the correct frequency of 10.000 MHz, BUT here we should have a precision of 2 x -^11 ….much much more than a normal quartz. So just for now I’ve checked the operational temperature. I’ve used a TMP36 sensor with an Arduino UNO, i’ve modified the temperature program code adding 35 (3.5 in the conversion formula from 10mv per degree with 500mv offset , you can find it here http://sidmonitor.rf.gd/gallery/temperaturefinalEXT.ino ) to fit external circuit from the breadboard (in the lower side of the rubidium to check if with the heat sink works. It does not exceeded the 40* C. (ext) .
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What is the initial current for the oscillator? I’ve got one of these units that I’ve just fired up after many years and it is current limiting my supply (that’s limited to 1.5A). The manual says 10 W that implies < 0.5A but perhaps it exceeds this at start-up. I see you use a 3A supply. Before I open it up, I want to check if its my setup that's wrong. (currently I don't have a 28V supply that's good for 3A)