Quite a few years ago now I joined the “time-nuts” mailing list – a “Time Nut” as the gentle reader may be aware is an individual who tinkers with precision time keeping and/or precision frequency references, measurements and the like for hobby interest. It’s a great list, always an interesting discussion and very high SNR indeed – fun place to virtually hang out.
Through this I became interested in playing around with some precision references at home and this in turn lead to me picking up a couple of Rubidium frequency standards off eBay from a local seller – one known working, the other known faulty. These devices are usually ex-mobile phone base station installations where they’re commonly used to provide a precision 10MHz reference signal to drive the timing circuitry of the base station itself. They’re usually a secondary source, the primary being a GPS Disciplined Oscillator (GPSDO) – the Rb standard is a hot spare in case the GPSDO goes out of lock for too long.
The Efratom SLCR-101
The SLCR-101 is made by US firm Efratom (now part of Symmetricom by way of Datum best I can tell) and is designed for OEM applications – so it’s basically a bare module that you feed power into and it gives you a nice clean stable 10MHz sinewave output as well as some status signals. I’ve been unable to find any specific data on the SLCR-101 but it seems very similar indeed to the LPRO or LPRO-101 units – just lower profile – about 25mm/1″ high versus the LPROs 38mm/1.5″ height.
A bit of searching turns up a PDF of the LPRO-101 “User’s Guide and Integration Guidelines”. Another document to seek out is Fred de Vries “Efratom LPRO-101 Repair Reference Guide” The most recent revision of this excellent reference seems to be Revision 7, January 2011 – Fred kindly sent me an email with the latest version of his guide which, with his permission, I’ve placed here. I am trying to find a definitive upstream source for the offical LPRO document, for now a little googleFu will get you there.
One immediate take home from these documents is that these units must not be operated for extended periods (say more than a half hour) without a proper heatsink (less than 2C/W thermal resistance to ambient) – the Rubidium lamp inside runs at around 100C so good heatsinking is required to ensure the rest of the electronics is kept within its safe operating temperature.
Removing the cover wasn’t difficult – I’ve a bunch of photos shared here but in particular this one shows a unit mounted on a heatsink and this one how once the connector assembly is removed (undo screws, break the slight sealing and pull straight out) With the connector removed, the cover can simply be eased up with a flat blade working around the perimeter of the casing. Note that the unit will not operate properly without the Rubidium lamp assembly being shielded from AC lighting. Perhaps also worth noting that you don’t need to disassemble the unit unless it’s faulty or you’re curious :)
The internals are well covered in Fred de Vries document, but by way of a quick “cooks tour”, referring to this photo; The rubidium lamp assembly is the machined section top left – lamp in the brass coloured section, photodiode etc. in the right hand side. Slots in the PCB are to accommodate a shielded section that fits over the lamp as visible in this picture. The cylindrical port in the top left of the shield photo is used to gain access to the frequency trimpot on the control PCB (blue, bottom right in this shot).
Will have more to write on my tinkering with these units in subsequent posts – for now hopefully the little note about how to remove the connector will save someone some time :)
Edited 20170423 to include link to the repair guide kindly provided by Fred de Vries.