I made a Symmetricom X72 breakout board - Page 1
Forum section: electronics / general

author: danderson
author_rank: Contributor
timestamp: May 25, 2025, 08:22:49 pm
content: Many years ago I picked up a Symmetricom X72 rubidium frequency standard off ebay, and then didn't do anything with it. I dug it up recently, and learned that Molex discontinued the connector the X72 uses for I/O. So, I made myself a breakout board to pull the timing signals out to SMA connectors, and the power and digital signals to a standard 2.54mm header.This isn't novel in itself,https://www.eevblog.com/forum/metrology/symmetricom-x72-(and-other-rubidium-oscillator)-signal-breakout-boards/msg1369137/did this many years ago. But, I missed that run of boards, and was looking for an excuse to do some PCB design, and so I made my own. I also open-sourced the design, so that any future X72 adventurers don't end up kicking themselves that they missed a one-time buy:https://codeberg.org/danderson/symmetricom-adapterHere's the breakout board in situ, with my X72 solidly bolted to it. To my great relief it works great, with nice clean waveforms at the expected frequencies. Amusingly my scope's frequency measurement reports a standard deviation of a few kHz on the 10MHz sine wave... Because as the X72's manual points out with a slightly exasperated tone, the frequency standard output is so stable that I am in fact measuring the phase noise of my scope's internal frequency referenceThe Molex connector they used seems to be a mild variation on the popular micro-ribbon aka "Centronics" connectors of the halcyon days of parallel SCSI and printer ports, but with slightly different contact spacing just to make sure no common connector can work with it. Thankfully, the geometry is such that a 1mm thick PCB with a card edge connector fits just about perfectly.Initially I built a single-board breakout that plugged directly into the X72's port, and had all the RF and 2.54mm headers... Until someone pointed out that putting all the mechanical strain of cables pulling and wrenches torquing on a teeny 1mm board that's also trying to remain still and provide a solid, short-free connection to a precious oscillator... maybe not the best idea.So I ended up with this rev2, in which the X72 is rigidly mounted to a standard 1.6mm thickness PCB (with 4xM3 screws on the underside), and a 1mm thick mezzanine card is stacked at the right altitude to connect to the module's I/O port. From there, the signals are pulled down to the mainboard, and routed out to the various connectors. As a bonus, I stuck a couple logic gates and a flop in there to provide some handy status LEDs: whether the module is locked onto frequency, an early warning that it's approaching the limits of its tracking ability and is likely to fail soon, and a blinkenlight for the 1pps output (it toggles on every 1pps pulse for simplicity, so I suppose technically it's a DDR 1pps display).This is my first foray into "medium-speed" digital design, with noise-sensitive signals in the double-digit MHz range and a 4ns rise time on the 1pps output. Very leisurely by professional standards, but an opportunity for me to learn. Thankfully the board doesn't carry much in the way of signals, so both inner layers provide completely continuous ground reference planes for a nice simple field coupling story. I also went a bit overboard with the via stitching around the timing signal traces, according to the rules of thumb my stitch spacing form a waveguide up to a little over 5GHz, when the fastest edges on the board translate to around 88MHz (Great Scott!). But the fab pricing included a fairly large number of via holes per square centimeter, so I took them up on it. As a reviewer of the design said, nobody's ever sent a board back complaining that there's not enough EMI!I did make a couple mistakes in this first run: it turns out NAND and NOR have different truth tables, so the health LED illuminates in bright angry red to let you know that everything is fine, oops. I also got the wrong size of stacking connector when I asked Samtec for a freebie, so the mezzanine card is hanging in the air by its solder joints instead of sitting on the plastic stacking standoff, which is half a millimeter too low. Doesn't sound like much, but it puts an uncomfortable amount of strain on the X72's connector.I already have enough interest from other Symmetricom X72 owners to do a rev3 production run, with those errata fixed and a couple additional tweaks incorporated: screw-down terminals for the 24V power input (currently routed to the 2.54mm 2x4 header), an onboard 3.3V LDO to drive the blinkenlights (currently has to be supplied externally, on the same 2x4), and a few more ground pins on the 2x4 connector (currently has a single one, making it a pain in the ass to hook up supplies and other test gear while referencing them all to the same ground).If you own an X72 and would like one of the rev3 boards, let me know and I can add more units to the order. Cost is $65 (USD), plus shipping to wherever you are, from western Canada. Any import fees and the like that your country charges are on you. I expect to place the order in a week or two. And of course, the design being open source, you're welcome to produce your own boards directly if you prefer, in accordance with the license terms. The readme athttps://codeberg.org/danderson/symmetricom-adapterhas some fabrication notes, in particular BOM sourcing info and a fab house agnostic rundown of the expected board stackup. Also note that the current design in that repository doesn't yet incorporate all the rev3 changes I mentioned above, I'm still working on it.