Datum/Symmetricom TS2100-IRIG OCXO & Heol N024 GPS Upgrade
Our story begins as a tale of two oscillators, and eventually involves a GPS module, and some miscellaneous hardware:
A couple of years ago, I was able to acquire a TS2100-IRIG for under $100 on eBay. While an older time server (the first units shipped in 1996), it could still be useful to me at home, particularly since I already had a Exactime 6600 and IRIG distribution amp providing time to my home office.
Then, last year, a couple of things happened:
A lot of new old stock OEM Symmetricom MTI OCXOs for this unit appeared on eBay for around $40, and I picked one up.
The factory original Trimble ACE III GPS units hit their rollover limits, and warped back to 1996. As a result of this, the folks at Heol in France started to produce a version of their N024 GPS module for the TS2100. This was great news, since I’d been unable to find an ACE III, and was wary of spending too much on a 20 year old GPS module with the knowledge that many of them would experience rollover problems after 19 years.
Based on some discussions I’d seen before on the time-nuts mailing list, I knew that it was relatively easy to upgrade the TCXO equipped units to OCXO, and that it was even easier to install a GPS module. So I broke down and ordered one of the Heol N024-TS modules.
And then...it all sat for several months until I had an hour or two to throw at this project:
Unfortunately, I forgot to take a photo of the entire stock PCB out of the chassis before I started making changes.
Materials used:
Symmetricom BC11736-100 OCXO (MTI 240-0530-D)
Heol N024-TS GPS Module
Gold plated, bulkhead female SMA connector
Right angle SMB plug to male SMA pigtail (4-6″ would be optimal, this one is too long. I’ve ordered a shorter replacement.)
4 #2-56 1/2″ screws, 8 #2 flat zinc washers, 4 #2 serrated lock washers.
A 0.1″ jumper to set JP4 and enable writes to the EEPROM.
2 PCB mount SMB jacks (these are optional, but I decided to install them for later experimentation since I had 100 on hand.)
An OCXO sized square of Kapton® tape. (Stolen from my 3D printer supplies.)
Solder, soldering supplies, desoldering braid and vacuum tools, 99% isopropyl alcohol and Q-tips.
Step 1: Remove the old TCXO
Since everyone has their own techniques and preferences, desoldering is left as an exercise to the reader:
Step 2: Remove the Kapton® tape underneath the TCXO:
Step 3: Prepare the U17 outline:
Desolder the bridges over the pads for the new OCXO (the U17 outline is the one we need for the MTI 240), then clean the flux off the board, and clean any residue, adhesive, or finger oils off the top of the board for the U17 footprint. Install the new square of Kapton® tape, and punch holes through it for the OCXO’s pins:
Step 4: Install the new OCXO & optional SMB jacks
Install, solder, and trim the leads of the new OCXO, and then, optionally, the SMB jacks adjacent to the footprint (I suspect these are the oscillator output, and EFC input, so they might make good test points later.):
Step 5: Reinstall the PCB into the chassis.
When installing the PCB, secure it properly using all nuts, screws, and make sure the flat washers are beneath the lock washers, to avoid damaging the PCB.
Step 6: Install the GPS:
Install the Heol N024-TS GPS module on the ACE III header, using the #2-56 1/2″ screws and washers (to keep the header well connected and not damage the PCB, use no washer underneath the board, and a stack consisting of flat washer/star lock washer/flat washer on top of the module):
Step 7: Install J6
J6-ANT is an SMA-sized hole on the rear panel. Install the bulkhead female SMA connector here, and then use the pigtail to connect it to the GPS module. The cable routing pictured here is circuitous since I didn’t have a shorter SMB/SMA pigtail on hand (now on order.):
This completes the hardware work, to reconfigure the unit, we’ll need that jumper installed on JP4, and a serial adapter plugged into serial port A to make the necessary adjustments.
Step 8: EEPROM changes.
Enter the "root eng ee info" command, and note the hex value it returns (00000000 on my TS2100-IRIG.) TS2100-GPS (TCXO) units are reported to return 00000024, so we'll set that here via "root eng ee info 0x00000024"
Also change the default gain from “2″ to “-20″ to pad the output of the OCXO, which is much hotter than the VCXO the unit shipped with, and don’t forget to set gain in the timing menu as well.
An item for further research are what all of the bits in the 'info' field in the EEPROM are, at least a couple should indicatate oscillator type, and for holdover NTP purposes it would be nice to get this set correctly.
Digging around in the timing/utils directory, my factory TCXO equipped unit reported the following:
tfp 0 = d/a = 0x01f4 tfp 5 = gain = 2 tfp 6 = KM = 0.9499970 tfp 7 = KO = 0.9499970
I also reset the filter setting to match parameters given on time-nuts for the MTI OCXO.
At this point, we have a happy little TS-2100, syncing to GPS, and reporting lock:
In a future post, after I've had some time to test this unit, I may adjust some of these parameters once I've figured out a good offset for TOD accuracy, as well as what the info bits mean.
Acknowledgements:
Greg Dowd & Jason Rabel posted some very helpful things to the time-nuts list in 2011. I'd also like to thank Olivier and the rest of the staff at Heol for producing a module to help us all keep these running.
