My Racal-Dana 1992 with the OCXO upgrade installed.
Introduction
I have always been interested in the Racal-Dana 199x series of frequency counters. Though I have plenty of counters already, I was on the lookout for one at a good price to add to my lab. Part of my interest stemmed from the fact that the timebase in the counter is very easy to upgrade, so finding one with the OCXO option was not necessary. Several weeks ago I picked up a model 1992 counter in good shape. It came with the standard XO reference as expected, so I set about designing a small upgrade board.
Gerry Sweeney has some excellent videos on his Youtube channel documenting a very similar project. He created an OCXO upgrade board for his own counter on perf-board that seemed to work well. He also showed how to replace the buttons on the front panel of the counter, which are known to wear out. Thankfully, the buttons on my counter are fully functional (for now).
Check out Gerry's excellent videos on his Racal-Dana 1999 counter:
OCXO Upgrade
Gerry presented a circuit schematic for his upgrade that was very straight forward. For my own board, I used the same LM4140 voltage reference. However, for the control voltage adjustment, I used two resistors and a 20 turn potentiometer in series instead of a single pot. With the values I picked, I have an adjustment range from 0.6V to 3.5V. This works fine for the OCXO I chose, an NDK ENE3311 (datasheet for a similar unit).
The header in the counter where the timebase board mounts is a five pin, 0.1" rectangular male connector with one pin missing. It gives you 5V and Ground and gets 10 MHz back. What could be simpler? I found the mating 5pin connector on Digikey for my board to exactly replicate the original connection.
The 5V rail in my counter is very clean, so I didn't feel it was necessary to do a lot of filtering.
The 5V rail in my counter is very clean, so I didn't feel it was necessary to do a lot of filtering.
Here's what I came up with:
OCXO Upgrade board schematic.
OCXO side of the upgrade board.
Other side of the upgrade board.
The upgrade mounts exactly like the original timebase, and the trimmer is accessible
through the hole on the rear panel.
Assemble Your Own
If you'd like to assemble one of these boards for your own counter, I have shared the PCB on OSHPark. At current prices, it only costs $16.25 for three copies of the bare board.
Most of the parts can be purchased on Digikey or your favorite distributor. For the OCXO, your best bet is to find a surplus unit on eBay. Purchasing a factory new OCXO will increase the cost of the upgrade by...a lot.
Parts List:
- OCXO: 10MHz 5V, Square Wave out, 1"x1" standard footprint (example listing on eBay)
- U1: Texas Instruments LM4140 4.096V VREF SO8 (Digikey listing)
- R1,R2: 1k ohm, 0603, 50ppm/*C or better preferred
- R3: 1 ohm, 0603
- VR1: 5k ohm, 20 turn potentiometer, 50ppm/*C or better preferred
- C1: 47uF tantalum, 1411, 10V rated minimum
- C2: 100nF, 0603
- C3, C4: 1uF, 0603
- C5: 10nF, 0603
- Connector: 5 pin 0.1" SIL Horizontal (Digikey listing)
Note: R1, R2, and VR1 values may be adjusted to customize the divider as you like. Using series resistors reduces the adjustment range (giving you finer adjustments) at a degraded temperature coefficient. If you want to use only a potentiometer, place 0 ohm jumpers in the R1 and R2 positions.
A special note about the 5 pin connector: It can be put on backwards! Make sure to test which direction faces down to mate with the pins in the counter before soldering it to the board. If it is installed upside down, the pins will not make solid contact with the header.
Watch the notes on the silkscreen. The OCXO installs on one side, the connector and trimmer install on the opposite side.
TCXO Version?
I was also curious how a TCXO version of this board would perform, so I designed one. Here it is next to the original timebase from the counter.
My own TCXO upgrade board next to the original timebase.
The TCXO performs fairly well. It swings about +/- 30 ppb over a typical warmup cycle. The counter is immediately ready to use from a cold start if I accept that uncertainty. Also, power consumption is low (about 50mW versus 1W for the OCXO). That might be handy if I ran the counter from batteries. (Does anyone actually do that?)
Now the downsides. This TCXO version of the board is actually more expensive than the OCXO version! I had to pop for a really nice $23 VCTCXO to get this level of performance (Connor-Winfield M100V-010.0M). Also, the OCXO I used warms up in less than 3 minutes to an equal if not better level of performance than the TCXO. And of course for long-term stability, and stability over wide temperature ranges, there is no comparison at all. The OCXO wins. Even at the 9 digit resolution of the counter it is noticeable.
So there's really no sense in continuing to explore the TCXO version. Ah well, it is still a nice little board. Maybe I'll find some use for it in the future.
Conclusion
I'm quite happy to have my upgraded Racal-Dana 1992 on the bench. In total, the OCXO upgrade only cost about $26 for a single unit. Though this particular project is wrapped up, I am now working on a similar upgrade for the Fluke/Philips 66xx series of counters.
Good luck if you decide to order PCBs and assemble your own board. Let me know if you need any further details in the comments below. Also, please post any comments or corrections.
Thanks for reading!
- Dan W.
More outstanding work, Dan. Well done.
ReplyDeleteLovely work Dan, makes me wish I had a need for such an accurate timebase.
ReplyDeleteAs a relative novice at board design, I am intrigued by the cut-out you have made around the VREF. What is the thinking here, thermal isolation?
Hi, it's also for mechanical isolation of the IC. Flexing of the package can cause output shifts. On a board this small with the IC in the corner it's not entirely necessary, but I had space so I did it. Performance of the board overall is great so far.
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