Saturday, September 19, 2015

Custom PCB for KS-24361 REF-0 Standalone Operation

In this post I will describe a custom PCB I designed to enable standalone operation of the Lucent KS-24361 REF-0.

My custom board to enable REF-0 standalone operation.


In my last post on the KS-24361, I described a prototype of a GPS retrofit board I made for the REF-0. I will eventually turn that into a proper PCB with a u-blox GPS onboard. However, I decided to make an intermediate board that does not have the GPS module. It merely contains a microcontroller and some miscellaneous circuitry. This board has allowed me to test dimensions, footprints, and a host of other things that will aid development of future boards.

The Design

Even though this will not be the final board design, I still wanted it to be useful. It has an ATTINY85 microcontroller for sending the serial strings to the REF-0 and synchronizing to the PPS signal. It also contains inverters and buffers. The serial transmissions to the REF-0 have to be inverted, so that is taken care of automatically. Also, the board will accept either polarity of PPS signal, at 3.3-5V. Selecting if the PPS to the REF-0 is inverted or just buffered is handled with solder jumpers that can be easily changed. The setting for the PPS polarity to the microcontroller can also be independently set. Finally, I put a 3.3V regulator on the board to accommodate lower voltage GPS modules. The board has a 5V hookup on the header as well.

The board mounts neatly using two mounting holes that would normally support an Oncore GPS in a REF-1. I described in my previous post how to solder some jumpers to the REF-0 mainboard so that nothing needs to be connected externally (accept for the antenna if you are using GPS). The PPS signal can be connected via a 0.1" header pin or the SMA connector. There is a spot for a termination resistor (R2) on the incoming PPS line if needed. Overall, the board is very flexible.

Potential Uses

There are several ways this board could be used. As it takes care of the serial strings, the only thing that needs to be provided to make the REF-0 hum is the PPS signal. The most likely source is GPS, and a module could be installed inside or outside of the case. But GPS is not the only option. The REF-0 could potentially be used as a clean-up oscillator for a Rubidium standard. This board would handle that quite easily.

I decided to go with GPS in one of my REF-0s and mounted a u-blox NEO-6M module inside the case. This little GPS board can be had for $12 to $15 USD online; it's a real bargain. I arranged the two boards in a stacked configuration as shown, and kept the wires as neat as possible.

My custom board and a u-blox NEO-6M make the REF-0 operate quite happily. The messy jumper wires will be completely eliminated in the next board design.

The only connection that needed to be made externally was for the GPS antenna. I used a uFL to SMA pigtail to accomplish this. The SMA bulkhead connector mounted nicely in one of the slots in the cover. It required a tiny amount of filing to fit it through, but by opening up the slot just enough it locked into place quite securely. I think I will use this approach in the future.

Here you can see the SMA bulkhead connector mounted nicely at the rear of the REF-0, and the pigtail connecting to the u-blox GPS module inside.

Wrap Up

I'm very happy with how this little board turned out. As I have time over the next few weeks (engineering students have nothing but free time of course...) I will merge this design with a u-blox GPS layout I have already completed and take this project to the next step. Gradually this is moving toward a single-board solution that will have a very professional appearance when installed, and a high degree of performance and reliability.

I don't know if this will ever turn into a kit. Currently my goal is to document what I am doing so that others can take it as a starting point and go even further. If I eventually get to a board design that I am happy with, I may consider figuring out a way to make it available to others.

Thanks for reading!

- Dan W.