Time/Frequency Standard Distribution System

The Extron ADA 3 80 is described in another page on this site (Ref 2).

I had been looking for a suitable enclosure for one of my Trimble Thunderbolt GPS Disciplined Oscillator and its power supply, and the GPS Monitor circuit described in another page (Ref 3).

The Extron ADA 3 80 Distribution Amplifier comes in a relatively large enclosure, which is essentially empty, because all the electronics is on a single PWB along the rear panel. The space left is just enough for a Thunderbolt, it's power supply, a small front-panel mounted PWB and display with some room left over for additional circuitry as required (see below).

This project is not complete, but it is close enough that I wanted to describe it for those who might be interested in doing something similar. I will update this page as I go.

The first step was to remove the strain relief on the power supply output cable. Then I soldered an AC cord directly to the AC terminals inside the power supply so that I would not have to use an IEC connector. I snaked the AC cord through the opening in the rear panel for the previous power cable.

Then I glued the power supply on top of the Thunderbolt and built a spliced wiring harness for the power. The splice is necessary to get the +5 and +/- 12V to the Thunderbolt, and the +12V separately for the display assembly.

Note: if you intend to use an LCD display, you can use the 5V from the power brick to power directly the front panel assembly. However, if you want to use a Vacuum Fluorescent Display like I did, the current draw will be too much for the 5V output, so I used the +12V output driving a linear regulator. The 5V linear regulator is bolted to the baseplate using one of the Thunderbolt mounting screw. The 3 terminal voltage regulator visible on the back of the display board is not used.

An alternative would be to use a small integrated switcher to convert the 12V to 5V with negligible dissipation.

Then the front panel had to be drilled to receive the two BNC connectors and the 9 pin DA-9 connector for the serial port.

Short cables had to be made to connect the two BNC connectors from the Thunderbolt to the front panel.

To Do:

1. At the moment, the distribution amplifier itself is not used. I need to route the power and 10MHz to it.
2. A Frequency Reference distribution system should use transformer isolated outputs to eliminate ground loops. The current metal shell connectors of the Extron ADA 3 80 will be replaced with plastic shell connectors and isolation transformers installed on the back of the PWB. There is not much room, but surface mount low profile 10Base2 or 10BaseT Ethernet transformers should fit.
3. The distribution amplifier was originally designed for video signals, and therefore has much wider bandwidth than necessary for a 10 MHz reference distribution system, which will cause increased noise level. Fortunately, the actual circuit of the Extron Distribution Amplifier is very similar to the one described by Bruce Griffiths in his Timing pages (Ref 4), so it should be sufficient to change the feedback and coupling capacitor values to get pretty close to Bruce's circuit.
4. In my intended application, some outputs will be used for precision timing but others will be used simply for frequency reference for lab equipment, and therefore do not have the same requirement for low noise. It is my intention to optimize one set of outputs for very low noise while the others will remain closer to the original design for simplification.
5. The serial port connector on the front panel is not yet wired.

References

1. Thunderbolt GPS Disciplined Oscillator: www.leapsecond.com/pages/tapr-tbolt/
2. Extron ADA 3 80 Distribution Amplifier: www.ko4bb.com/ham_radio/Extron_3_80/
3. GPS Monitor project: www.ko4bb.com/Timing/GPSMonitor/
4. Bruce Griffith's Distribution Amplifier page: www.ko4bb.com/~bruce/IsolationAmplifiers.html