Paste, place and reflow all of the components on the board.

It may seem a bit harsh for that to be one instruction step, but there's really nothing particularly special about the process for this board. As always, make sure the polarized components are placed the correct way 'round.

Install 2 JST XH headers in the output port positions.

If desired, install a mini-DIN 6 connector in the diagnostic port. One pin is ground, one is a copy of the PPS signal from the GPS module, one is the transmit data pin of the GPS module, one the the receive data pin of the GPS module, one the diagnostic serial transmit pin of the controller, and the last 2 are 5 volts and ground. You can send commands to the GPS module if you like, but altering the NMEA message structure or timing may cause the firmware to malfunction. The firmware makes no effort to update the UTC reference year, so the date stamps in the diagnosis log may wrap at some point in the future. This should not have any impact 

For the FE-5680A board, add a DB9 female connector with PC board pins. It mounts on the edge of the board with the 5 top pins soldered on top and the 4 bottom pins soldered on the bottom. Bend the pins together slightly to make the mechanical fit better. Solder one pin and then examine the jack from the side to make sure it's straight before soldering the rest of the pins.

Using a pogo pin adapter and a PDI programmer, program the controller. Be sure your programmer operates at 3.3v. The XMega32E5 controller is not 5v tolerant.

There are no fuses to set on the XMega controller. Just upload the code to flash.

For the FE-5680A variant, connect the oscillator to the board first.

Apply 5 VDC to the 2.1mm barrel jack (center positive) for the OCXO/TCXO variants. For the FE-5680A variant, the power must between 17 and 24 VDC at 30W+.

You should see the FIX LED turn on and the 0 and 1 LED blink back and forth. This will continue until the GPS receiver obtains a 3D fix. When this happens, the FIX LED will transition to blinking at 1/2 Hz and the 0 and 1 LEDs will go out. For the FE-5680A variant, 0 and 1 will continue to blink back and forth until the oscillator achieves and indicates a physics lock. When the unit is cold, this may take up to 15 minutes.

At this point, it may take the unit anywhere from the better part of an hour to several hours to obtain and keep a lock. How long it takes depends on how long the unit has been powered off and how old it is. As it steers the oscillator, you will see the 0 and 1 LED show various states. The two LEDs form a binary number, and from 0 to 2 they represent the coarse tuning mode, the "fast" PLL mode, and the "slow" PLL mode, respectively. The OCXO and FE-5680A variants have an additional "3" mode, which is the final "long" TC PLL. Higher numbers are desirable. In general, after first power-up, it will take at least a couple days for the (non-5680A) oscillator to become maximally stable.

In general, if any of the mode lights are on, you can count on the frequency being fairly close. Mode 0 is an FLL intended only to get the frequency close enough for the PLL to be able to maintain a lock. Modes 1, 2 and (if applicable) 3 represent varying time constants for the PLL. Shorter time constants (lower modes) mean that the frequency will be adjusted more quickly in the face of phase shifts. Longer time constants keep a looser rein on the frequency, but the result of that can be somewhat more phase drift before correction. In general, the result of that is better frequency stability at higher modes, but you can probably still count on the frequency being fairly accurate at lower modes. Unless the oscillator is disturbed, it should maintain the highest mode. "Disturbed" can mean any of

• GPS reception outage
• Physical disturbance (being moved or knocked around)
• Load changes (adding or removing output load)
• Input power voltage changes (of sufficient magnitude)

If any of those happen, you should allow the unit to settle for a few minutes. If the unit decrements the mode, then that's a clue that it's having to recover phase control.

One output is a 5v 50% duty cycle square wave at 10 MHz. The other is a 10 dBm sine wave. Both outputs are 50Ω.

To use the clock to drive a microcontroller on another board, try using a 10 pF cap in series, followed by a self-biased inverter (that is, an ordinary CMOS inverter with a 1MΩ resistor between the input and output). The output of that can be used to feed any desired logic. The self-biased inverter can be fed from either the square or sine output.