This is a brief description of a modification made to a 
PLX31-08 "brick" oscillator to use an external reference input 
rather than the internal crystal oscillator of the brick.

What formerly was the reference monitor BNC connector on the 
end of the brick, now becomes the ~100 MHz reference input.

The details are mostly in two pictures:
Ext-Ref-Mod.jpg   --  a labeled picture of the modified board
Ref-Osc-circ.png  --  a schematic of the oscillator and mod

Steps
------

1) Open the back of the brick (4 screws).

2) Unsolder and remove the existing coax to the monitor BNC.

3) Unsolder the orange -19 V wire from the large resistor to 
the right of the crystal oven. Insulate the end of the wire and
tuck it to the side of the case. This disables the crystal 
heater to save a little power.

4) Lift one end of the 100 ohm resistor that runs vertically 
and is just to the left of the Crystal oven. This removes DC 
power to the crystal oscillator transistor.

5) Remove the 100 ohm resistor that runs horizontally, to the 
right of the oven and just below where the orange wire was 
removed in step 3. You should be able to do this in small 
steps of heating and lifting, from the top, without pulling 
the circuit board. This resistor was the path for the reference 
signal. The right-end pad becomes the input for the external 
reference. It is point 'A' in the schematic.

6) You need to build the input circuit shown at the top of the 
Ref-Osc-circ.png schematic. I soldered the capacitor to right-end 
pad where the 100 ohm resistor was removed (point 'A'). I used 
1000 pF, but it is not critical. Any "medium sized" value with 
low Z at 100 MHz is fine. The 62 ohm resistor is to provide 
loading in the vicinity of 50 ohms for the external ref. Not 
critical; higher value or no resistor would probably work. I 
got ground for this resistor and the coax shield by soldering 
to the ground plane of the board, just below where the 100 ohm 
resistor was removed.

The more detailed picture Mod-detail.jpg may give a clearer 
view of these modifcations.

7) Cut and route a coax from this new input circuit to the BNC. 
You could probably adapt the original monitor cable, but I 
made my own new cable.

8) I used a 106.5 MHz input to get 10224 MHz out of the brick. 
I think this reference was around 0 - 10 dBm. If your brick was 
designed for the 10 GHz range, then you may not need to tune 
the multiplier section filter.