The QS1R was the only SDR here at USA-Satcom that was not referenced to an external precise 10MHz timing signal. To remedy this situation a custom 125MHz PLL that uses an external 10MHz timing signal as a reference was added to the QS1R hardware platform. The instuctions from the QS1R website are:
To use the external encode option, disable oscillator U9 by placing a zero ohm jumper at J5. Place a zero ohm jumper at J26 to enable the external clock input. Supply a 0 to 130 MHz clock to J1. The external clock can be sinewave or squarewave drive, 3.3 V pk-pk maximum.
Below are a few images documenting the modification.