Locking the Adalm Pluto to an external reference (Part 1)

 Having finished boxing up the Langstone Transceiver  which is based on an Adalm Pluto SDR, the next step was to improve frequency stability.

Some have tried changing the reference 40 MHz oscillator to a more stable device. The preferred device is very small and i can imagine the damage caused by taking out the old oscillator.

Others have tried feeding in an external 40MHz GPS locked signal. One discovery from here was that the existing oscillator has an enable/disable pin so it can be left in place when an external signal is injected. This is thr chosen route

Note that C123 and C124 form a potential divider to reduce the output level of the oscillator. However C124 is not fitted but it does provide solder pads to mount another 180pf 0603 capacitor (I am not brave enough to try an 0402 size!) through which to inject the external reference

This is how mine ended up:-

Note that the braid of the coax cable through which the reference is injected is soldered to the groundplane in two positions. This is to prevent the 0603 capacitor being ripped off the board if the cable moves.