This is  an injection locked PLL for use with the RX888 v2.  It is one that I think has some advantages over simple injection of an external clock. It provides  automatic switching between undisciplined/internal and disciplined/external reference operation,  can provide sub-ppb frequency accuracy and low spurious and spectral noise power density clocks forthe ADC. It has an advantage that even if there is an interrupted connection with an external disciplining source, perhaps because of a cable or a frequency change, the RX888 will NOT lose a clock which can cause a host SDR program to crash and require complete restart.  The automatic switching within this PLL will simply cause the free-running but settable frequency of its VCXO to be used until the break is restored.

It is in the form of an add-on PCB that can mount inside an RX888  as shown below.  It uses  a Type I PLL and replacement VCXO to replace the OEM one. 

[Caution]    Many installations will require internal soldering probably  in an SMD environment.

This may beyond what many should choose to attempt !

In the RX888 it is possible to install it without any soldering but does require connecting a u.fl cable to the connector on the RX888 PCB. This itself may be difficult, u.fl connectors can be tricky. It receives 3.3V power from the 3 pin jumper/header inside the Rx888. The jumper is replaced with a connector that provides a single connection the Vcc on the new PCB.