Now the practical part.
For the test the circuit was simply soldered together, quick and dirty, without a printed circuit board.
U1 and the Pluto-XO are almost pin-compatible. However, the metal housing of Y3 prevents a simple piggyback mounting.
The transistor Q1 is placed upside down, with the emitter at C121. R1 consists of five parallel-connected 1k resistors, because they were just available.
A level of about 6.5dBm is just enough to switch to external clock at the given impedance ratio of the input transformer. At more than 12 dBm, however, U1 is increasingly overdriven.
With this circuit the Pluto was able to operate with external clock frequencies between 20 and 50 MHz. 10 MHz unfortunately did not work.
The currently applied clock frequency (in the example 25 MHz) must be submitted to the Pluto with the following well known commands:
fw_setenv ad936x_ext_refclk "<25000000>"
fw_setenv xo_correction 25000000