Please look at the AD9520 evaluation board schematic. C41 and C42 will be removed and R35 and R37 will be placed to use the external passive XTAL. The values of C4 and C3 are dependent on the crystal used.
If you are looking to use a crystal with a driver (XO) then you terminate the output based on the driver structure and ac couple in to our receiver. If it is a 3.3V CMOS driver, the output can be dc coupled.
Have you looked at the evaluation board user guide?
If you are using software version 2.1.x and the evaluation board is not being recognized by the software, you may have to burn the evaluation board EEPROM by following the instructions located in the evaluation software tools section of the evaluation board page.
A square wave is expected at 50MHz. A CMOS output driver is high impedance, so you want to terminate into a high impedance load. If you make the frequency lower does the waveform become more square? Keep in mind that your oscilloscope must have 3-5x the bandwidth of your signal to be able to sample the necessary harmonics and reconstruct the square waveform.
Retrieving data ...