ADF4110 REF keep floating

Please help me how to use ADF4110 in above scenario for better understanding follow the attached schematic

Hi, 

Just to understand the application better. What is the reason for using the ADF4110 as an enable for the 20MHz Xtal? 

Without a reference connected, I don't think the device would be expected to operate correctly. It would simply rail up or down and remain fixed in this state. 

Regards,

Kieran 

I want to drive a MAX2871 frequency synthesizer, But I have to use ADF4110 in such a manner that If no external reference frequency (20 MHz) is available then ADF4110 have to drive the 20 MHz xtal followed by MAX2871, whereas if any external reference frequency (20 MHz) is available at ADF4110 (PIN 8_ REF) then it should again drive the Xtal followed by MAX2871 as previously shown in figure.

Please help me to complete this project. this circuit is useful to provide stable frequency from the vco and good protection for the other circuits. 

I want to drive a MAX2871 frequency synthesizer, But I have to use ADF4110 in such a manner that If no external reference frequency (20 MHz) is available then ADF4110 have to drive the 20 MHz xtal followed by MAX2871, whereas if any external reference frequency (20 MHz) is available at ADF4110 (PIN 8_ REF) then it should again drive the Xtal followed by MAX2871 as previously shown in figure.

Please help me to complete this project. this circuit is useful to provide stable frequency from the vco and good protection for the other circuits.