The LMX
2592 requires an external loop
filter that is application-specific and can be configured by the
PLLatinum™ simulation tool found
here. For the LMX
2592, it matters what impedance is seen from the
Vtune pin looking outwards. This impedance is dominated by the component C3 for a third
order filter or C1 for a second order filter. If there is at least 3.3 nF for the
capacitance that is shunt with this pin, the VCO phase noise will be close to the best
it can be. If there is less, the VCO phase noise in the 100-kHz to 1-MHz region will
degrade. This capacitor should be placed close to the Vtune pin.