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LMX2572LP: LMX2572LP ground planes

Part Number: LMX2572LP
Other Parts Discussed in Thread: LMX2572

Hi,

I am considering the eventual PCB design of an RF circuit employing the LMX2572LP PLL.

The datasheet specifies 3 types of ground pins, the VCO Ground, the Digital Ground, and the Charge Pump Ground.

The stack usually used has Layer-2 as the RF and system ground plane, with all various types of grounds eventually providing current return paths to this large copper ground plane on the PCB.

There are a few local ANALOG ground planes on other layers with a single return path to the main system/RF ground plane on layer-2 as mentioned, but these are mainly for audio components and amplifiers etc.

With regards to the LMX2572 PLL, to what extent should one discern between the PLL VCO and Charge Pump Grounds and the system/RF ground (including a few Digital Ground pins) ? Is it recommended to have a specific localised Charge-Pump return ground for passive components forming the Loop filter around pins 11, 12 & 13 ? Equally for the pins 34, 35,& 36 should I dedicate some type of isolated ground with a single return path for currents to the main system/RF ground, or is it adequate to simply tie all grounds to the main system/RF ground plane on Layer -2 of the board with multiple micro-vias (this layer being the one otherwise serving as the main ground plane for all microstrip transmission lines that will be traced on the board) ?

Thanks in advance for your advice.

Regards, Mike