Regardless of whether the part is used in internal or external LO mode, all synthesizer VCC and GND pins must be connected properly. If VCC and GND pins are not connected on the synthesizer, the internal power-up procedure may not execute properly. Noise may also be coupled into the mixer from the synthesizer circuitry if power and ground are not properly connected.
Place bypass capacitors, whenever possible, to minimize the inductance of the current loop formed by the capacitor and the IC. Placing the bypass capacitors on the same surface as the IC allows one terminal to be connected closely to the IC, minimizing this loop. The ground connection can also be made low-inductance by placing a ground via to a plane layer immediately below. Placing capacitors on the opposite surface substantially limits the effective frequencies they can bypass, since the current must travel through two vias. The loop area formed by placing a capacitor on the opposite surface is almost always larger than the loop area formed by placing a capacitor on the same surface.
Consider the path that ground currents will take. For optimal performance, supply and bypass capacitor currents must not flow underneath high-frequency traces.
Use as many ground vias as possible to connect the IC pad to the ground plane. This is required for optimal thermal performance.
Connect ground pins back to the pad. Aside from routing convenience, the inductance through the bond wires tends to be very high, and the inductance through the ground pad tends to be very low.
Avoid connecting different VCC pins in such a way that the current paths overlap. Overlapping current paths can inject common-mode noise into the supply pins, degrading performance.