Electronic – Top Ground Copper pour in PCB having three RF Sections

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I am designing my second RF board (First one had BLE). This board has LoRa Components (SX1257 and SX1301), GPS (MAX-6) and 4G LTE Module (SIM7600). All the components are on top layer (red layer).

My STACKUP is:
Layer 1 (Red): Signal layer with Ground copper pour
Layer 2: Ground Layer
Layer 3: Power and Ground Layer (There are multiple sections for 3V3, 1V8, 4V and the section just below RF traces is Ground)
Layer 4 (Blue): Mostly Ground having few signals.

Below is the picture of Layout:
enter image description here

My question is I can pour ground copper on my first layer with via fencing and stitching at a distance of wavelength/20. Should I keep the ground pour separate for LoRa, GPS and LTE module? They will be connected to each other via layer 2. Or I can have a big copper pour covering the top layer as distance with three RF section is sufficient.

Best Answer

My question is I can pour ground copper on my first layer with via fencing and stitching at a distance of wavelength/20. Should I keep the ground pour separate for LoRa, GPS and LTE module?

There are a few problems with separating ground planes:

Return currents on the ground plane have to travel back to the source via a different pathway (usually). If the rue is a high frequency current, if a trace crosses a slot in the ground plane this can add inductance to the transmission line or trace (so don't cross over the boundary of a ground plane with a high frequency trace or you'll suffer the consequences)

The separated ground planes make great dipole antennas. This is usually not a super big deal, but it can be if your trying to pass FCC testing, and you have a radiator that you didn't account for.

Seperated ground planes can also add inductance if the portion connecting them is small or through a via and trace on another layer (a bad idea in almost all cases).

If you need isolation or if parts require a separated ground plane (I've seen one GPS module that recommended a small one), then go for it and separate the planes. Otherwise a continuous ground plane is a good idea.

There has been a few times where I have used a slot to re route large return currents away from sensitive analog electronics, but that may be the only time I've found separating a ground to be beneficial (other than isolation)