Circuit design

The microwave transmitter consists of following blocks:

Board layout

Layer stackup

Two-layer PCB won't be enought for the circuit of this complexity. Luckily, OSH Park where I plan to do the fabrication, offers 4 layer PCBs with the following characteristics:

1.4mil1oz Cusignaling and powertop layer (L1)
0.7mil0.5oz Cusolid signal ground
1st inner layer (L2)
0.7mil0.5oz Cusignaling,coupler and 3V3 power plane2nd inner layer (L3)
1.4mil1oz Cupower plane / groundbottom layer (L4)

Both core and prepreg are FR408, with a dielectric constant of 3.66 at 1GHz. Total board thickness is 1.6mm.


The most sensitive part of PCB layout are microwave traces. These need to be routed with special care and well shielded. I decided to go for the so called planar waveguide topology on the top layer and used Saturn PCB calculator for dimensioning. Knowing that layer thickness between the L1 and L2 (GND plane) is 6.7mil, to get 50 Ω line I'd need 14mil wide conductor with 10mil gap. It is important to have solid noninterrupted return (GND) plane beneath the RF tracks on the L2. I did via stitching at the PCB edge and next to the waveguide to achieve the best possible shielding.

The coupler is designed in the stripline topology, meaning that it consists of the 2 coupled lines on the L3 that are ground-shielded between top and bottom layers. There is a gap in the L2 layer just above the coupler structure. This makes a rather asymmetrical situation as the stripline has 47+6.7=53.7mil of dielectric above the structure and only 6.7mil of dielectric below. But,if it has worked for Henrik, I see no reason why it won't work for me as well.

Power tracks should be as short as possible and far away from the sensitive RF and signaling tracks. It would be the best if the power tracks would also have noninterrupted return path. The return paths of power and digital signals should not interweave in any case. I used power plane for 3.3V on the L3 and L4 and one smaller 5V plane on the L1. All other planes on L1, L2 and L4 are dedicated to the GND.