label: PCB layout,PCB design,Printed circuit board
For the most part, PCB layout that include RF and/or microwave circuits are difficult to design. However, regardless of their difficulty, the rule of thumb is to start with the basics and follow the laws of physics. Up front, the PCB designer needs to understand that microwave signals are highly sensitive to noise. The possibility of incurring ringing and reflections must be treated with great care.
When working with very high-speed digital signals in the gigahertz range or 10Gb per second, for example, the PCB designer has to follow certain guidelines and rules. When RF and microwave enter the layout, the PCB designer must have the same mindset, yet multiply that mindset many times simply because RF is far more sensitive than very high-speed digital signals. Second, impedance matching is extremely critical for RF. Digital signals -- even if they are very high-speed -- have a certain tolerance. But for RF and microwave, the higher the frequency, the smaller the tolerance becomes. For example, the PCB designer must keep it at 50 ohms -- 50 ohms out from the driver, 50 ohms during transmission, and 50 ohms into the receiver. Third, the return loss must be minimized. This loss is caused by signal reflection, or ringing. The return is the path taken by the return current. For example, take a single-ended signal going from the driver to the receiver. Obviously, there has to be a return signal from the receiver to the driver. If it’s a single-ended signal, then the return typically takes the path of least impedance.
At very high microwave frequencies, the return signal takes the path of least inductance. Ground planes underneath the signals are good at providing this path. Therefore, there should be no discontinuities in the plane underneath the signal all the way from the driver to the receiver. However, If there is a cut in the ground or the ground plane doesn’t exist underneath that trace, the signal will still somehow find its way to the driver. It will go through power planes, through a PCB’s multi-layers or through some other route - it will definitely find a return path. But such a path won’t be ideal, and thus will cause reflection and ringing since it will no longer be an impedance controlled signal.
