RF Transmission Lines
The impedance transmission lines are useful for transferring power to and from IC pins.
Here, we will look at the different types, including microstrip, suspended stripline, and grounded lines.
1.Microstrip
Microstrip transmission lines have fixed-width metal routing and unbroken ground plane that is placed on the next layer.
The characteristic impedance will depend on the type and thickness of the dielectric layer. It usually ranges between 50O and 75O.
2.Suspended Stripline
This one is made up of an inner layer and a routing with of fixed width. It has solid grounds over and under the center conductor, usually in the middle of the ground planes or offset.
3.Coplanar Waveguide (Grounded)
When designing RF PCB, a coplanar waveguide helps you to better isolate between RF lines and other signal lines.
It has a center conductor and ground planes on one side. Also, it should have via fences on both sides.
Characteristic Impedance
There are different ways when it comes to calculating and accurately setting the width of the signal conductor line for target impedance.
Only note that the dielectric constant for the outer laminate layers is usually lower because of lower glass content.
You should, therefore, be cautious when entering the dielectric constant of the layers to achieve an optimal balance.
Corners and Bends
The corners and bends of the transmission lines should never be at right angles. All cornered transmission lines should have rounded edges.
Sharp (right angle) cornered transmission lines are prone to higher losses.
The bend radius for the round edges should be at least 3 times the width of the center conductor. This helps to ensure steadiness of impedance while current passes through the bend.
In cases where, for whatever reason, you cannot come up with a curve, you can use an angled meter to reduce impedance fluctuations.
Layer Changes for Transmission Lines
To help reduce inductance loading, use at least two via holes for every transition of a transmission line between two layers.
Use the largest diameter via compatible with the width of the transmission line.
You can as well use three vias if you are not able to use the largest diameter vias due to space limitations.
Signal Line Isolation
Keep the transmission lines as far apart as possible. Never route them close to one another for long distances as that can also increase coupling.
Ensure there is a ground plane to keep apart any lines that cross on separate layers. Keep away high-power signal lines from all the other lines.
Ground Planes
Use a continuous ground plane for layer 2. Strip lines and offset striplines will require that you have ground planes over and below the center conductor.
Do not use these planes for signal or power nets.
If you have to use partial ground planes, they must be below the components and transmission lines.
Never break ground planes or place them under the transmission lines.
To avoid ground return paths that can cause parasitic ground inductance to increase, add enough ground vias between layers.
This will also help in preventing cross-coupling.
Selection of Decoupling or Bypass Capacitors
Any capacitor operating above the self-resonant frequency.(SRF) is inductive.
They, therefore, cannot be effective in decoupling. SRF means the capacitors have limited capacity ranges.
If you need broadband decoupling, use many capacitors with higher capacitance.
Bypass Capacitor Layout Considerations
The parasitic inductance on the AC ground path must be minimized because the supply lines must be AC ground.
Parasitic inductance usually occurs based on the choice of component orientation.
Grounding of Shunt-Connected Components
An example of these components is a power-supply decoupling capacitor.
For each of these components, use more than one grounding via to reduce the impact of parasitic inductance. For a collection of shunt-connected components, you can use via ground islands.