High speed Designs - Part 1

A signal can be considered high speed, if the signal frequency that need be transmitted approaches 100MHz. There are many  parameters to decide if a signal can be considered high speed or not but let us for now consider 100MHz or greater frequency as high speed signal. When we talk in terms of bit rate it comes to 100Mbps if each cycle carries a single bit. as a straight forward definition, a signal can be treated as high speed if the speed is the major reason for the loss of the signal. Putting it the other way, the minimal frequency at which the signal starting degrading if improper care is not taken while routing it. All the frequencies above it are known as high speed frequencies. In another terms, frequency at which all the line parameters (R,L,C) are to be considered for analyzing the signal or the channel need to treated as transmission line can be called high speed signals.

                Routing a high speed signal is not so easy. There must be appropriate care taken starting from the PCB material selection to final routing stage. this implies from component selection, placement, stack up and routing each and every stage is critical in determining the quality of the board. We should take care that no signal integrity issues come up on the board. So, whether it is a high speed or low speed there are few factors that contribute to the loss of a signal on the PCB. The below are the list of items that are major contributors of loss in a PCB:

1. Package loss

2. Connector loss

3. Losses due to PCB traces (channel loss)

4. Losses due to via

The parasitics that the ic package introduces is the major contributor of losses. A package need to be treated as electrical model as such a model behavior need to be used in simulations. Consider, the electrical model of any ic (Ex: .ibis models), they list the R,L,C of the package pins. The Power distribution network of these ics and the modelling of these package connectivity is very crucial in signal integrity simulations.

Connectors are a must on any PCB to connect the board to the external world. Connectors are not always ideal. The characterization of the connector determines the accuracy of loss calculation. It is always preferable to select a low loss connector for high frequency designs.

The Channel loss or the PCB trace loss is due to 2 main factors: PCB material selection, PCB routing. The PCB material (dielectric material) is the crucial selection factor for high speed board designs. It is preferable to have a low loss dielectric for high speed signals. Also, narrower the PCB traces, higher the losses. Have as much wider trace as possible for lower losses. Again there is a trade-off between PCB material to be used, impedance of the board, width of the trace. The designer needs to make a careful selection to have a lowest loss signal.

Vias are one of the major loss on the PCB. Vias are the main reason for discontinuities on the board. The via type selection is vital at the preliminary stages of the high speed design. The types that can be used are blind via, buried via, micro via, differential vias.  Back drilling is used in some boards in case of micro vias to reduce losses. The via length in these cases is the major loss contributor. The return path for the signal path which has vias is very crucial for maintaining the integrity of the signal. If the vias are not laid properly, the introduced noise may cause substantial damage to the end product in use causing endless breakdowns. The via annular ring need to be designed after considering the amount of loss that is tolerated.

If you are working on high speed board, these PCB factors are very important and some designers calculate the loss budget with these parameters before proceeding with the manufacturing. Loss budget in these cases is the sum of the loss parameters (Connector loss, via losses, PCB trace losses, package parasitic losses) that we have talked of here.