Signal Loss is the loss of strength of a signal as it propagates over a medium. Generally, in PCB design and manufacturing, the term refers to loss of signal strength in guided media, such as impedance controlled lines.
Signal Loss = 27.3 x (f/c) x (εeff)½ x tanδ
εeff = Effective Dielectric Constant (Er or Dk)
tanδ = Dissipation Factor (Df)
f: Frequency
c: Light Speed
As the frequency increases, the loss of signal will also increase. Therefore, high-frequency and high-speed boards are mostly designed and manufactured with materials with low dissipation factors. Also see Propagation Delay.
When talking about the design and manufacturing of printed circuit boards (PCBs), customers mainly focus on the controls and signal frequencies that the PCB will perform for the specific application. Another topic to also take into consideration is the insertion loss.
Insertion loss occurs when there is a loss in the signal’s strength or power when passing through components, systems, and materials. This issue may also be referred to as attenuation when discussing the power loss within the length of a transmission line. Improving the insertion loss during the PCB stack-up ensures that the application will operate within specifications when receiving higher signals of 10Ghz to 30Ghz or more.
What Causes Insertion Loss?
There are three main causes of insertion loss:
- Copper Losses: Copper losses occur when the connector’s conducting surfaces cause the power to become dissipated. This problem is usually the result of the plating and the material that is used in the PCB.
- Dielectric Losses: Dielectric losses occur when the dielectric materials used in the PCB layers causes the power to be dissipated.
- Reflected Losses: Reflective losses occur with the connector’s voltage standing wave ratio (VSWR) when in transmission lines. It occurs when the power does not become accepted by the load, as it becomes reflected back along the line.
These losses are measured in decibels and is represented as a positive number. For PCBs using lower speed rates, such as 3Ghz up to 5Ghz, a mismatch in impedance usually results in signal loss. Yet, with the higher speeds of 10Ghz to 30Ghz+, the materials used to create the dielectric constant will be the top issue that could be creating the insertion loss. To deal with these many issues, selecting the appropriate materials for the core, PP, and copper foil becomes essential when building the PCB stack-up.