Effects of PCB on Signal Rate

With the increase of the signal rate, the PCB also has a certain influence on the transmission of the signal. The impact of PCB can be divided into the following parts:

1. 1. PCB board: PCB material
      PCB board is the substrate of the signal transmission link. The PCB consists of a core layer (core), an electrical layer (dielectric layer) and a copper foil (copper foil), etc., which are finally laminated together by gluing to form a complete PCB laminate structure. Among them, the main factors of signal attenuation caused by PCB board are dielectric loss, conductor loss, reflections, mismatched impedance, etc. The above losses are related to various properties of materials, which directly affect the link. They can be summarized as the following properties:
          Loss Tangent and Dissipation Factor (tan( δ)/Df)
          Dielectric constant: Dielectric constant (Er/Dk)
          PCB fiber weave compactness: Fiberglass weave composition
          Copper foil surface roughness: Copper surface roughness

•  Loss Tangent (Dissipation Factor): Loss Tangent and Dissipation Factor (tan(δ)/Df)
         Dissipation factor can be used to measure the loss of the transmission line. When the signal is transmitted from the transmitting end to the receiving end on the transmission line, electric and magnetic waves will be generated. Electromagnetic waves are absorbed in the electrolyte layer, and this loss is called dielectric loss. As the signal rate increases, the electrolyte loss also increases proportionally.
• Dielectric Constant: Dielectric constant (Er/Dk)
         Dielectric constant is a measure of the insulating properties of a material. A low dielectric constant provides better insulation, so that the signal travels faster, the impedance of the transmission geometry is larger, and the parasitic capacitance is smaller.
• PCB fiberglass weave compactness: Fiberglass weave composition
          Phenomenon: A PCB board consists of fiberglass yarn strands woven into bundles with epoxy resin to form a fiberglass board that constitutes a core and a prepreg laminate. Due to the different dielectric constants of fiber bundles and epoxy resins, the density of the fiber structure affects the uniformity of the dielectric constant of the laminate.
• Surface roughness of copper foil: Copper surface roughness
          phenomenon: High frequency is due to the skin effect, which will reduce the effective area of ​​current flow and increase the effective resistance of the transmission line, thereby increasing the transmission loss of the transmission line;
  
  2.PCB stack design : stack-up design
  
  •     Transmission line type:
          The signal line with the signal path on the surface of the PCB is called a microstrip;
          the signal line with the signal path on the inner layer of the PCB is called a stripline;
  
  •     Differences due to transmission line type:
          The characteristic resistance of stripline and microstrip line is different, so the corresponding matching resistance is different (in the case of the same matching resistance, the width and height may be different);
          at the same frequency, the insertion loss of stripline is lower than that of microstrip The insertion loss of the line, generally the high-speed signal is placed on the inner layer, and the stripline wiring is used;
          when the material is constant (the dielectric constant is the same), in most cases, the transmission delay of the microstrip line is lower than that of the stripline. ;
     3.PCB link design: channel design
  
  • • •   Single-ended signal:
        all traces are perpendicular to each other as far as possible;
        the spacing between traces is large;
        the distance between traces and adjacent planes is small; the
        clock line is as straight as possible, if there are bends, use arcs instead of broken lines ;
        Change layers less for the clock line; the via hole will change the impedance and cause the reflection of the signal;
        the outermost layer is placed next to the ground layer to reduce noise; the clock line of the inner layer should be wrapped by two reference layers;
        
        • Difference signal:
        The spacing between differential pairs should be greater than 2 times the intra-pair spacing; the intra-
        pair spacing is equal to 3 times the distance to the reference layer ; the intra-pair
        spacing should be kept as constant as possible;
        the deviation and phase difference within the differential pair should be minimized;
        avoid using vias.
        4.PCB power noise filtering: Power Noise Filtering

            5.PCB impedance matching: Termination Schemes