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

PCB plate: PCB material

PCB board is the base plate of signal transmission link. PCB is composed of core layer, dielectric layer and copper foil. Finally, it is laminated together by adhesive to form a complete PCB laminated structure. The main causes of signal attenuation caused by PCB are electrolyte loss, conductor loss, reflections, mismatched impedance, etc. The above losses are related to various characteristics of the material, which have a direct impact on the performance of the link. They can be summarized as the following characteristics:

Loss tangent and dissipation factor (tan)( δ)/ Df)

Dielectric constant: dielectric constant (ER / DK)

PCB fiber weaving compactness: fiberglass weave composition

Copper surface roughness

Loss tangent and dissipation factor (tan)( δ)/ Df)

The 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, electrical and magnetic waves will be generated, and the electromagnetic waves will be absorbed in the electrolyte layer. This loss is called electrolyte loss. As the signal rate increases, the electrolyte loss also increases in proportion.

Dielectric constant: dielectric constant (ER / DK)

Dielectric constant is a factor to measure the insulation characteristics of materials. Low dielectric constant can provide better insulation, so that the signal can be transmitted faster, the greater the impedance of the transmission geometric line and the smaller the parasitic capacitance.

PCB fiberglass weave composition

Phenomenon: the PCB board is woven into bundles by glass fiber yarn strands and epoxy resin to form a glass fiber board forming a core and prepreg laminate. Because the dielectric constants of fiber bundle and epoxy resin are different, the density of fiber structure affects the uniformity of dielectric constant of laminate.

Copper surface roughness

Phenomenon: high frequency is due to skin effect, which will reduce the effective area of current flow and increase the effective resistance of transmission line, so as to increase the transmission loss of transmission line;

2. PCB stack up design

Transmission line type:

The signal line whose signal path is on the surface of PCB is called microstrip line microtrip;

The signal line whose signal path is in the inner layer of PCB is called stripline;

Differences caused by transmission line type:

The characteristic resistance of stripline and microstrip line is different, so the corresponding matching resistance is different (when the matching resistance is the same, the width and height may be different);

At the same frequency, the insertion loss of stripline is lower than that of microstrip line. Generally, high-speed signals are distributed in the inner layer, and stripline wiring is selected;

When the material is constant (the dielectric constant is the same), the transmission delay of microstrip line is lower than that of stripline in most cases;

3. PCB link design: channel design

Single ended signal:

All routing lines shall be perpendicular to each other as far as possible;

The distance between lines is large;

The distance between the route and the adjacent plane is small;

The clock line shall be as straight as possible. If there is a bend, an arc shall be used instead of the broken line;

Less layer change of clock line; The via will change the impedance and cause the reflection of the signal;

The outermost layer is placed close to the stratum to reduce noise; The clock line of the inner layer shall be wrapped by two reference layers;

Differential signal:

The spacing between differential pairs should be greater than twice the spacing within pairs;

The internal spacing is equal to 3 times the distance to the reference layer;

Keep the spacing inside as constant as possible;

Ensure that the deviation and phase difference within the differential line are minimized;

Avoid using vias.

4. PCB power noise filtering

5. PCB impedance matching: termination schemes

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