PCB Technical - PCB traces and dielectric attenuation at the end of the trace

Signal attenuation due to PCB traces and dielectric

The amplitude of the signal will be distorted by the wiring resistance and the loss factor of the circuit board dielectric. This effect is more prominent at high frequencies, because the signal tends to propagate along the trace surface. Attenuation causes slower signal rise time and increases the possibility of data errors.

The high frequency transmission channel makes it difficult for the receiver to interpret the actual information. Due to the influence of the transmission medium, the following transmission loss will occur:

Dielectric absorption: When high-frequency signals propagate on the surface of the circuit board, the dielectric material will absorb the signal energy. It reduces the signal strength that can only be controlled by choosing the perfect PCB material. Choose materials with low loss tangent to reduce dielectric absorption.

To learn more about material selection, please read PCB Material Selection: Electrical and Manufacturing Considerations.

Skin effect: The skin effect is a phenomenon in which high-frequency components begin to be closer to the outside of the circuit board conductors rather than the inside. High-frequency signals are also responsible for generating waveforms with varying current values. Such signals have their self-inductance value, and as the frequency increases, the self-inductance value will increase. It is responsible for reducing the conductive area on the PCB surface, resulting in more resistance and attenuation of the signal amplitude. The skin effect can be reduced by increasing the trace width (surface area), but this is not always feasible, because changing trace geometry may cause impedance problems.

As the signal range increases, the attenuation also increases. The factors listed below are the causes of signal attenuation:

Noise source: RF frequency, leakage current and current interference cause attenuated signals. More noise, more attenuation!

The distance between the transmitter and the receiver: When the signal traverses a longer distance, its strength will decrease. The greater the distance between two points, the higher the attenuation.

Trace width: The signal attenuates less when passing through a wider trace.

Crosstalk: Crosstalk in nearby traces is also a cause of signal attenuation.

Conductor and connector: When the signal passes through different conductive materials and connector surfaces, it will be attenuated.

Transmission frequency: The shorter the wavelength, the greater the attenuation of radio waves. Such signals are transmitted by 2.4GHz or 5GHz electromagnetic waves. Electromagnetic waves have high frequency and short wavelength. Therefore, the attenuation of radio signals is large and cannot be transmitted over long distances.

Resistance loss associated with conductor materials: Conductive materials such as copper used in the manufacture of transmission lines introduce resistive losses, leading to attenuation of signals propagating on the copper traces.

Loss associated with dielectric materials: The loss of dielectric materials sandwiched between transmission lines causes dielectric loss. This dielectric loss forms conductance across the substrate, also known as reverse resistance, and absorbs part of the propagating signal energy, resulting in signal attenuation.

Copper surface roughness: The copper surface roughness on the PCB can also hinder signal propagation. Rough copper wires increase resistance, because the topography of the copper surface moves the signal up and down. Surface spikes also increase capacitance. Smooth copper is the solution to this problem, but at a higher cost.

Ground loop resistance: As the frequency increases, the ground loop becomes narrower and uses less copper area, resulting in an increase in resistance.

How to reduce signal attenuation?

The signal attenuation can be reduced by using the following techniques:

Use a repeater: If the received signal is weak, use a repeater to regenerate the original signal by reducing attenuation. It also enhances the range of the signal so that it can be transmitted over longer distances without failure.

Use an amplifier: If the received signal is weak, use an amplifier to increase its amplitude, which is different from a repeater that regenerates the entire signal.

Correct material selection: Careful selection of low-loss dielectric materials and low-resistance traces can minimize signal attenuation.

Use programmable differential output voltage (VOD) settings: Programmable VOD ensures that the drive strength is synchronized with the line impedance and PCB trace length. Increasing the VOD of the driver will enhance the signal of the receiver.

Pre-emphasis: Using an amplifier to enhance signal strength is not the only solution for PCB signal attenuation control, because it also amplifies related signal noise and jitter. Pre-emphasis only enhances the high frequency components of the signal by increasing the level of the first transmitted symbol. If subsequent symbol levels are transmitted at the same level, they remain unchanged. For example, if the signal transmits a high level of three symbols, only the first symbol is enhanced. The next two symbols will be transmitted at the usual level.