Electronic Design - Tutorial to reduce signal attenuation in high-speed PCB

When the signal is transmitted from the source through the PCB conductor to the load, it will experience attenuation due to energy loss caused by trace resistance and dielectric loss.

When high-speed signals propagate on the circuit board, signal attenuation is the most common term. It is one of the main factors that cause signal degradation and signal integrity problems.

What is signal attenuation?

Signal attenuation is a measure of the decrease in signal strength (amplitude and strength) as the signal propagates through the transmission medium. It is an important attribute in telecommunications applications because it calculates signal strength as a function of distance.

When the information provided by the transmitter remains unchanged when the receiver is decoded, lossless signal transmission can be achieved. A sufficient threshold level should be met to extract the correct information from the signal.

How is the signal attenuation calculated?

The signal attenuation is estimated in decibels (dB) per unit length of the transmission medium. It can be calculated based on power (A p) and voltage (A v ).

To avoid the chance of attenuation, the transmitter sends multiple signals to ensure that at least one signal reaches the final destination, the receiver. However, due to the need to send those additional signals, this approach will reduce the overall network speed.

P s is the signal power at the source

P d is the signal power at the load

V s is the signal voltage at the source

V d is the signal voltage at the load

The lower the attenuation, the higher the efficiency of the transmission medium. Higher attenuation means more signal loss and reduced amplitude at the receiving end.

Attenuation coefficient or attenuation coefficient

The attenuation factor determines the distance the signal can travel and still provides enough data bits or information. It quantifies different transmission media based on how the amplitude of the transmitted signal decreases with frequency. It is given by:

AF = P output/P input

The signal attenuation factor depends on:

Transmission medium length

Transmission medium material

Physical conditions

Signal attenuation in the transmission line

In a transmission line, attenuation loss is a combination of two types of loss: conductor loss and dielectric loss. Conductor loss is due to imperfect conductivity and trace resistance, while dielectric loss is due to dielectric materials.

The signal attenuation coefficient of a transmission line of length "l" is given by:

In dB, the signal attenuation is expressed as:

It can also be expressed as the dB loss per unit length, namely:

Note: Ignore the minus sign and remember that it is a dB loss.

The above formula represents the total insertion loss per unit length of the transmission line, which can be written as:

R/Z0 is the loss component proportional to the wire resistance R per unit length, which is called conductor loss. It is represented by α C. The component GZ 0 is proportional to the conductance of the G-dielectric material and is called the dielectric loss. It is represented by α d.

Compared with conductor loss, dielectric loss is negligible. Up to 20GHz, the loss tangent related to the PCB material (ie FR4) will not change significantly. This is the main reason why the dielectric loss curve is almost in line with the frequency. The distance between the transmitter and the receiver in the PCB circuit board is generally less than 1m. Therefore, it can be assumed that the dielectric loss remains constant throughout the frequency range. As the sum of conduction loss and dielectric loss, the total loss is dominated by conduction loss.

The loss tangent of the FR4 material used in the circuit board design is approximately 0.003.