Signal Transmission


Signal transmission is critical in understanding high-speed digital design. Digital ICs send digital bits "0110" down on the wires of the PCB trace at increasing high speed. As speed goes up, the transmission path becomes a challenge in overall digital design due to signal reflection and noise immunity issues that arise with voltage waves exhibiting transmission line effect and the signal starts to attenuate at high frequency.


Basic Transmission

The basic digital transmission consists of a driver, a transmission medium, and a receiver.

Logic level


Noise Margin

When the transmitter output a voltage high (VOH) for logic level "1" and output voltage low (VOL) for logic level "0", the receiver correspondingly has its input voltage high level for detecting a logic level "1" and input voltage low (VIL) for detecting a logic level "0". In a good communication design, the transmitter's VOH needs to be higher than the receiver's VIH in order for a receiver to correctly detect a  logic level "1"; on the other hand, the receiver's VIH needs to be higher than the transmitter's VIL in order to correctly detect a 0. The difference between VOH and VIH and VIL and VIH is the noise margin of the receiver that it can tolerate before compromising logic level detection accuracy.

Factors Influencing Transmission

For low-peed interface:

For High-speed interface:

Signal reflection

the voltage ringing often seen on the receiver is caused by an impedance mismatch, poor layout, etc. The signal reflection can happen on either low-speed or high-speed signals, but it mostly has much higher issues for high-speed signals due to smaller logic level transition windows (i.e. clock period). Remember reflection happens when the trace exhibits a transmission line effect where the characteristic impedance, based on the geometry and material of the trace, does not match termination resistors and vice versa.

Transmission loss

External Noise over a long transmission

How do we solve this?

What is differential signaling?

Differential driver outputs equal but opposite signals effectively doubling the voltage seen by the differential receiver hence increasing the noise margin. Similarly, differential voltage swings are generally a few hundred mV, hence the driver can drive the transmission line at a  much faster speed compared to single-ended signaling where logic levels are driven between 0 to 1.8V.

What is common mode noise?

A common mode noise generally refers to noise from either internal (crosstalk and ground bounce) or external environment (60 Hz light flickers) that induce a noise voltage that is in the same direction (i.e equally in amplitude and phase) onto the transmit and return path of a signal. Common mode refers to the characteristic of a signal where it is the same  (i.e. common) state (i.e. mode) between transmit and return wire.

It's important in the context of differential signaling with two transmission wires. The positive transmission terminal drives a positive signal while the negative transmission terminal drives an equal but opposite signal. Because of this differential drive waveform, the current goes from the positive transmission wire and returns from the negative transmission wire.

How do you minimize external common mode noises?

To minimize common mode noise on a signal, differential signaling should be used. In order to ensure that the interference has the exact same voltage noise and phase on two traces, the differential signals should be routed as close as possible to each other next to the inference source. Then a differential amplifier is used to reject the common mode noise and leaves only the differential signals which is the desired signal.

Commonly encounter design problems in signal transmission

Different I/O voltage levels between two digital ICs

A level shifter is commonly used to bridge two communication systems with different I/O voltage levels. For slow and short interfaces, a simple level shifter can be designed discretely using an N-type MOSFET with resistors.

Radiated noises due to timing skew between differential transmission signal paths.

Mismatching if positive and negative transmission (i.e. timing skew/phase noise) carries high-frequency voltage pulses of common mode noise seen in the traces. These high pulses radiate when travel over a exposed wire and long pins such as a connector.

Summary and Conclusion

We learn the basic building block of signal transmission and receiving systems. To ensure that we have a good design quality of digital communication, signal transmission paths Must be evaluated and designed in correctly.