Signal transmission is critical in understanding high speed digital design. Digitial ICs sends down digital bits "0110" down on the wires of the PCB trace at increasing high speed. As speed goes up, transmission path becomes an challenge in overall digital design due to signal reflection and noise immunity issues that arises with voltage waves exhibit transmission line effect and signal starts to attenuate at high frequency.
The basic digital transmission is consist of a driver, transmission medium, and a receiver.
- VOH; driver output high representing logic "1"
- VOL; driver output low representing logic "0"
- VIH; receiver input high representing logic "1"
- VIL: receiver input low representing logic "0"
- rising edge; it's characterized as the time it takes from the signal to rise from 10% to 90% of maximum driver voltage
- falling edge: it's characterized as the time it takes from the signal to fall from 90 % to 10% of maximum driver voltage
When 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 receivers' VIH in order for receiver to correct detect a logic level "1"; on the other hand, receivers' VIH needs to be higher than transmitter VIL in order to correctly detect a 0. The difference between VOH and VIH and VIL and VIH are the noise margin of the receiver that it can tolerate before compromising logic level detection accuracy.
Factors Influencing Transmission
Low speed interface
- long trace
- additional capacitive line due to long trace loading increase rising and falling time and hence affecting the speed of transmission. One should route the trace as short as possible to reduce the amount of parasitic capacitance
High speed interface
- High bit rate
- high bit rate requires driver to switch at higher speed, that means sharper rising and falling edges are required. Sharper edges contains very high frequency content, which gives rise to reflection problems due to transmission line effect.
the voltage ringing often seen on the receiver is caused by impedance mismatch, poor layout, etc. The signal reflection can happen on either low speed and high speed signals, but it mostly has much higher issue 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.
- high frequency content of a signal attenuates as frequency goes up due to dielectric loss and resistive path loss. This signal attenuation is an important problem in long cable transmission that needs to be addressed.
External Noise over long transmission
- cable running a long distance can pick up common mode noise as it travels. This presents a challenge problem for inter device communication.
How do we solve this?
- Differential signaling comes to the help!
What is differential signaling
Differential driver outputs equal but opposite signals effective doubling the voltage seen by the differential receiver hence increasing noise margin. Similarly, differential voltage swings are generally few hundred mV, hence the driver can drive the transmission line at much faster speed compared to single ended signaling where logic levels are drive between 0 to 1.8V.
what is common mode noise
A common mode noise generally refers to a 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 are. 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 positive transmission wire and return from negative transmission wire.
How do you minimize external common mode noises?
To minimize common mode noise on a signal, a 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 route 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 on signal transmission
Different I/O voltage levels between two digital ICs
A level shifter is commonly used to bridge two communication system with different I/O voltage levels. For slow and short interfaces, simple level shifter can be designed discretely using a N-type MOSFET with resistors.
Radiated noises due to timing skew between differential transmission signal path.
Mismatching if positive and negative transmission (i.e timing skew/phase noise) carries high frequency voltage pulses of common mode noise seen the traces. These high pulses radiates when travel over a exposed wire and long pins such as a connector.
Summary and Conclusion
- Basic communication is consisting of driver, path, and receiver.
- Long trace and high throughput degrades signal transmission
- Common mode noise are the most common noise type within a signal path
- Differential transmission ensure robust transmission and noise immunity for high speed and long transmission path.
We learn the basic building block of signal transmission and receiving system. To ensure that we have a good design quality of digital communication, signal transmission paths Must be evaluated and designed in correctly.