Embedded Technology


It means that a signal edge travels on a PCB trace with a speed of 15cm per nanosecond. We can also say that the pulse travels a distance of 1 km in a time as short as 6.7μs. Thinking in PCB dimensions, the propagation speed of the clock signal edges is 67 ps per cm [0.067 ns/cm].


Consequences for PCB design Based on the assumption that a signal trace can be seen as a lumped trace if its length is shorter than 1/6th of the wavelength of a fast pulse travelling on that trace, we can


actual / measured rise & fall time*


(ns) 5


speed on PCB trace


(cm/ns) 15


415 3


15


215 1


0.5


15 15


estimate the critical length of a trace versus the rise and fall time of a clock signal, if the rise and fall time is given at 10% ~ 90% of the voltage swing. If the rise and fall time is specified at 20% and 80% of the voltage swing, a calculation based on 1/4th of the wavelength is acceptable. *Note that specifi ed maximum rise and fall time in XO datasheets may be different from actually measured rise and fall time.


The source impedance termination As a termination at the receiver end is not


wavelength of pulse on PCB trace


(cm) 75 60 45 30 15


7.5


trace requires a termination, if longer than 1/6th of wavelength


(cm) 12.5 10


7.5 5


2.5 1.25


allowed, an impedance matching between the oscillators output and the transmission line is recommended to avoid refl ections to travel in both directions of the PCB trace. Assuming that the output of the oscillator has a source impedance RI, and the transmission line has a characteristic impedance Z that is higher than the RI, a series resistor RS should be added. For best impedance matching, the sum of RI and RS should be equal to the characteristic impedance Z. The best value of RS might be determined experimentally, especially if the impedance Z of the PCB trace is unknown.


Summary


If using clock oscillators with a HCMOS output, a series resistor RS should be placed


very close to the output of the oscillator. The sum of the


source impedance RI and the series resistor RS should be equal to the impedance Z of the transmission line.


The purpose of this recommended


confi guration is to avoid a further refl ection of pulses that are refl ected at the receiver end of the transmission line. Typically, termination resistors at the receiver side are not recommended, as they will reduce the voltage swing of the clock signal, and required threshold levels at the receiver side might no longer be reached. https://www.jauch.com/en


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