transmission line effects

[rajesh13]

I want to know when transmission line effects start to dominate. is there exist a criteria for frequency. if yes, what we should do from circuit design point of view to suppress these effect. I want to look from I/Os (input and outputs) circuits point of view.

 

[yjkwon57]

Hi rajesh.

I think there is no exact criteria for that purpose. But, I think the dimensions relative to the signal wavelengths are important. In IC design, the dimensions of the components including the connecting wires are relatively small, thus, the transmission line effect would not occur unless the connecting wires are long compared to the signal wavelength to be handled. But, the off-chip components are very large compared to the on-chip components. Thus, if the signal frequency is high, then, you will attach the impedence matching circuits to the I/O pins outside the chip.

Bye~~~

 

[rajesh13]

Hi rajesh.

I think there is no exact criteria for that purpose. But, I think the dimensions relative to the signal wavelengths are important. In IC design, the dimensions of the components including the connecting wires are relatively small, thus, the transmission line effect would not occur unless the connecting wires are long compared to the signal wavelength to be handled. But, the off-chip components are very large compared to the on-chip components. Thus, if the signal frequency is high, then, you will attach the impedence matching circuits to the I/O pins outside the chip.

Bye~~~

Hi yjkwon57,

Is there some thumb rule with which I can find out that after this much frequency I need to take these transmission line effects.

 

[zorro]

One effect is delay in the signals. Another effect is reflexions that produce ringing.
Transmission line effects (reflexions) start to be important when the round-trip delay is comparable to rise and fall times.
These effects are mitigated with terminations in the lines: they should be loaded with the characteristic impedance of the transmission line in order to avoid reflexions.
TTL logic IC's designs have an internal diode that reduces ringing.
Regards

Z

 

[yjkwon57]

Hi, rajesh13.

The transmission line effects always occurs even for the low-frequency cases, although we need not detect the effects.
The reason for the effects is the existence of the parasitic inductances and capacitances. These parasitic components are affected by many factors such as the material properties, the component shapes, the physical relationships with the near-by components, etc.
Thus, I cannot say the exact answer to your question. And the reuirements to tolerate this effect depend on the apllication specification of your project.
But, I woould like to show one figure that is obtained from a brief simulation. But, unfortunately, I cannot make the figure posted. If you like to see the figure, let me know your e-mail address or any URL where I can upload it. In the transmission line, the impedence matching is very important, thus, I think the reflection coefficient is to be investigated so as to get the answer to your question.
Accoding to the figure (sorry), the larger the values of the parasitic components, the lager the amount of the reflected signal, i.e., the lesser the circuit operation efficiency. If you get the relavant parameter values, then, you can determine the frequency at which the transmission line effects become apparent.
Since these parameters is hard to be measured, they use some typical parameters, thus, you will consider the amount of margin for the safety.

Bye.