pcb board signal traces problems

[zenghong]

Hi all,

I am new here and found out it's a really amazing forum.
I am trying to learn some pcb testing technique and there is a question that i was asked is if the signal traces signal have a very large rise time and small fall time(there is also a vdd trace and gnd traces).  I think it might be too much capacitance to load?! But no idea to how to fix it.
It might be operating in high frequency but I think it also have this problem in low frequency. In addition, what if a vice versa situation?
I hope to know not only the possible answers but also any references I can search for such kinds of problems?

thank you guys!

[Kremmen]

Not quite sure what you are asking here, exactly. But i assume your problem is that a signal in your PCB exhibits too long rise and fall times at both high and low frequencies.
If this is the case, then typically your load capacitance is too large in relation to the source impedance of whatever drives the load.
You get high frequency signal transition issues when your traces act as transmission lines but are not terminated correctly, causing reflections and standing waves. This phenomenon however is not seen on low frequencies so it will not explain your problem if it is independent of frequency, as i understand from your post.

Can you reword your question in a more specific way? Then it might be easier to give a more specific answer.

[ptricks]

Really need information on what frequencies are involved.

[free_electron]

Long rise time and short fall time means that the pull-up current is too low.

Nothing wrong with the capacitance.  If capacitance is a problem then both rise and fall would be problematic ( assuming symmetric current handling capability in all driver stages ).

Add a pull up resistor. That will speed up rise time.

[zenghong]

How to connect a pull-up resistor? From vdd to signal traces?

[SeanB]

Correct. Depending on speed and the output drive capability of the drivers it can be any value from 150R to 10K.

[zenghong]

Thank you. So if the problem is at fall time a pull-down resistor can solve the problem?