Electronic – How to route a trace on PCB with 50 Ohm impedance


I used some online calculator to calculate parameters of trace so it has 50 Ohm impedance.

I use FR-4, so H=1.5mm, T=0.035mm and Er = 4.5. 2-side PCB, one side with signals, the other with GND. I set the calculator to "microstrip" and for this data it told me to make trace width = 2.73 mm…

That's more than 100mils for width, most of my traces have 8-10 mils. I'm connecting DRAM to uC, there's no possibility I can me those traces so wide.

So how people achieve 50 Ohm impedance with reasonable traces width?

Best Answer

DRAM will NEVER work with transmission lines and terminations because the termination (50 ohms) will kill the digital signal.

The whole point about memory and micro slumming it in the same place (or very close) is that you can get away with teminations because the length of the trace is so short.

25mm of trace represents a signal delay of about 150p seconds and a rule of thumb is that you invert the 150ps to get a frequency of 6.7 GHz then divide that by ten to get an acceptable top limit speed for clk and data i.e. 670 MHz.

Is this too low?