A

#### [email protected]

- Jan 1, 1970

- 0

Let's say I have a 50R termination resistor at the end of a 5 mils

wide 50 ohm line on an outer layer on this particular PCB. Now when

the line hits the pad of the resistor, the impedance of the pad is

lower than the line, since the pad is about 12 mils across (0201

part). Now the signal is "in" the resistor, which has a 50 ohm

resistance, and now the signal goes back out the other pad, back to a

short line, and then ends up in the 18 mil via to ground....

1) What difference does it make if the 3 mil long line that connects

the resistor to the via is a "50 ohm" line or not? The prop delay is

0.3 PICOSECONDS. What frequency does the signal need to be to be able

to "see" the impedance in such a short trace? 100GHz +???

2) Isn't it worse to have 2 steps (pad, 50 ohm line, via) than no

steps (line is as wide as the pad)??

3) Something I never quite "got". Parts must have an impedance as

well, is there such a thing as a 50 ohm 1K resistor? A 0201 1K

resistor is wider than a 50 ohm trace on this PCB, so does the 1K

resistor present an additional mismatch on the line due to the

physical size as well as the electrical resistance? What happens to a

signal that encounters a large part on a narrow 50 ohm line? I

understand that the impedance basically tells me about the ratio of

electrical to magnetic field, or something to that effect, while the

resistance tells me about turning electrical energy into heat.

Question is, if a resistor is sufficiently mismatched, will the

impedance effects dominate the part? Will a sufficiently large (1206

say) part on a sufficiently small (5 mils) trace, reflect most of the

signal instead of actually going in the part??

4) My brain is burnt. It looks like a bowl of oatmeal left in the

microwave for 3 hours then tossed into a volcano. May I ask you to

keep the answers short, with 2 or 3 syllable words? Grunt, or use sign

language if appropriate.

Thanks!