About the Series Termination for matching the characteristic impedance

See Fig. 11 in

Another poster gave a link to a pdf describing the theory and application of termination techniques. It would be in your best interest to read and understand this.

Here's the short version for so-called 'source termination'.

First, assume the fastest case where the entire edge of your signal fits (timewise) into the transmission line (pcb trace).

The basic idea is that you want to launch a half-voltage signal into the trace. This will happen if your net source impedance (of the driver and corresponding series R) is equal to the characteristic impedance of the trace. That is, if your driver would generate a signal from zero volts to V volts when it's not connected to anything then you will get a zero to V/2 signal, into the (long) trace, if you drive the trace as stated.

When this V/2 edge hits the open trace (at the end) the following will happen:

1) A same phase signal, but double in amplitude (2x as compared with what was traveling down the trace), will appear at the end of the trace 2) An opposite phase (-1x) signal will be reflected back toward the source driver

Since a V/2 edge is traveling in the trace (in the forward direction) and it will be doubled (2x) at the end, you'll end up with an amplitude of V at the end -- which is exactly what you want (for source termination).

The -1x reflection (in this case -V/2) heading back toward the source will be completely absorbed by the source termination (assuming it's close to the source driver) and no further reflections (of this particular edge) occur. Life is good.

Please read that pdf and/or other related material so you'll really understand this stuff.

Bob

Oops...one correction.

In #2 above, it's a same phase 1x signal that is reflected back toward the source driver, not a -1x, so V/2 is reflected back toward the source. A -1x is reflected only if it runs into a short circuit.

Bob

thanks for your help, I will read the pdf file carefully.

Jack. X