I have several old computer backplanes which were originally designed for
only modest (Typ Z-80) speed signals which I'd like to use with more
vigorous signals and I'd like to have the lines properly terminated. Is
there any simple experimental procedure to determine the characteristic
impedence of the backplane?
TIA
Norm
Let's say you drive the transmission line with a realy fast logic chip in
series with a known resistor. Until the reflection comes back from the far
end of the transmission line, it looks like a resistor of its
characteristic impedance. So, if you can generate a fast enough edge, and
have a fast enough scope, you can use the simple voltage divider rule to
calculate the characteristic impedance.
For example lets say the resistor is 49.9 Ohms. If you are driving with
some kind of CMOS logic at 3.3 Volts, you should see (roughly) a single
step up to almost 3.3 Volts on the driver side of the resistor. But on the
transmission line side of the resistor, you will see some other voltage.
Maybe 1.65 Volts, for example, if the backplane is 50 Ohms. But when the
reflection comes back from the transmission line, the 1.65 Volts will
change to something else (probably 3.3 Volts.).
So the waveform on the transmission line side of the resistor has a step
in it like this:
------------
/
V1 -----/
/
----------------/
time --->
You can use V1/V2 = Z0/(Z0 + R)
Where V1 is the lower step level (on the transmission line side), V2 is
the first step level on the driver side of the resistor, Z0 is the
characteristic impedance, and R is the known resistor.
I'll let you solve for Z0.
HTH
--Mac