[SI-LIST] Matching impedance

Hi! For most applications the impedance requirement is
say 50ohm single ended or 100ohm differential and so
on a PCB the traces are etched to meet this impedance.
Now, for some signals it is not critical to maintain
absolute 50/100 and for higher speed devices, chips
have equalisation circuitry that takes care of some
amount of noise to a certain degree. But the problem
comes for chips that don't have equalisation.
I have an interconnect that has 75ohm output impedance
driver & 50 ohm input receiver. Hence I need to put
matching network. So, the design of the interconnect
is Driver-->75ohm trace-->matching network-->50ohm
trace-->receiver. Now, I keep the 75ohm trace as short
as possible. But the problem I have is that if I look
at the impedance from the driver upto the matching
network it will not be 75ohm but lower because of
added parasitic capacitance of pin, via, pad of
matching network. Hence, I was thinking of removing
the non-functional pads on the driver pin(thru-hole),
keep the trace impedance slightly above 75ohm say
80ohm, that way the effective impedance seen by the
driver is around 75ohm and then I can plug in the
values of resistors to give 75ohm termination at that
point.On the other side since the trace is 50 ohm it
will terminate into the 50ohm termiantion at the
receiver.
Any thoughts or am I on the wrong track?
Also, what is the best way to measure impedance of
assembled board. I solder an SMA cable to the pin(pt.
of interest) and look at the impedance.
Thanks in advance
Bob

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