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Topic: PCB Impedance Question (Read 1 time) previous topic - next topic


Lots of things to consider at higher frequencies! Ran into this issue with a GPRS board I made... had to go to a thinner FRP design than usual (1mm vs. 1.6mm thickness) to accommodate the microstrip requirements of the cell phone antenna. Also consider grounding and EMI carefully - I tried and appear to have succeeded. It took a while, but the board has a almost continuous ground plane except for things like the SPI pin headers, for example.


You may have a top pcb 20mil trace 5 mm long, shorted to bottom's ground plane via a single via, and its impedance could be 80ohm (at few GHz for example).
Z = 0.0000001 + j80
So the impedance is "inductive" - it means the "via" behaves as a small inductor - that is important when you want to ground a decoupling capacitor though a single via for example, thus you must consider the physical sizes and material properties as well..

No, in this case, Amilobe is correct. The manufacturer is talking about a traces impedance as a section of a transmission line. This does depend on frequency (within limits), that trace should remain a 50 ohm impedance transmission line at 1MHz or 3GHz. It is called a stripline or microstrip.



So if you feed one end with a driver with a 50 ohm impedance, and the other end is connected to a receiver with 50 ohm input impedance, the result is no reflections and an SWR of 1:1, not dependent on frequency or length of transmission line trace.

Steve Greenfield AE7HD
CET Consumer Electronics and Computer
Please don't read your attitudes into my messages




And the solitary genius who first understood transmission lines (and many other implications of
Maxwell's equations) was Oliver Heaviside.  He worked out that you could add regular load inductors
to telegraph wires to allow them to carry information much faster, invented the coaxial cable,
invented iron-cored copper wire for increasing distributed inductance, invented a practical
duplex telegraph circuit for long distance (IIRC), reformulated Maxwell's 20-or-so equations
into the modern 4 vector calculus equations (Hertz also did this independently I think), codiscovered
the Poynting vector...

He is much underrated in my opinion:


Note that he invented the terms impedance, reluctance, admittance, permeability, conductance, inductance....
[ I won't respond to messages, use the forum please ]


A line characteristic impedance is a theorical impedance so it is always real.
If at one end an impedance controled line is correctly matched by is characteristic impedance , on the other end the actual impedance is also real.
It is for this property they were invented.

If it is not matched, the actual impedance could be complex, capacitive or inductive depending of the line lengh.
Unmatched a controled impedance line is also dangerous in analogic than in digital, depending of the higher frequency and the complexity of the design.

FR4 is a "Flame Retardant" norm, not an electronic norm, so with high frequency (>500 MHz) the same design will have different behavior with two PCB from two different FR4 manufacturers diffrence with epoxy resin).

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