Impedance unlike resistance varies with frequency.  Impedance can be defined
as "the resistance offered by an electronic circuit to an AC signal".  The
reaction between the Capacitive (dielectric),  Inductive (circuit volume),
and AC signal frequency of the circuit in question relates to "Circuit
Impedance".  As frequency approaches infinity this "Circuit Impedance"
approaches a set value or "upper limit" which can be loosely definined as the
"Characteristic Impedance" of the circuit.  

Impedance can be measured in one of two ways.  1) Direct Measurement of
"Circuit Impedance" with an Impedance Analyzer.  2) Derived Measurement of
"Characteristic Impedance" by Time Domain Reflectometry (TDR). 

The primary difference between the two methods is that a TDR does not
directly measure "Characteristic Impedance", but derives it from the behavior
of a pulse, which is injected into the circuit under test.  The Impedance
Analyzer directly measures "Circuit Impedance" at a given frequency using a
current-voltage technique.

Each of these methods has positives and negatives, but overall the TDR method
is most widely accepted as the "method of choice" for measuring the
"Impedance" of PWB's.  It is necessary to understand whether your customer is
looking to find "Circuit" or "Characteristic" Impedance of his PWB's before
deciding on a test method.

The primary advantage of TDR measurement is the ability to characterize the
entire length of a circuit, directly viewing variations along the test trace.
 The ability to resolve small defects is only limited by the rise time of the
injected TDR test pulse.  The lower the pulse rise time, the greater the
ability to resolve small differences in adjacent areas of the circuit under
test.  The primary disadvantage in the TDR method is that it will not give
the relationship between a given frequency and "Circuit Impedance".  The
frequency of the TDR test instrument does not relate to "Circuit Impedance"
at a specific frequency, and therefore is not a consideration when your
customer says he wants "Impedance" at "XXX" frequency.

The primary advantage of Direct measurement using an Impedance Analyzer is
that the actual "Circuit Impedance" can be measured directly at a specific
frequency.  Until recently these Analyzers were limited to a test frequency
of about around 100Mhz, but the HP 4291 can now go up to 1.8Ghz in test
frequency.  This is an expensive piece of equipment, and fixturing is a
nightmare, but measurement of "Circuit Impedance" are possible with this
instrument up to 1.8Ghz.  There also may be other equipment recently on the
market which can provide Ghz "Circuit Impedance" measurement.  The primary
disadvantage of this method is that the unit reports a "normalized" average
of the "Circuit Impedance", and cannot resolve differences within the test
circuit.

Bob Neves
Director of Technical Services
MICROTEK LABORATORIES
(800) 878-6601
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