Hi all,
Previous message in this thread (in part):
>Subject: Re: PXI or SCXI impedance analyzer?
>From: "Uwe Frenz" <[EMAIL PROTECTED]>
>Date: Wed, 02 Jun 2004 10:05:15 +0200
>George,
>you asked on 1 Jun 2004:
> > I need to measure the impedance (low frequency, low power) of
<snip>
>
>I do not really know what an impedance analyzer does other than measuring
>the resistance or impedance = 1/resistance of an object. IMHO a resistance
>measurement should be sufficient.
<snip>
>HTH and
>Greetings from Germany!
>--
>Uwe Frenz
Uwe, et. al.,
Having invested 10 years of my career teaching electronics, I just
had to throw in my two cents worth here. First however, Uwe, I must say
that your posts are always excellent and I learn much from each one. Thank
you for your contributions to this list.
The comment "impedance = 1/resistance" is not correct. In fact
conductance = 1/resistance, units Siemens (formerly "mhos").
Resistance is a DC parameter, i.e. the opposition to the flow of a
DC current (mechanical analog might be friction).
Impedance is an AC parameter, i.e. the opposition to the flow of an
AC current (above mechanical analog extended might be stiction). Since
there are an infinite number of AC frequencies it is more complex to
measure.
Theoretically impedance is generally thought of as being comprised
of a DC (resistance) portion combined with an AC (inductive or capacitive)
portion. To measure impedance one would need to measure the AC voltage
across and the AC current through the device under test (DUT) and form their
ratio. Note that these measurements would need to be conducted at every
frequency of interest and that the voltage waveform and the current waveform
would (for any not purely resistive DUT) have a phase shift between them.
This phase shift is caused by the inductance or capacitance (termed the
reactive component) in the DUT and must be measured as well as the amplitude
of the v & i waveforms.
Since by Fourier analysis it can be shown that any periodic waveform
is comprised of a weighted sum of sinusoidal waveforms at the fundamental
frequency and integer multiples thereof, the above DUT would typically be
excited by a purely sinusoidal voltage source. In time domain notation,
v(t) = A * sin(w*t + theta), where A is the peak amplitude, w is really
omega, the radian frequency of the sine wave, and theta is the phase shift.
Dealing with this form of equation is challenging from the bookkeeping
standpoint (read that "pain in the rear") so this is where complex numbers
are introduced to simplify the bookkeeping aspect. I won't go there as the
proof is trivial and left to the student as an exercise.
As a closing note, the reciprocal of impedance is admittance
comprised of a conductance component (1/resistance) and a susceptance
component (1/reactance, either inductive or capacitive).
Having waxed sufficiently long-winded, I'll stop now.
HTH and greetings from the U.S.A.
Mark Watson
Staff Engineer
Philip Morris
[EMAIL PROTECTED]
(804) 752-5631
(804) 752-5600 (fax)
(804) 215-5631 (pager)
