Atec Solar-6220-1A - Manual (Page 3)

APPLICATION NOTE AN622001
USING TYPE 6220-1A TRANSFORMER FOR THE MEASUREMENT OF LOW FREQUENCY EMI CURRENTS
INTRODUCTION
“There is more than one way to skin a cat” your
great grandfather and my father used to say. The
evolution of methods of measuring conducted
interference illustrates this homely expression in a
distorted kind of way.
To start with, a clever and versatile propulsion
engineer named Alan Watton at Wright Field early
in WW
II created an artificial line impedance which
represented what he had measured on
the d.c. buss in a twin-engined aircraft. Probably a
DC-3, but memory is dim on this point. Watton’s
work was sponsored by a committee headed by
Leonard W. Thomas (then of Buships) with active
participation by Dr. Ralph Showers of University
of Pennsylvania and others.
So the Line Impedance Stabilization Network
(LISN) was born. It was a pretty good simulation
of that particular aircraft and the electrical
systems it included. But then someone arbitrarily
decided to use this artificial impedance to
represent any power line.
radiation, particularly at the lower frequencies,
since the coupling between power leads at low
frequencies is inductive, not capacitive.
As it turned out, Stoddart was successful in
developing a current probe based on Alan
Watton’s suggestions regarding the torodial
transformer approach which is still the primary
basis used today. However, the development of
the voltage measurement probe suffered for lack
of sensitivity. Watton’s hope had been to provide
a high impedance voltage probe with better
sensitivity than was then available for measure-
ment receivers designed for rod antennas and
50 ohm inputs. Since this effort failed and
Watton’s funds (and probably his interest in the
subject) faded out of the picture, the program
came to a halt.
This meant that the RFI/EMI engineer could either
measure EMI voltage across an artificial imped-
ance which varied with frequency, or he could
measure EMI current flowing through a circuit of
unknown r.f. impedance. Either way, the whole
story is not known. In spite of the unknown
impedance, the military specifications began
picking up the idea of measuring EMI current
instead of voltage. The test setup was simpler and
the current probe was not as limited as the LISN
in its ability to cope with large power line
currents. And the current probe measurement
was also a measurement of magnetic field
At any rate, this impedance suddenly began
appearing in specifications which demanded
its use in each ungrounded power line for
determining the conducted EMI (then known as
RFI) voltage generated by any kind of a gadget.
The resulting test data, it was argued, allowed the
government to directly compare measured
RFI/EMI voltages from different test samples and
different test laboratories. No one was concerned
about the fact that filtering devised for suppress-
ing the test sample was based on this artificial
impedance in order to pass the requirements, but
that the same filter might have no relation to
reality when used with the test sample in its
normal power line connection.
Not until 1947, that is. At that time, this same
Alan Watton, a propulsion engineer having no
connection with the RFI/EMI business, decided
to rectify the comedy of errors which had
misapplied his original brainchild. He was in a
position to place a small R and D contract with
Stoddart for the development of two probes;
a current measuring probe and a voltage
measuring probe. Obviously, he felt that one
needed to know at least two parameters for a true
understanding of conducted interference. The
current probe is not only a measure of EMI
current, it is a measure of the magnetic field
radiation from the wire or cable under test. This
is a more meaningful measure of magnetic
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