APPLICATION
The Solar Type 6220-1B Audio Isolation Transformer was especially designed for screen room use in making conducted audio frequency susceptibility tests as required by MIL-STD-461/462 and other EMI specifications.
The transformer may also be used as a pickup device to measure low frequency EMI currents at lower levels than conventional current probes.
INTRODUCTION TO SOLAR’S AUDIO ISOLATION TRANSFORMERS
by Solar Electronics Company founder A.T. Parker, circa 1962
“There is more than one way to skin a cat,” your great-grandfather 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 World War II created an artificial line impedance which represented what he had measured on the DC buss in a twin engine aircraft (probably a Boeing DC-3, but memory is dim on this point). Watton’s work was sponsored by 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.
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 suppressing 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 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 measurement receivers designed for rod antennas and 50 Ω 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 impedance which varied with frequency, or he could measure EMI current flowing through a circuit of unknown RF 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 radiation.
The current probe was somewhat better than the LISN for measurements below 150 kHz and above 25 MHz but, even so, the technique was not very sensitive at the lower frequency end of the spectrum.
A young Boeing EMI engineer named Frank Beauchamp was the first to apply the current probe to wideband measurements from 30 Hz to 20 kHz. He was smart enough to realize some of the problems in this range so he incorporated the sliding current probe factor into the method of measurement he spelled out in the Minuteman Specification, GM-07-59-2617A. The test method required that the probe factor existing at 20 kHz should be used for obtaining the wideband answer in terms of “per 20 kHz” bandwidth. This meant that the specified limit was not constant throughout the 20 kHz bandwidth, but was varying as the inverse of the probe factor. A very sensible solution at the time. Regrettably, later specifications did not follow this lead.
When later EMI specifications extended the need for measurement of EMI currents down to 30 Hz without taking into account the sloping probe factor, the problem of probe sensitivity became critical. Attempts to compensate for the poor current probe response at low frequencies by using active element suffer from dynamic range difficulties and the possibility of overload.
This led to another way of “skinning the cat,” with the aid of the audio isolation transformer already available and in use for susceptibility testing. The technique described in the following paragraphs indicates how to obtain considerably greater measurement sensitivity for conducted narrowband EMI currents and a means for obtaining a flat frequency characteristic without the use of active elements for broadband or “wideband” EMI current measurements.
BASIC CONCEPT
The application described herein has grown out of a suggestion by Sam Shankle of Philco Ford in Palo Alto. He and his capable crew first tried this scheme using H-P Wave Analyzers as the associated voltmeter. Our work with the idea has concentrated on conventional EMI meters with 50 Ω inputs.
Basically, the test method consists of using the secondary (S) of the Solar Audio Isolation Transformer as the pickup device. The transformer winding normally used as the primary (P) is used as an output winding in this case. The method provides a two-to-one step up to further enhance the sensitivity.
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