LISNFILTER

A Calibrated and Safe EMI Solution for Overpowering Signals

The CE Mark and Military Standard 461 process now requires that just about everything be measured and controlled for EMI interference. Most notably appliances, laboratory instruments, military and medical devices are using switcher supplies and power factor correcting circuits to meet the latest standards of Mil Std 461F, Cispr 11, 22 and EN 61000-3-2. On the commercial side, since the frequency of the conducted emissions standard starts at 150 KHz for Europe and 450 KHz for FCC, these manufacturers typically control only as much as necessary to bring their devices into compliance leaving the lower frequencies uncontrolled. This only makes sense because there is expense in mass production to add EMI components and/or use larger component values unnecessarily.

During the measurement phase and typically with line impedance stabilization network (LISN) measurements of high power devices of 750 watts or more, a condition can be reached such that the low frequency emissions from the equipment under test can overpower the spectrum analyzer generating harmonic spurs at multiples of the switcher frequency. This will add into the noise of the EUT (equipment under test) and can easily lead to a conclusion the EUT has failed a conducted emissions test. To compensate and correct this, resistive pads and filters may be used to discriminate against the low frequency energy. However if the match is not good to the load, the switcher may behave nonlinearly forcing a change in harmonics that can lead to serious inaccuracies in the measurement. Although this is one situation many others may occur where it is desired to discriminate against low interfering signals so that signals of interest may be safely measured.

Thus the problem exists to 1) discriminate against the low frequency energy aggressively but still provide an excellent resistive match, 2) protect the analyzer or measuring device against large signals that can potentially damage the front end or cause erroneous results, 3) match the loads as well as possible so that harmonics of the switcher or characteristics of the signal source stay the same versus frequency, 4) provide a wideband pass to the signal of interest so that they may be analyzed by sensitive EMI receivers, 5) guard against catastrophic failure of components in the LISN which can allow line power into the measuring apparatus following it, 6) provide an inexpensive and easily replaceable component to be used whenever the characteristics of the signal source are not known.

After trying many alternatives in a lab setting, a new tool has emerged that takes care of all those problems and provides as well a very portable solution that can be used during investigation or final test. This so called LISNFILTER design provides the following advantages to the user:

1. Reduction of the signal level by approximately 21 dB from input to output
2. Brick wall filter taking the noise down an additional 30 dB below the passband (150 or 10 KHz).
3. Fuse protection of the input that can be easily replaced if the signals become too high
4. Greater than 20 dB return loss on input and output to provide an excellent match to the LISN and the measuring equipment following it. Excellent 50 ohm match.
5. TVS protection of the output so that ESD, EFT, Surge and other one shot events do not cause damage the receiver following the filter. The TVS device does not become effective unless the power rating exceeds one watt at the output of the filter.
6. Type N connections on input and output for easy interface into most measuring devices
7. Very miniature size of only about 2.2 by 2.2 inches excluding the connectors
8. Low cost and portable which can easily fit in a tool pouch.
9. Very wideband operation to 600-1000 MHz depending on the model

Although primarily made for EMI measurements, the design can be adapted to many other uses. Each design is serialized and comes with an individual network analyzer plot for use in correction factor files. We can repair devices on short notice. There are both 150 and 10 KHz versions available now in the same size.

For further information contact:

Midwest EMI Associates
21234 W Commercial Drive, Mundelein, Illinois 60060.
Website: www.midemi.com, info@midemi.com.

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