EMC Filters/EMI Filters / RFI Filters Frequently Asked Technical Questions
Can an EMI filter help meet RE102 of Mil-Std 461?
The standard EMI filter is designed to filter conducted emissions over the frequency range of 10KHz to 10MHz. This would lead one to think that the filter will not help much for the RE102 requirements.
This is true unless the filter is electrically and mechanically designed to attenuate the higher frequencies.
The power cord is often the major source of RE102 emissions. The good news is that this is generally an easy problem to solve. There are a few things that are necessary to implement the solution.
- The enclosure should be metal and “well-sealed”. Essentially you want to have a Faraday cage.
- The power line needs to have a filter that will block or send the high-frequency signals to a low impedance ground plane.
- An absolute requirement is to use feed through capacitors in the filter. These low impedance devices shunt the high frequencies to ground. They also block the propagation of HF signals along current-carrying wires bypassing the wires through interior bulkheads that present orifices small enough to block signals below 20 GHz.
The figure below shows a possible installation of a Mil-Std-461 filter, but there are many more configurations that will also work. The key thing is that the input to the filter must be completely isolated from the output of the filter. JMK has many of these filters. The filters can also be modified to meet almost any installation.
Is it possible to build a filter to enable a system to meet the CE101 conducted emissions of Mil-Std-461?
We have built such filters and they are not particularly economical or efficient. The problem with designing a filter for a system to meet CE101 is the low frequency of the noise signals involved. Typically, the 3rd , 5th and 7th harmonics are the problem frequencies. This means that for 60 Hz power the filter must attenuate 180, 300.and 420 Hz at a minimum. In the case of 400 Hz power, attenuation of 1200, 2000 and 2800 Hz will be necessary.
These filters will involve large components and some damping may be necessary, resulting in power loss. The best approach is to select a power supply which incorporates active harmonic attenuation. The harmonic attenuation will, indeed, add noise back on the incoming power lines. This noise will need to be attenuated but the frequencies are much higher, and the filter components required are much smaller.
The cost of filtering the active harmonic attenuation is considerably lower and the efficiency does not drag down the overall power factor. Our recommendation is to avoid trying to meet CE101 without harmonic attenuation built into the power supply.
This approach allows standard design approaches for the power supply and filter with reasonable size and cost.
How To Install Mil-Std-461 and Tempest EMI Filters?
How are the various insertion loss limits measured?
What is insertion loss?
Filter performance is also dependent on the input and output impedance seen by the filter. Insertion loss is therefore measured with 50 ohm source and load impedances as a standard. While this provides a standard for comparison, it does not necessarily reflect the actual performance of the filter within your system.
Since filter performance is specific to the attached system, JMK maintains a test facility to test the individual needs of our customer’s power supply to select or design an appropriate filter. Normally such testing is offered by JMK free of charge.
What is leakage current?
This current is limited by the international safety agencies to prevent a danger to personal safety.
What is Hi-pot testing?
What aspects of filter design and performance are regulated by safety agencies?
How are EMI tests performed?
How does the 50 Ohm data compare with actual performance in my system?
What criteria are used to select the proper EMI filter?
How does an EMI filter work?
What is an EMI filter?
What standards apply to EMI and filters in the EU?
|Industrial, scientific and medical equipment||EN55011|
|Broadcast receivers and associated equipment||EN55013|
|Electrical motor-operated and thermal appliances for household and similar purposes, electrical tools and similar apparatus||EN55014|
|Electrical lighting and similar apparatus||EN55015|
|Information technology equipment||EN55022|
What are the applicable US requirements?
Class A: Commercial, Industrial and Business Applications
Class B: Residential Applications