EMC Filters/EMI Filters / RFI Filters Frequently Asked Technical Questions

he answer to this question is a very “qualified” yes.

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 3^rd , 5^th and 7^th 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.

To learn more details on our filters, or download the Application Note, that include instruction on how to install them, click here.

See JMK’s Insertion Loss Page.

Insertion loss is quite simply a measure of the effectiveness of an EMI filter.  Insertion loss is defined as the ratio of voltages across the load without the filter inserted in the circuit and with the filter inserted in the circuit.

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.

When a filter has capacitors connected between the conductors and ground, small amounts of current flow through the capacitors to ground. This current can create an electric shock hazard if the system is improperly grounded.

This current is limited by the international safety agencies to prevent a danger to personal safety.

Hi-pot or High Potential testing is also known as dielectric-withstanding testing. The testing is performed by applying a known potential or voltage between the power lines or between a power line and the case to expose any electrical deficiencies in the construction of the EMI filter. As part of our testing, all JMK products are tested for dielectric withstanding (Hi-Pot) between the power lines (DM) and between the power lines and the case (CM).

Safety agencies define limits for the maximum leakage current, operating temperature rise, dielectric strength and the mechanical design to protect end users.

EMI tests are performed using passive networks called LISNs, Line Impedance Stabilization Network, connected in series with the power lines to the equipment. The LISN establishes a consistent impedance to allow for repeatability of test results. Conducted emissions are measured via an RF connection to a port on the LISN. Radiated emissions may are measured using an antenna or current detecting probe.

The 50 Ohm insertion loss data which appears in our catalog represents an industry standard to allow comparison of different manufacturers’ products against a known standard. This is generally the impedance used to test conformance of a system to various military and other EMI standards. In practice, filters are source and load impedance sensitive. The actual load impedance varies greatly from system to system. It is therefore often easier to test the filter and system to determine the actual combined performance.

The proper selection of an EMI filter encompasses the entire electromechanical configuration of the filter. The filter’s mechanical footprint including mounting and terminations may impact the effectiveness of the filter in your system. Primary considerations also include leakage current, insertion loss, rated voltage and current, and agency approvals required for proper usage in the end application. We do not anticipate the standard catalog products to fulfill every application and welcome the opportunity to customize any product to best suit your needs. For additional information, please reference our electronic or print catalogs.

An EMI filter has a high reactive component to its impedance. That means the filter looks like a much higher resistance to higher frequency signals. This high impedances attenuates or reduced the strength of these signals so they will have less of an effect on other devices.

An EMI filter is a passive electronic device used to suppress conducted interference present on any power or signal line. It may be used to suppress the interference generated by the device itself as well as to suppress the interference generated by other equipment to improve the immunity of a device to the EMI signals present within its electromagnetic environment. Most EMI filters include components to suppress both common and differential mode interference. Filters can also be designed with added devices to provide transient voltage and surge protection as well as battery backup.

The EMC Directive 89/336/EEC mandates that all electronic equipment must comply with the applicable EN specification for EMI. Some typical EN specifications follow:

Electronic Equipment	Spec.
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

Conducted and radiated emissions for computing devices are regulated by Part15, Subpart J of the US Federal Communications Commission, (FCC). The limits for conducted and radiated emissions are based upon the end application of the device.

Class A: Commercial, Industrial and Business Applications
Class B: Residential Applications

EMI is regulated to allow today’s sensitive equipment to function properly without suffering degradation in performance due to interference generated by other electronic devices. The EMI spectrum is a limited natural resource that must be maintained to allow reliable radio frequency communications. The successful regulation of EMI interference will allow future electronic devices to operate as defined, in the intended environment, without suffering any degradation in performance due to interference, and without disrupting the performance of other equipment.

Differential mode interference, a.k.a. symmetrical interference, is a noise signal which exists between the line and neutral conductors.

Common mode interference, a.k.a. asymmetrical interference, is a noise signal which is found in phase on both the line and neutral conductors with respect to ground. Common mode noise also typically has equal amplitude on both line and neutral conductors.

The most common offender in the radiation of EMI is the electrical power cord of the electronic device itself. Since the power cord can act as an antenna, conducted EMI can also become radiated interference.

Typical sources of conducted interference include switching power supplies, ac motors, and microprocessors. In short, just about any electrical and electronic device has the potential to generate conducted and radiated interference.

EMI or RFI propagate through conduction over signal and power lines and through radiation in free space.

Radio frequency interference, RFI, is any undesirable electrical energy with content within the frequency range dedicated to radio frequency transmission. Conducted RFI is most often found in the low frequency range of several kHz to 30MHz. Radiated RFI is most often found in the frequency range from 30MHz to 10GHz.

Electromagnetic interference, EMI, is any undesirable electromagnetic emission or any electrical or electronic disturbance, man-made or natural, which causes an undesirable response, malfunction or degradation in the performance of electrical equipment.