RFI Filter Technical Considerations

Meeting Emissions Standards
The emission limits that a piece of equipment must meet will depend on the intended market for that piece of equipment. If more than one market is being aimed at, more that one emission standard may have to be met. This can have a substantial effect on the circuit, size, and cost of a filter. Standards like VDE0875 or the FCC Rules Part 15 Subpart J have low frequency limits of 150 kHz and 450 kHz respectively.

Equipment meeting these specifications can utilize a filter with a fairly high cutoff frequency. Other standards like VDE0871 with a low frequency limit of 10 kHz will result in the equipment using lower cutoff filters. As might be expected, the lower the cutoff frequency, the larger the physical size and the higher the cost of the filter.

Conducted RFI Susceptibility
The problem of susceptibility can be extremely difficult to deal with because the amplitude and frequency of the offending RF noise are seldom known and are oftem intermittent. If the malfunction can be duplicated by isolating the equipment from the power line with LISN's (Line Impedance Stabilization Network) and using signal generators to inject RF of varying amplitude and frequency, some insight can be gained as to the nature of the problem. However, the criteria fro acceptable performance will have to be decided upon so that a filter yielding this level of performance can be obtained from the test procedure. Unfortunately, this still does not eliminate the need for final testing in the actual operating environment which, in many cases, occurs in the field.

Selection of a suitable filter can best be based on the type of power supply or input impedance of the equipment and on the mode of the offending RFI noise.

Noise Modes
The RFI voltages that must be reduced by the line filter can be broken down into two components. The common mode or line-to-ground noise is that common part which is measured between the power lines and ground potential. The differential mode or line-to-line noise is that part which exists between the lines of the power mains. Power line filters are designed to take care of either one or both components of the noise. The need for one design over another will depend on the magnitude of each noise type present. The selection guide can identify which of our products has been designed for each noise type.

Circuit Configuration
Power line RFI filters are generally built with two or three-pole filter networks. As the number of poles and the corresponding component count increases, the cost will increase also. Tryign to typify an equipment's impedance as either high or low for purposes of filter selection may not be successful. If it is a complex impedance, it would probably be low at some frequencies and be high at others and be some intermediate value at still other frequencies

Although we have been generally successful in recommending a two-pole network for linear power supplies and three-pole networks for switching power supplies and synchronous motors, you should not limit your testing to just one circuit type if either additional circuit performance or lower cost is desired. Consider the following: If the equipment looked strictly capacitive, the performance of a two-pole network would be reduced to that of a single-pole filter.

Obviously a three-pole filter would be preferred for maximum performance. Likewise, if the equipment looked struictly inductive, the performance of a three-pole network would be reduced to that of a two-pole network.

Undoubtedly the two-pole filter would be a more economical choice with probably equal performance in this application. Since the equipment is not likely to be equivalent to either one of these simple cases, the only way to find the best cost-effective solution is to test the filters in your equipment and base your judgement on these these test results.

Leakage Current
The maximum leakage current that a device is allowed depends on the requirements of the particular safety agency involved. Here, selection of the filter is quite easy since either the filter is designed to meet a given level or it is not. Although there is no compromise when it comes to safety specifications, it should be understood that for a given level of performance, as the leakage current is reduced, the physical size of the package will increase.

Insertion Loss
DO NOT use the insertion loss specifications to make your final decision. Power line filters are two-terminal pair passive networks whose attenuation characteristics can be defined by a complex transfer function. How that transfer function will react in a particular system and at specific frequencies will depend on the complex impedances connected to each side of the filter. The equipment impedance and the impedance of the power line, even if a 50 ohm LISN (Line Impedance Stabilization Network) is being used during emission testing, will not generally be equal to the resistive 50 ohms used during insertion loss measurements. Therefore, the performance of the filter in the equipment cannot be related to the published insertion loss data.

Minimum Insertion Loss
Do not be alarmed that the insertion loss figures we have published may be of lower value than those of our competition. You will only find guaranteed minimum insertion loss figures in this catalog, without any mention of typical values.

Insertion loss test data measured in a 50 ohm system is a valueable incoming inspection tool to assure you that consistent product is being shipped. The only figures of any importance are those that specify the criteria for acceptance or rejection of that product, and those figures are the minimum values.