1. What is Radio Frequency Interference (RFI)?

RFI is unwanted electromagnetic energy in the frequency range generally used for radio communications. For conducted phenomena, the frequency ranges of interest are 10kHz. For radiated phenomena, it’s 30MHz to 1Ghz。

2. Why Be Concerned with RFI?

People must concern themselves with RFI for two reasons: (1) Their equipment must operate properly in the application environment, however it always in the presence of significant levels of RFI, and (2) Their equipment must not emit RFI to make sure not to interferes with RF communications which often vital to health and safety. The necessity for reliable RF communications has given rise to legal regulations ensuring RFI control for electronic equipment

3. What are the Modes of Propagation of RFI?

RFI is spreaded via radiation (electromagnetic waves in free space) and by conduction over signal lines and AC power systems.
Radiated - One of the most significant contributors to radiated RFI from electronic equipment is the AC power cord. The power cord is often an efficient antenna since its length approaches a quarter wave length for the RFI frequencies p resent in digital equipment and switching power supplies.
Conducted - RFI is conducted over the AC power system in two modes. Common mode (asymmetrical) RFI is present on both the line and neutral current paths with reference to the ground or earth path. Differential mode (symmetrical) RFI is present as a voltage between the line and neutral leads.

4. What is a Power Line Interference Filter?

With the fast development of today’s world, more electrical energy is being generated at increasing levels of power, and more and more low power energy is being used for the transmission and processing of data. The result is caused increasing noise or interference, which can disrupt and even destroy your electronic devices.
A power line interference filter is a main tool to control conducted RFI both into the equipment (potential equipment malfunction) and out of the equipment (potential interference to other system Elements or RF communication). By controlling the RFI conducted onto the power cord, a power line filter also contributes significantly to the control of radiated RFI.
A power line filter is a multiple-port network of passive components arranged as a dual low-pass filter; one network for common mode attenuation, another network for differential mode attenuation. The network provides attenuation of RF energy in the stop band of the filter (typically above 10kHz), while passing the power current (50-60Hz) with little or no attenuation.

5. How Does a Power Line Interference Filter Work?

As passive, bilateral networks, Power line interference filters have complex transfer characteristics, which are extremely dependent upon source and load impedance. The magnitude of this transfer characteristic describes the attenuation performance of the filter. In the power line environment, however, the source and load impedance's are not defined. Therefore the industry has standardized practices to verify filter uniformity through measurement of attenuation with 50ohm resistive source and load terminations. This measurement is defined to be the Insertion Loss (I.L.) of the filter:
I..L. = 10 log * (P(l)(Ref)/P(l))
P(l)(Ref) is the power transferred from the source to the load without the filter;
P(l) is the power transferred when a filter is inserted between the source and load.
The Insertion Loss may also be expressed in terms of voltage or current ratios as shown: I.L. = 20 log *(V(l)(Ref)/V(l)) I.L. = 20 log *(I(l)(Ref)/I(l))
V(l)(Ref) and I(l)(Ref) are measured without a filter ;
V(l) and I(l) are measured with a filter.
It is important to note that Insertion Loss does NOT describe the RFI attenuation provided by a filter in the power line environment. In the power line environment the relative magnitudes of the source and load impedances must be estimated and the appropriate filter configuration selected such that the greatest possible impedance mismatch occurs at each termination. This dependence of filter performance on terminating impedances is the basis for the concept of "mismatching networks".

6. How to Perform Conducted Tests?

Conducted testing requires a quiet RF environment (usually a shielded enclosure) with a line impedance stabilization network, and an RF voltage measurement instrument such as a tuned receiver or spectrum analyzer. The RF ambient of the test environment should be at least 20dB below the desired compliance limit to get accurate results. The line impedance stabilization network (LISN) is required to establish a defined source impedance for the power line input. This is an important part of the test procedure, since this impedance directly affects the measured emission levels. The correct bandwidth for the measurement receiver is a critical test parameter as well.