Ignoring all the poles and zeroes in the filter textbooks: filters work by creating an intentional discontinuity in the characteristic impedance of a current path, reflecting radio frequency (RF) energy away from a protected circuit, or absorbing the RF energy (converting it to heat) – rather like a shield does, as will be described in Part 4 of this series.
The greater the discontinuity, the greater the attenuation. So if the source impedance of an unwanted signal (noise) is 100Ω and we put a 1kΩ impedance in series with it, only about 10% of the signal gets through the high impedance – an attenuation of around 20dB. A similar effect can be created by instead connecting the 100Ω noise to the ‘RF Reference’ via an impedance that is much lower than 100Ω: for example, 5Ω would provide an attenuation of around 26dB.
Filters use electronic components such as resistors (R), inductors (L), and capacitors (C) to create the desired impedance discontinuities over the ranges of frequencies of concern. R, L, or C can be used as filters on their own, but combining them gives better attenuation. LC types can give better attenuation than RC types, and are often used in power circuits because of their lower losses, but all LC filters are resonators that can produce gain at some frequencies, so they need to be carefully designed, taking their actual source and load impedances into account, to ensure attenuation over the desired range of frequencies. RC types generally provide more reliable filter performance.
A range of basic schematics exists for low-pass filters based on R, L and C, and is shown in Figure 1. There are high-pass equivalents, and band-pass or notch filters can also be achieved with passive components like these – but the low-pass filter is the one that is mostly used for EMC so that is the type that is shown in Figure 1 and discussed in this article.
Simple inductive filters (chokes, ferrites, etc.) have no RF Reference connection, so are especially useful where no RF Reference Plane exists, or if it exists but does not have a structure that provides a low enough impedance at the highest frequencies of concern. Unfortunately, such very simple filters are generally unable to achieve very high attenuations – typically between 3 and 20dB, depending on the frequency.
Capacitors can also be used on their own as very simple filters (by creating a ‘high-to-low’ impedance discontinuity), or as part of a more complex filter circuit that includes inductors and/or resistors. But the effectiveness of a capacitor filter depends upon the impedance of the RF Reference it is using as its ‘ground’, and also upon the impedance of the interconnection between the capacitor and the RF Reference (e.g. wire leads, PCB traces). As a result, manufacturer’s data sheet figures for capacitive filters are rarely achieved in real-life because they were tested with RF Reference Planes that were solid copper sheets covering an entire bench-top, and so had a lower impedance than is usually possible in real life.
Many a well-designed and expensive filter has had its performance wasted by being connected to a poorly performing RF Reference, or by being bonded to an excellent Reference by a short length of wire instead of the direct metal-to-metal contact that was needed.
An example of a common use of RCR filters is to connect computer boards to displays via flexible circuits, to reduce the emissions from the ‘flexi’. The resistor values in these filters are often chosen as much for transmission-line matching (see section 2.7 of [6]), as they are for filtering.
Filters must pass the wanted signals/power, while attenuating unwanted ‘noise’. So filter specification must begin with knowledge of the full spectrum of the wanted signal or power. It is very common these days for the spectrum of a wanted signal to contain very high frequencies that are not required, caused by the very fast switching edges of modern digital and switch-mode devices. Analogue signals are also polluted with such noise, due to stray coupling from digital and switch-mode circuits nearby. These very high frequencies can be removed by filtering and/or shielding, and it is good EMC practice to remove them at their sources, rather than wait until they have polluted many more conductors, and this was discussed in section 1.1.2 and Figure 1B of Part 1 of this series [7].
Active filters can be designed, based upon operational amplifiers (opamps), using feedback techniques to achieve remarkable attenuations. But the phase-shifts inherent in all opamps converts the attenuation of feed back circuits into amplification, above some frequency. So unless you have the experience and skills to really know what you are doing, and unless you are using op-amps with gain-bandwidth products measured in many GHz – always use passive filters based on Rs, Ls and Cs to control frequencies above 1MHz.
