EMI has been the bane of many designs and will affect your product’s ability to pass EMC certification tests. Your design needs to have some resistance to noise, as well as emit low noise to be compatible with FCC and EU requirements. Suppressing EMI radiated to external devices can be a difficult task, while suppressing received EMI requires an EMI filter circuit design on some critical components.
The two principle types of time-invariant linear filter circuits are passive and active EMI filter circuits. These circuits can be placed on a PCB as discrete components, or they can be lumped into an integrated circuit. Many SoCs include an integrated EMI filter circuit design that is specifically designed for a particular application. Let’s look at how these different types of filters work and how they provide resistance to EMI.
As the name suggests, passive filters make use of passive circuit elements to provide filtration over broad or narrow frequency bands. These filters are easy to construct as higher-order filters without requiring active devices. This keeps power consumption low and component costs low. These filters can also be cascaded to provide multi-band filtration (i.e., for a wideband antenna). As long as you are operating below the self-resonance frequencies for the discrete components in your EMI filter circuit design, your filter’s behavior can be reliably predicted using the basic series and parallel impedance equations.
The simplest way to place this type of EMI filter circuit in your board is with capacitors and inductors. These components will provide reactive impedance to your system and can be tuned to desired frequency ranges simply by adjusting the component values. These circuits are quite useful for removing EMI from specific frequency ranges. Some common applications include removing radiated EMI from a noisy clock or from another analog component that emits strongly at specific frequencies. The table below shows some common configurations. Any of these circuits can be easily analyzed with SPICE simulators.
Wideband EMI filter circuit design with inductors and capacitors
Although arranging passive components in series/parallel configurations is quite easy from a design standpoint, the operation of these filters may not always be intuitive, especially when you are aiming for multiband filtration. Take a look at the image below for an example where mutual resonance between a multiband filter creates an additional resonance. In this filter design, there is a desired stopband that needs to be filtered, and there is a pass band. The filter is cascaded using linear elements. The problem with this type of filter is that the two sections of the filter can mutually resonate, which creates a second passband in the transfer function and the output voltage. This can be clearly seen in the output voltage (purple curve). The stopband can be seen in the cyan curve and is centered around ~1.4 MHz.
Example cascaded bandpass/bandstop EMI filter circuit design created in Altium Designer.
They type of filter shown above is still useful as an EMI filter circuit for removing noise between 1 and 2 MHz. In particular, if you know there is a noise source near ~1.4 MHz, that noise will experience ~100 dB attenuation in the desired stop band. You can then use one of the two passbands to pass your desired signal and even apply gain. This is one way to provide some filtration and gain on the output a transceiver, mixer, antenna, or other RF component using only passive linear elements (note: be careful with impedance matching!).
More complex filter circuits can include op-amps to provide linear or non-linear broadband filtering. These types of circuits are not band-limited in the same ways as a passive EMI filter circuit design. Instead, the active components can be the limiting factor governing EMI suppression as they have some well-defined bandwidth.
Active filters are constructed using op-amps or more complex circuitry, although op-amp circuits are most common. The great thing about these circuits is they can be easily configured as higher order filters for common mode and differential mode noise. This provides a simple way to filter out near-field EMI in the design using discrete components with small footprint. Note that the near-field distinction is critical here as EMI will be spread across the entire circuit in this regime. The design below provides common mode and differential mode noise suppression over a broad bandwidth by simply using some capacitors and resistors along with an op-amp.
EMI filter circuit design with op-amp and RC elements. Image source.
Designs like this are easy to analyze; take a look at the reference link to see how to analyze the above EMI filter circuit design. At the ultra-high frequency level (i.e., microwave and mmWave frequencies), parasitics in your components and board start to become critical. Your passives will stop behaving as ideal components due to self-resonances; your capacitors will start exhibiting inductive impedance, and your inductors will start exhibiting capacitive impedance. Be sure to check the self-resonance frequencies of components when designing EMI filters to eliminate strong high frequency noise from specific sources.
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