Suppress Noise and EMI in Your PCB With the Right Analog Filter Design

April 19, 2018 Altium Designer

A simple low pass filter

 

The electronics industry continues to pack more capabilities onto smaller PCBs and devices are being run at lower power and at higher frequencies. Noise suppression becomes even more important as operating frequencies rise and signal levels fall, this becomes more manageable with an EMI PCB filter design. Adding filtration to your PCB designs can enhance signal integrity in EMI-prone environments with large stray magnetic fields and in low power RF applications.

 

Industry standards even require that your device include noise suppression, EMI, and EMC filtering capabilities. To fulfill conducted emissions standards, EMI noise must be suppressed at frequencies from 150 kHz to 30 MHz. Some products have more stringent standards and the lower limits start at 9 kHz. IoT applications require ripple filtering at 1 MHz to maintain data and signal integrity.

Active vs. Passive Filters

One of my first PCB designs required building a device for measuring external low-frequency signals. My first run resulted in a scrambled mess of data points when I expected my measurements to be pretty consistent. Soon, I found the culprit: my low-quality power supply output a voltage with significant noise. Rather than spring for a major power supply upgrade, I was able to solve this problem by designing a filter directly on my PCB.

 

Even if you followed all the best design techniques for noise suppression and EMI reduction, your design may still be susceptible to noise. To further improve signal integrity, active and passive filtering methods can be used to reduce both EMI and EMC. Before selecting which filters will be used in your PCB, always test your filter designs and ensure that the filter meets the applicable noise reduction and EMI standards for PCBs.

 

Passive filters use the impedance of standard electronic components to prevent noise in circuits at certain frequencies. Active filters combine passive filtration components with powered components like amplifiers or transistors. Active filters can also be packaged as a surface-mounted device with a small footprint.

 

Before designing EMI PCB filter or noise suppression, you need to know something about the frequency bands you are trying to filter from your signals.

 

PCB designed for microwave applications

PCB designed for microwave applications

 

A simple example of an active filter is the first-order low pass active filter. A low pass RC filter can be connected to a non-inverting operational amplifier. This topology is also applicable to a bandpass or high pass filter. Second order active filters have a more complicated design. Third and higher order filters are easily constructed by daisy chaining multiple first and second order filters in series, and these filters a provide steeper cutoff at the filtering band edges.

 

The main advantage of using an active filter is the gain that can be provided. Amplification can be applied by including feedback and pulldown resistors on the inverting input.

 

The small footprint of op-amp ICs allows powerful filters to be placed on your PCB, leaving plenty of leftover real estate for other components. The drawback to active filters is that op-amps have high attenuation at high frequencies, and active filters can only be used in lower frequency applications.

Passive Microstrip Filters

Microstrip traces can be used to build passive filters that are embedded directly into the PCB. The center frequency and bandwidth can be adjusted based on the geometry of the microstrip. These filters are easy to manufacture, but they tend to have a larger footprint than other passive filters.

 

Analyzing these filters is also rather simple as their geometry allows them to be modeled as a circuit of inductors and capacitors. If circuit analysis comes naturally to you, then these filters can be quickly reduced to an equivalent circuit and you can figure out the formulas for the filtering properties by hand.

 

Different microstrip geometries and layouts will function as a bandpass, low pass, or high pass filter. Genuine high-pass filters are extremely difficult to fabricate using distributed microstrip elements. One way to form a high pass filter is using a band-pass design with an extremely high bandwidth and upper cutoff frequency. Filters that seem to have a high pass topology turn out to be bandpass filters when their high-frequency behavior is analyzed.

Power Supply Filters

DC power supplies typically convert AC to DC power using a rectifier circuit with a smoothing capacitor. The power supply output may contain some residual ripple voltage, even if the power supply contains built-in filtering. The residual ripple voltage can be suppressed by designing a simple passive power supply filter.

 

Linear regulators can suppress much of the low-frequency ripple voltage from a power supply, but they lose effectiveness for noise components above about 10 kHz. Higher frequency components in the 100 kHz range can be suppressed with an LC filter. Filtering even higher frequency components in the MHz range can be accomplished by placing bypass capacitors among the ICs.

 

High voltage power supply

Keeping in mind the right voltage requirements will help you manage EMI

 

Filtering voltage ripple and its higher order harmonics up to 1 MHz becomes important in IoT devices. During data transmission in IoT devices, the data gets sent to a baseband module that encodes data into a 1 MHz signal. This 1 MHz signal will be mixed with the carrier signal in the RF transmitter module. Removing voltage ripple and noise up to MHz frequencies maintains signal and data integrity during wireless transmission.

PCB design software like Altium Designer makes it easy to add noise suppression capabilities to your device. Their extensive component libraries and rules-based design interface make designing your filters a breeze. Talk to an Altium expert today to learn more.

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Altium Designer

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