An experienced person tells you how to choose a common mode noise filter

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An experienced person tells you how to choose a common mode noise filter [Copy link]

This article shares how to select common mode filters and examples of improving reception sensitivity in smartphones.

Selection of common mode filters

The method for selecting a common mode filter is described; in principle, there are two steps: not attenuating digital signals and attenuating noise.

First, in order to pass the digital signal without loss, a step with low insertion loss in differential mode is selected. As shown in the figure, the shaping of the digital signal waveform requires at least 3rd to 5th harmonic components, so a cutoff frequency of 3×fo or 5×fo or more with the fundamental frequency fo of the digital signal is usually selected.

In addition, in recent years, in order to compensate for the loss of high-frequency components, interfaces that perform output-side reinforcement and receiving-side compensator processing are more common. At this time, if there are no problems in actual use after passing the prescribed compliance test, it may be adopted at frequencies below 3×fo.

Matching of differential mode characteristic impedance

Another important parameter in differential mode transmission is the characteristic impedance of the differential mode. (Characteristic impedance) Transmission lines are usually designed based on the characteristic impedance specified in the impedance specification. For example, for USB, the characteristic impedance is 90Ω, and for HDMI, it is specified as 100Ω.

If the common mode noise filter deviates greatly from the characteristic impedance within the transmission band, reflection and loss of the differential signal will occur, resulting in signal degradation.

The figure shows the comparison results of the HDMI-1080p-Eye mask test with two filters with the same common mode cutoff frequency of 3GHz but different characteristic impedance.

Compared with a product with a characteristic impedance of 100Ω, the signal of the filter with a characteristic impedance of 80Ω is visibly degraded. In this way, it is also necessary to pay attention to the characteristic impedance matching in the differential mode.

Methods for evaluating the characteristic impedance matching include TDR measurement that evaluates on a time axis and S-parameter that evaluates on a frequency axis.

By calculating the S-parameter, the frequency-axis characteristic impedance information of the common mode filter can be obtained.

In TDR measurement, a step pulse is input from the connector side, and the characteristic impedance of the differential mode can be measured by observing its response. If there is a part with a characteristic pulse different from the step pulse with the specified characteristic impedance that has been input in the measured line, reflection and loss will occur at this part. It is reprocessed into the value of the characteristic impedance and returned as information on the line. The advantage of TDR characteristics is that the characteristic impedance can be observed together with the position information in the state of assembling the components, and the unmatched parts on the substrate can be grasped.

Differential mode characteristic impedance matching by TDR measurement

The following is an example of TDR measurement, showing the TDR characteristics of our common mode filter related to HDMI impedance. It can be seen that the characteristic impedance information of the connector, common mode filter, HDMI-IC, and their respective parts can be obtained.

It can be seen that our common mode filter fully meets the characteristic impedance of 100Ω+/-15Ω of the HDMI transmission line and has excellent matching in differential transmission.

Next, based on the common mode noise attenuation frequency characteristics, select a product that matches the target frequency band for common mode noise elimination.

As shown in the figure, our company has products that require high attenuation in the wireless communication band for smartphones, as well as products that require attenuation of broadband noise such as HDMI, and can provide the optimal filter according to the application.

Example of improving reception sensitivity on smartphones

The following is an example of measuring the cellular reception sensitivity of a smartphone when our common mode filters are used.

This is an example of configuring a common mode filter in the Mipi interface of an LCD to improve reception sensitivity by suppressing radiation from flexible wires.

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