Use of common mode chokes in high speed data links
Introduction
Common mode chokes are widely used in high speed serial data transmission, especially when the transmission medium is a cable connecting two sub-systems. They are used to reduce electromagnetic radiations from the cable and to help obtain compliance with regulatory requirements. Since the common mode chokes are directly in the data path, their electrical characteristics can impact the performance of the differential signal transmitted through the cable. This paper outlines the construction, the electrical properties and the common uses of common mode chokes, providing hardware engineers with information for selecting the correct components for a given application.
High Speed Serial Data Links
The FPD-Link II and III from National Semiconductor are serial interfaces commonly used in automotive Infotainment sub-systems. The DS90UB901Q/DS90UB902Q and DS90UB925Q/DS90UB926Q are two serializer/de-serializer chipsets offering FPD-Link III serial interface with a high speed forward channel and a bi-directional control channel for data transmission over a single differential pair. Common mode chokes are sometimes used in the shielded twisted pair cable connecting the serializer and the de-serializer sub-systems. Figure 1 shows a typical FPD-Link III interface implemented with the DS90UB901Q and DS90UB902Q.
Figure 1. Typical FPD-Link III Interface with the use of DS90UB901Q and DS90UB902Q
Construction of Common Mode Chokes
Common mode chokes appear in many forms. A ferrite clamp wrapped around a cable forms a common mode choke that becomes part of the cable. Many suppliers1 produce miniature common mode chokes with physical sizes as small as a 0805 or even a 0603 resistor designed for printed circuit board mounting.
A common mode choke is built with two identical wires symmetrically wound on a ferrite core. Figure 2 shows the construction of a 0805-size common mode choke. The structure is a pair of highly symmetrical mutual inductors. The equivalent circuit diagram is shown in Figure 3.
Figure 2. Typical Construction of a Surface Mount Common Mode Choke
Figure 3. Equivalent Circuit Diagram of a Common Mode Choke
Characteristics of Common Mode Chokes
An ideal differential signal is defined by a pair of signals with equal amplitudes but exactly 180 degrees out of phase. When a differential signal is applied to a common mode choke, each signal creates magnetic flux that is equal but opposite polarity. The magnetic fluxes cancel each other and therefore the choke appears to be transparent to the differential signal. In reality, there will be a small reduction in the signal amplitude due to circuit parasitic.
Similarly, a common mode signal is formed by a pair of signals with equal amplitudes and exactly in-phase. When a common mode signal is applied to a common mode choke, the signals create reinforcing magnetic flux in the ferrite core and increase the choke’s impedance. Therefore the choke provides large attenuation to the common mode signal.
In simple terms, a common mode choke is a magic component that presents high impedance and provides attenuation to reject common mode signals, while presenting low impedance and introducing a small insertion loss to the differential signal that passes through the device. Figure 4 depicts the properties of a common mode choke pictorially.
Figure 4. Electrical properties of a Common Mode Choke
Specifications of Common Mode Chokes
Suppliers usually specify common mode chokes in terms of their common mode impedance values at a reference frequency. Chokes with larger common mode impedance provides common mode attenuation at a lower frequency. Table 1 shows the specifications of the DLW21S series of common mode chokes from Murata. They are designed for high speed serial links such as HDMI or FPD-Link II and III.
Table 1. Typical Specifications of Common Mode Chokes
A better way to describe a common mode choke’s characteristics is via its frequency response plots. Figure 5 depicts the choke’s common mode insertion loss. It shows the choke’s ability to attenuate the unwanted common mode signals at frequencies in the stop band. The common mode signals in the pass band will not be attenuated.
Figure 6 and 7 show the choke’s differential mode insertion loss and the return loss. These are the parameters that are important to the differential signal passing through the choke. The differential insertion loss determines the usable bandwidth, while the return loss determines if the choke will preserve the characteristic impedance of the transmission medium that connects to the choke.
Figure 5. Common Mode Insertion Loss
Figure 6. Differential Mode Insertion Loss
Figure 7. Differential Mode Return Loss
Use in Emission Mitigation
Differential signaling strives for symmetry in both the complimentary signals and the transmission medium that carries the signals. Differential signal source are designed to achieve good matching in the amplitude, the rising and falling transition times, propagation delays and loadings. In the real world with physical constraints, there is always some amount of mis-match that makes the differential source or the transmission medium a little unbalance. Unbalance in the differential signals results in the creation of common mode signals. Depending on the shielding effectiveness of the equipment enclosure and the cable harness, a small amount of the unwanted common mode signal radiates to the air as electromagnetic emission.
Common mode chokes, by virtue of their properties in attenuating common mode signals but leaving differential signal unaffected, is an effective remedy to reduce electromagnetic radiation by reducing the common mode signal source of the radiation. As shown in Figure 8, a common mode choke (L1) placed before the connector of a Transmitter board is used to reduce the unwanted common mode signals before they reach the cable harness.
Figure 8. A typical High Speed Serial Data Link
Use in Interference Mitigation
To ensure reliable operation among electronic sub-systems, they are tested for compliance to the electromagnetic compatibility (EMC) requirements. Some of the commonly used EMC tests are Bulk Current Injection (BCI) and radio frequency interference (RFI). BCI test induces a large amount of common mode signal on the cable harness through the use of a current injection probe. RFI testing involves the use of an antenna to deliver strong interference signal to the sub-system under test.
Depending on the shielding effectiveness of the cable harness, some amount of the electromagnetic interference may appear at the input pins of a high speed differential receiver as common mode interference signal. If the common mode disturbance is excessive, a common mode choke can be placed near the receiver’s input pins to filter the common mode interference signals.
Use in Cable Unbalance Mitigation
High speed differential signaling requires good symmetry between the two wires in the cable used for transmitting the differential signal. The factors affecting performance depend on the matching of the physical dimensions such as wire length and wire diameter, the material properties of the conductors and their dielectric insulations, and the characteristic impedance of each conductor. The cable’s construction highly influences the mutual coupling between the two conductors, which determines the cable’s characteristic impedance. When the mutual coupling varies a little along the length of the cable, as in the case of a twisted pair cable, their electrical properties varies along the wires and impacts the cable’s degree of balance.
A differential signal transmitted through a well-balanced cable can maintain the differential properties of equal amplitudes and opposite polarities. Any unbalance results in mode conversion, in which a small amount of the differential signal is converted into common mode signal. Along the length of the cable, there are multiple mode conversions between the differential and the common mode signals. At the end of the cable, the presence of the unwanted common mode signal distorted the differential signal and appears as unequal amplitude and not exactly opposite polarities. These impairments are commonly called amplitude mis-match and intra-pair skew.
Figure 9 illustrates the amplitude mis-match and intra-pair skew at the end of a 10-meter shielded twisted pair cable.
A common mode choke placed at the end of the cable, reduces the amount of common mode signal, and improves the cable’s balance. Figure 10 illustrates the ability of a common mode choke in restoring the differential signal to equal amplitudes and negligible intra-pair skew.
Figure 9. Distorted Differential Waveforms after a 10m Unbalance Cable
Figure 10. Restored Differential Waveforms with a Common Mode Choke
Performance Trade-off
A common mode choke provides an effective tool for hardware engineers to improve emissions, system robustness to interference, and cable imbalance. An ideal choke is one with very high differential bandwidth so that there is no negative impact to the differential signal, and it also has attenuation for the lowest frequency common mode signal that may occur in the system. Figure 11 depicts the frequency response of an ideal common mode choke.
Figure 11. Frequency Response of an Ideal Common Mode Choke
Common mode chokes are not perfect and their use may sometimes contribute slight performance degradation for the differential signal. One contributor for the performance degradation is the differential insertion loss added by the common mode choke. Higher insertion loss introduces slightly higher inter-symbol interference jitter. Another contributor for the performance degradation is the incomplete filtering of common mode signals at lower frequencies. Common mode choke is effective to improve a cable’s intra-pair skew for higher frequency differential pulses, but less effective for very low frequency pulses. Incomplete filtering of common mode signals may introduce small amount of inter-symbol interference jitter caused by the disparity in the group delay between the lower frequency and higher frequency pulses.
Selecting Common Mode Chokes
Depending on the pixel clock frequency, the DS90UB901Q transports parallel video data into serial bit stream up to 1.2Gb/s, while the DS90UB925Q transports parallel video data into serial bit stream up to 2.975Gb/s. For common mode rejection consideration, fCCmin should be set to the pixel clock frequency, which represents the lowest frequency common mode signal introduced by the mode conversion from the rare occurring lowest frequency pulses. For differential bandwidth consideration, fDDmax should be set to half the serial bit rate, which represents the shortest duration pulses in the serial bit stream.
Common mode chokes come in many forms, physical sizes and impedance values. The choice of the proper chokes depends on two main criteria: maximizing the attenuation of the unwanted common mode signal, and minimizing the performance degradation to the differential signal. To maximize the attenuation of the unwanted common mode signals, it is important to understand the frequency of the unwanted common mode signal, and the amount of attenuation needed. In general, higher common mode attenuation at lower frequency is achieved with the use of higher common mode impedance and usually associated with lower bandwidth and higher insertion loss for the differential signal. To minimize the performance degradation to the differential signal, the common mode choke should have small insertion loss at the maximum speed that the receiver will operate. Return loss should be high (< -20dB) without adversely affecting the cable’s characteristic impedance.
The frequency characteristics in Figures 5-7 show Murata’s DLW21SN121HQ2 is sutiable to support DS90UB925Q/926Q FPD-Link III interface at 1.5-2.975Gb/s and provides attenuation to common mode signals starting at about 30MHz. Similarly, Murata’s DXW21BN7511S is suitable to support FPD-Link III interface at 0.5-1.0Gb/s and provides attenuation to common mode signal starting at about 15MHz.
Layout Considerations
High speed layout techniques should be used to minimize the parasitic capacitance of the landing pads and the choke’s body to the ground plane below the component. Minimizing parasitic capacitance supports the goal of maximizing the differential bandwidth with minimum insertion loss.
Figure 12 shows an example of a printed circuit board layout for a 0805 common mode choke. Ground relief is used in the ground plane underneath the choke for reducing parasitic capacitance to ground.
Figure 12. Printed Circuit Board Layout of a Common Mode Choke
Summary
A common mode choke is a component with the ability of attenuating undesirable common mode signals but letting differential signal to pass through with little negative impact. Electromagnetic emission and interference are caused by the presence of unwanted common mode signals. Cable unbalance introduces unwanted common mode signals that impact amplitude mis-match and intra-pair skew for the differential signals. By suppressing the unwanted signals, common mode chokes are widely used as remedy in aiding EMC compliance and balancing unbalanced cables. Users should understand the frequency characteristics of the common mode choke, and select the proper component that provides adequate common mode attenuation and adequate differential bandwidth to support the transmission of the differential data.
Acknowledgement
The author would like to thank Murata in providing their common mode chokes characteristics and approval in using the graphics provided by Murata.
About the author:
Tsun-kit Chin is member of Technical Staff, Applications Engineering, High Speed Products Division, National Semiconductor Corp. E-mail: Tsun-Kit.Chin@nsc.com
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