Common Mode Interference and Radio Reception

The high switching frequency of current data lines has made common mode (CM) interference a problem for radio reception. In normal or differential (single) mode, current travels on one line in one direction from the source to the load and in the opposite direction on the return line. This completes the circuit. However, in CM, the noise current travels on both lines in the same direction. Inductors create magnetic fields that oppose changes in current. An electrical filter, the CM choke, blocks the high frequency noise common to two or more data or power lines while allowing the desired DC or low-frequency signal to pass.

CM noise current is typically radiated from sources such as unwanted radio signals, unshielded electronics, inverters and motors. If left unfiltered, this noise presents interference problems in electronics and electrical circuits. Key players in the global CM choke market include AKEMET Corporation, EPCOS, Murata, Bourns, Schaffner, Pulse Electronics, TDK, Schurter, Abracon, API Delevan, Cooper Industries, Eaton, FASTRON Gmbh, Ferroxcube, Halo Electronics, Knitter Switch, KOA Speer Electronics, Laird, Littelfuse, PulseR, LLC, Sumida, Taiyo Yuden, RECOM Power, STMicroelectronics, TOKIN Corporation, Triad Magnetics, TT Electronics and Welwyn Components.

Würth Elektronik Presents WE-CCMF Ceramic CM filter

In May, Würth Elektronik announced the offering of a compact multilayer CM choke. The choke is characterised by high CM attenuation in the >30 dB at 2450 MHz WiFi frequency range. “The ceramic common mode choke WE-CCMF achieves excellent damping precisely in this range of 2.45 GHz. The LTCC design ensures low losses and high reliability. The reflow solderable SMT component has a high temperature resistance. The push-pull cut-off frequency of the choke is 8 GHz,” said the company. The ceramic choke is described as ‘the solution’ to improve the sensitivity of wireless LAN and Bluetooth-enabled devices (notebooks, laptops, routers, IoT devices) that also have interfaces with high-speed push-pull signal transmission. These include USB 3.1/3.2, SATA 3.0/3.2, HDMI 1.4/2.0 or Thunderbolt 2/3.

Coilcraft High-Speed and ‘Super-Speed’ Data Line EMI chokes

Coilcraft UK Ltd, a global supplier of magnetic components including RF chip inductors, power magnetics and filters, offers its RA6870-AL CM Choke developed, for infotainment applications, and its CM1394 Series IEEE1394 CM Choke. These high-speed and Super-Speed data line chokes effectively reduce common mode noise in high-speed interfaces. “They maintain excellent signal integrity for high-speed communications with -3 dB differential mode cut-off frequencies up to 6.5 GHz,” says the company. They provide greater than 30 dB common mode attenuation at 500 MHz and 25 dB in the GHz band.

Coilcraft CJ5100, CQ7584, and CR7856 surface mount data line CM chokes, meanwhile, are designed to attenuate common mode noise up to 100 MHz. The PDLF Series can reduce noise by a factor of 32 from 15 MHz to 300 MHz and are available in 2, 3 and 4 line versions. The PTRF Series is optimised for FCC and ITU-T (formerly CCITT) requirements. Coilcraft LPD, MSD and PFD families, meanwhile, are low profile, miniature footprint common mode chokes that can be used to attenuate common mode noise or differential mode noise in both data and power line applications. Coilcraft CMT toroid style common mode chokes are designed to provide the highest common mode impedance over the most extensive frequency range. “These parts are ideal for any application requiring a high DC current bias and are well suited for use in switch-mode power supplies. These common mode chokes are most effective in filtering supply and return conductors with in-phase signals of equal amplitude,” says the company.

Signal Transformer’s New Series of CM Toroidal Chokes

Signal Transformer, a Bel company and a leader in the custom design and manufacturing of linear transformers and SMD inductors, recently announced its new HCTC series CM toroidal chokes. It was created for design engineers and manufacturers specifying power supplies accommodating common-mode noise reduction and line filtering. It features a solid carrier base for stable vertical printed circuit board (PCB) mounting and fixed-pin spacing for easy PCB insertion. It is said to offer high current capacity and excellent electromagnetic interference (EMI) suppression.

Acal BFi in Partnership with Magnetec for Nanocrystalline Cores

Acal BFi, the provider of advanced technology solutions, recently announced a new pan-European partnership with the magnetic component supplier Magnetec. “The accord will provide customers across Europe with access to an enhanced portfolio of high permeability common mode chokes, with nanocrystalline cores and the direct design and integration support of Acal BFi’s expert engineers, “ said the company. The Magnetec CM choke range joins Acal BFi’s portfolio of magnetic components, which include both standard and custom solutions from all significant material classes, including Ferrites, Advanced Powder, Iron Powder, Nanocrystalline and Amorphous cores. Magnetec’s CM chokes offer proprietary nanocrystalline alloy material, Nanoperm, with high permeability and low losses at high frequencies. The material is made into a range of core shapes and sizes, enabling designers to use less copper wire in their designs, thus reducing copper losses, component size, and weight and material costs.

Replacing CM Chokes with Monolithic EMI Filters

One of the problems with CM chokes, though, is that they are heavy, expensive, and subject to vibration induced failure. Another disadvantage is the limited frequency range due to parasitic capacitance. Compared to CM chokes, monolithic EMI filters provide significantly more RFI suppression in a substantially smaller package. A monolithic EMI filter also rejects a much wider frequency band and is not affected by the amount of DC current required because it is mounted in shunt (between lines and ‘ground’). In April, STMicroelectronics unveiled its integrated common mode filter (CMF) and ESD suppression in new automotive-protection devices. The ECMF04-4HSM10Y and ECMF04-4HSWM10Y CMFs are claimed to be the first CMFs on the market with automotive-grade qualification and automotive surge compliance guaranteed.

“The filters replace discrete CM chokes or LTCC (low-temperature co-fired ceramic) devices that are typically larger and provide no ESD protection,” says the company. They are placed on high-speed data lines for camera, radar, display, multimedia and other connections to prevent interference with wireless-communication equipment. The ECMF04-4HSM10Y has 2.2GHz differential bandwidth to suppress common-mode noise on HDMI 1.4, MIPI, and connections. Deep attenuation, reaching -25dB at 900MHz and -14dB at 1.5GHz, prevents radiated noise from reducing the sensitivity of cellular and GPS antennas. The wider 3.5GHz bandwidth of the ECMF04-4HSWM10Y extends the application range to LVDS, DisplayPort™, USB 3.1, and HDMI 2.0 buses.