Conformity Magazine - September 2008 - (Page 42)

The common mode filter is a passive component constructed with 2 parallel windings over a ferrite material core. Some of the critical characteristics to be considered in high speed application are: • Characteristic impedance, similar to the TMDS (in the range of 100 Ohm) in the case of HDMI; • Its insertion loss (which provides the EMI emissions characteristic reduction for a specific frequency range); There is also the possibility of choosing between wire wound and thin film technology (multilayered type) which typically provide similar electrical characteristics. However, the thin film type provides more design space flexibility, due to its small profile (see Figure 7). The Common Mode Filter as a Noise Suppressor and Signal Integrity Solution For purposes of this analysis, we will use the HDMI application to showcase the effect of a common mode filter in the improvement and countermeasure of typical noise and signal integrity issues often encountered in high speed interfaces designs. As mentioned earlier, HDMI signal transmission system utilizes a method called TMDS (transmission minimized differential signal), which is a variation of the LVDS2 method (see Figure 8). The HDMI test specification 1.3 defines specific criteria for signal integrity and impedance matching conditions for HDMI circuit through the compliance test specification (CTS) document. The CTS specification breaks down the test into its major system components (source, medium, sink and protocol).1 In the following cases, special test setup and fixtures have been used to obtain real-life validation of the solutions (which is the intention of this article), and will be referring mainly to the source, and sink areas of the system (see Figure 9). EMI Suppression One common source for EMI noise generation can be the internal signal clock. For example, at 480p resolution (clock=27MHz), the EMI level might be very low. However, if the resolution increases to 1080p (HDTV resolution with clock=148.5 MHZ), the EMI level increases considerably. This is one of the main challenges that the designer faces in the PCB design. The main characteristic to be considered for the common mode filter is the cutoff frequency (typically specified at -3db). In the case of HDMI, -3 dB at 6 GHz is the target specification. A wire wound or thin film technology CMF can provide similar performance (see Figure 10). By choosing the correct common mode filter as shown in Figure 11, the high frequency harmonics can be adequately suppressed. During the testing of this solution, the effect of the noise suppression can be observed in the following EMI scan curves (with and without the use of the common mode filter). For simplicity, we only present the horizontal scan curves (see Figures 12 and 13). Circuit Representation Wire Wound Type Thin Film Type Figures 7a, b and c: Common mode filter symbol and types Figure 8: HDMI TMDS basic block diagram2 2 Conformity September 2008

Table of Contents Feed for the Digital Edition of Conformity Magazine - September 2008

Conformity Magazine - September 2008 Contents Editor’s Note Commission Adopts Regulations for Internet-Based TRS Services Another Company Fined for Non-Compliance with HAC Requirements FCC Extends “Do-Not-Call” Registry Compliance Requirements Marketing Company Gets $1.4 Million Fine for Junk Faxes EU Commission Codifies Directive on TTE Market Competition EU Commission Releases Updated Standards List for the R&TTE Directive Updated Standards List for the “New” Low Voltage Directive Released New Standards List Released for the EU’s Machinery Directive The ESD Association: Voluntary Standards in Action ESD Open Forum Medical EMC Standards: Towards the 4th Edition of IEC 60601-1-2 iNARTE Informer Pre-Pulse Voltage in the HBM ESD Model Digital Interfaces and EMC Performance ESD Design Buyer’s Guide Product News Commission Publishes Updated Standards List for the EU’s Personal Protective Equipment Directive FDA Links “Human Factors” to Adverse Events with Medical Devices CPSC Actions in the News IEC Standards Update UL Standards Update Product Reviews Telcordia Standards Update From Our “You Can’t Make This Stuff Up” Department Looking Back: Items from Past Issues of Conformity Advertisers

Conformity Magazine - September 2008

For optimal viewing of this digital publication, please enable JavaScript and then refresh the page. If you would like to try to load the digital publication without using Flash Player detection, please click here.