Conformity Magazine - September 2008 - (Page 45)

Signal Quality Improvement for Distorted Signals Another issue often encountered is called “undershoot” and “overshoot” problems in the TMDS signal. Sources of this problem can be impedance mismatching, capacitances introduced by ESD devices, stray capacitances due to PCB layout, etc. The HDMI criteria for under and overshoot is: Overshoot < 15% for 2*Vswing Undershoot < 25% for 2*Vswing Looking at Figure 16, the target value for the overshoot is less than 15%, and the undershoot value less than 15%. The waveform in its current state measured an overshoot of 38% and undershoot of 25% at 1080i video resolution mode (74.25MHz clock) and data transmission of 742.5Mbps. The over and undershoot can be significantly reduced with the use of a common mode filter. This brings the overshoot down to 14%, and the undershoot down to 12%. Impedance Compensation The use of a common mode filter in the differential pair will reduce the skew in these signals. This corrective measure is due to the generation of a negative voltage in the opposite line of where the skewed line is, bringing both signals into synch (see Figure 20). Impedance mismatching in the path of the differential signals (through the IC, PCB traces, connectors, cables etc) might result in signal reflections, thus affecting the signal integrity. The HDMI compliance test specification describes the applicable measurement method through its TDR (time domain reflectometry) measurement. The specified criterion is to maintain the 100 Ohm +/- 15% through the signal travel path (these limits shown in the chart as UL and LL values). The addition of a common mode choke can offset the mismatching condition due to the added inductance (see Figure 17). This particular test was performed by inducing an impedance mismatch in a test PCB, measuring the mismatch condition, and then using a common mode filter to improve the matching. Skew Compensation Skew in the differential signals is normally presented in the form of a delay in one of the differential signals (D+ and D-). Their switching point is not at the center line, as shown in Figures 18 and 19. Figure 17: Reflectometry diagram showing the effect of the common mode filter Comprehensive EMI Solutions, On Time & On Budget From design, quick-turn protos, and worldwide ISO9001 manufacturing, we are the superlative EMI solutions provider. The cornerstones of our mission are simple: Quality. Integrity. Service. Value. It is this commitment to total solutions without compromise that has led to our established position as a premier designer, manufacturer & distributor of EMI filters and magnetic products. When measured, the skew in the signal can be observed as in Figure 19. Furthermore, if we attempt to do the mathematical average of both signals, the resulting common mode amplitude is not zero. Plug into our comprehensive solutions. Radius Power is a leader in the design & manufacture of power products used to condition, regulate and govern electronic performance. The company provides a broad line of EMI Filters, Power Entry Modules, Power Transformers & Inductors to meet front-end requirements for AC & DC power line applications. We offer free pre-compliance testing for conducted emissions at our labs or yours! Bring us your EMI challenge & we will provide an optimized solution usually within 48 hours. Call Radius today. 714-289-0055 PH 714-289-2149 FAX 1751 N. BATAVIA ST. ORANGE, CA 92865 www.radiuspower.com September 2008 Conformity 5 http://www.radiuspower.com http://www.radiuspower.com

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

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