EMI Control Applications Notes
For Typical Computer Subsystems

Video Controllers & Cables

Introduction

High performance electronic monochrome and color image display has become a standard feature of many modern commercial products. Personal computers, video arcade games, and motion picture special effects equipment provide brilliant and crisp images that previously could be captured only through still photography. To render images in millions of colors at rates of up to 76 complete frames per second, these products must use timing circuits and operating frequencies from as low as 30 MHz to over 130 MHz. This characteristically high frequency architecture requires painstaking design effort to minimize the wide bandwidth EMI noise sources that are inherent to video subsystem design. Steward ferrite products can provide cost effective EMI suppression and signal integrity control to maximize system performance and minimize potential video signal EMI. This application note provides a brief outline of critical video signal and subsystem characteristics as well as suggestions for the proper use of EMI suppression ferrites in video circuits.

 

Design Considerations: General Description Of Signals

Successful design of high resolution video controllers requires achieving two seemingly contradictory goals: 1) providing an optimally fast analog output signal to the video display in order to construct a crisp image, and 2) providing sufficient high frequency roll-off beyond the passband of the video signal to reduce high frequency EMI source strength. If left unattenuated, the high frequency content of the output signal will capacitively and inductively couple into nearby circuits, causing radiated EMI test failures via external cables not associated with the video subsystem.

Modern high resolution video monitor design requires the use of large gain, wide bandwidth amplifiers with RF outputs of up to 50 volts peak to peak in desktop displays, and up to 150 volts peak-to-peak in projection systems. Since high performance shielded enclosures are difficult and costly to design for large, semi-RF transparent cathode ray tubes, it is critical to reduce the high frequency content of the video signal at the video controller, before it is transported to and amplified within the display.

The video digital-to-analog converter (VDAC) changes digital information into a high speed analog output in a timing format determined by the choice of pixel clock frequency. A typical video controller's analog output signal conforms to RS-343-A standards and is continuously valued from 0 to 1.0 volts or 0 to 0.716 volts peak-to-peak, for channels with or without synchronization information, respectively. The VDAC output in turn drives an external cable at the system characteristic impedance, usually 75 ohms.

The necessary bandwidth of a video signal is determined by the number of horizontal and vertical lines of resolution and the number of frames per second to be displayed. The smallest possible unit of information that can be displayed is called a pixel (an acronym for picture element). The resolution and pixel size of a given video subsystem determine the minimum video signal rise and fall times necessary to display acceptably sharp images. Many papers in the high resolution display literature cite a rule of thumb that states that a video signal's rise and fall times should be no greater than 1/3 to 1/2 of time necessary to illuminate a single pixel (often called a pixeltime) on a display. As shown in Figure 26, a pixeltime corresponds to one period of the pixel clock oscillator.

 

Design Example:

A typical high resolution personal video subsystem with 1024 horizontal lines by 768 vertical lines of resolution uses a pixel clock of approximately 75 MHz. The optimum rise and falltime for the video output signal is T0/2 for color display, or 2/(F0) = 6.66 nanoseconds, where F0 is the pixel clock frequency. As described in the "EMI Filtering" section of this application note, a combination of ferrite beads and capacitors may be used to slow the VDAC output to this speed for optimum EMI control.

Video Controller Component Placement & EMI Filtering

A simplified schematic of the output section of a video controller is shown in Figure 27. Most modern VLSI VDACs use high performance current sources to drive the video signal into an external video cable. The transmission line formed by the video controller analog signal path, the external video cable, and the monitor input is doubly terminated in 75 ohms. Since the output impedance of a current source is close to infinity in magnitude, the 75 ohm source terminating resistor R1 must be placed immediately adjacent to the red, green and blue (or monochrome) outputs of the VDAC to prevent signal reflections. A Steward ferrite bead Part Number(s) HI1812N121R-10 or LI1806E101R-10 (shown as component L2 in Figure 27) may be used in conjunction with shunt filter capacitors (47 pF or less) at these outputs to provide high frequency filtering.

To prevent undesirable impact jitter and visual noise patterns from appearing at the display, most VDAC manufacturers specify the use of a ferrite bead and capacitors to filter the VDAC power input pins. Since the pixel clock oscillator may drive excessive high frequency noise on to the video controller power pins or on to the video signal output through the parasitic capacitance of the VDAC IC die, many EMC engineers also specify Steward ferrites to filter the input power pin of the pixe clock oscillator. A Steward ferrite bead Part Number 28L0138-10R-10(shown as component L1 in Figure27) may be used for this application.

Radiated EMI Due To The External Video Cable

As discussed previously, many video related EMI problems can be avoided through the proper layout and filtering of the video controller output circuitry. However, many video subsystems will fail radiated EMI tests due to the presence of common mode current on the outer surface of the shields of the external video cable, as shown in Figure 28. Since the external cable is usually a significant fraction of a wavelength in overall length at many video frequency harmonics, it provides an efficient antenna structure for even the smallest common mode currents.

These common mode currents are often induced by poor cable shield terminations at the video cable plugs, monitor connection, or video controller interface connector. Additional causes include the use of poor quality video cable with low braid coverage and poor shielding effectiveness.

EMI Filtering Solutions For The External Video Cable

Electromagnetic theory dictates that the intended video signal current (on coaxial cable) flows on the inner conductor and the inside surface of the cable shield. Since equal and opposite currents exist inside the highly conductive cable shield, no net magnetic field (due to the intended signal) exists on the outside of the cable shield. If a ferrite cylinder is placed around the cable, it will "see" only the magnetic field associated with the common mode current that causes radiated EMI. The ferrite will thus insert a large lossy impedance in series with the EMI noise current, while not interfering with the intended high frequency video signal. Since ferrites are a cost effective alternative to expensive high performance video cable and connectors, many personal computer, monitor, and video cable assembly manufacturers select Steward ferrites to solve video cable related EMI problems.

Power Supplies

Introduction

Advances in the field of power supply design have led to dramatic reductions in the size, weight, and improvement of the energy efficiencies of electronic products. Most modern power supplies are generally described as either linear mode or switched mode, according to the method by which standard household and commercial AC line input voltage is transformed into a DC output voltage suitable for use by miniaturized electronics. Simplified block diagrams of both design topologies are shown in Figure 29.

The second major source of EMI that is actively generated in switch mode supplies is associated with the switching behavior of the rectifier diodes located in the converter transformer secondary circuit. These rectifier diodes are selected to pass the full output load current (often tens of amperes) and efficiently switch at twice the switching frequency for full wave rectification. During operation, the rectifier diodes can transition from forward bias (on) to reverse bias (off) in less than 35 nanoseconds, thus inducing a large, fast transient voltage impulse or "spike" in the highly inductive power supply secondary circuit. These transient events will increase the strength of any EMI from the first ten switching frequency harmonics, thus increasing conducted EMI.

 

 

Using Steward Ferrites To Reduce Radiated EMI On The AC Power Cord

A second effect of the rectifier diode transients is the excitation of high frequency oscillations or "ringing" in the transformer secondary circuit. In the frequency domain, the characteristics of the impulse waveform and additional ringing represent a broad band EMI signal that may have significant energy beyond 30 MHz. Since most AC line filters are designed for EMI attenuation below 30 MHz, any rectifier noise above 30 MHz that couples back through transformer capacitance may easily pass through the filter and be radiated by the AC line cord. The resulting AC power cord EMI can be significantly attenuated if a Steward ferrite cylinder is placed around the phase and neutral wires between the AC line filter and the input terminals of the power supply, as shown in Figure 30.

Note that for products with three wire single phase AC input (not double insulated), the preferred design does not pass the green/yellow safety wire through the cylinder, since the wire must be connected to chassis and hence zero EMI voltage exists between it and chassis. If the green/yellow wire must be considerably longer than two inches, then it also should be passed through the cylinder to prevent common mode current flow on this conductor. In all cases, the cylinder should be located as close to the AC input connector as possible, to prevent noise from coupling ahead of the ferrite and passing directly out through the AC line cord.

Applying Steward Ferrites To Reduce EMI Induced By Diode Switching

In some instances it may be possible to reduce switch mode diode EMI by "softening" or slightly slowing the edges of the switching waveform. This can be accomplished by placing a small ferrite bead over one lead of each rectifier diode, as shown in Figure 31. At the beginning of a diode transition from "off" to "on," the ferrite will present a high impedance, thus initially slowing the time rate of change of the current that passes through the diode. As the current continues to increase, the ferrite quickly saturates and "disappears" from the circuit. The modified switching waveform has slightly slower rise and fall times and therefore less power at its higher order harmonic frequencies. Other solutions utilizing series combinations of resistors and capacitors ("snubber" circuits) may benefit from the addition of ferrite beads to provide enhanced high frequency filtering.

Using Steward Ferrites To Reduce Passively Coupled EMI From Other Circuitry

The most common and difficult power supply radiated EMI problems result from the coupling of noise from nearby high speed devices back into the power supply. The noise may enter the power supply as a voltage or current that is conducted along the power supply output conductors, or through electric and/or magnetic field coupling. Once it has entered the power supply, the noise often appears as radiated EMI on the power supply AC line cord, or on the supply's DC output wires, as shown in Figure 32.

To greatly reduce passively coupled power supply radiated EMI, a Steward ferrite cylinder can be installed on the power supply AC input conductors as discussed in the previous section and shown in Figure 33. Since a significant amount of EMI may be directly injected into the power supply through its DC output cabling, power supply EMI can also be reduced by additionally installing a Steward ferrite cylinder on these conductors, as shown in Figure 33. For maximum effectiveness at high frequencies, the cylinder should be located as close to the noise generating circuitry as possible.

Small Computer System Interface (SCSI): Introduction

The Small Computer System Interface (SCSI) is among the most popular of industry standards for data storage and retrieval. SCSI controller circuitry is widely used on personal computers and desktop workstations. Almost all hard disk, floppy disk, tape drive, and optical storage device manufactures offer integral SCSI data interfaces for devices with storage capacities from 1.44 Mbytes to over 1000 Mbytes.

Like most industry standards, SCSI is being constantly changed and improved. At present there are three distinct physical and software implementations of SCSI referred to simply as SCSI-1, SCSI-2, and SCSI-3. SCSI-1 is a fully mature standard (ANSI X3.131-1986) that is supported by present day computers and storage devices. This application note primarily addresses the functionality and EMI considerations for SCSI-1, although most of the concepts and techniques presented will apply equally well to SCSI-2 and SCSI-3.