Electronics Guide

Termination Adjustments

Termination modifications are among the most common and effective corrective actions for addressing reflection-related signal integrity issues. Reflections occur when signals encounter impedance discontinuities, and proper termination matching can significantly reduce these effects.

Series Termination Optimization

Series termination places a resistor near the source to match the driver's output impedance to the transmission line characteristic impedance. When reflection problems are identified, the series termination resistor value can be adjusted to achieve better matching. Common adjustments include:

• Increasing series resistance when undershoot or ringing is observed at the receiver, indicating insufficient damping
• Decreasing series resistance when signal rise times are excessively slow or when the first incident wave amplitude is too low
• Fine-tuning based on TDR measurements that reveal the actual source impedance accounting for driver characteristics and PCB trace effects
• Replacing fixed resistors with resistor networks that provide better tolerance matching

Parallel Termination Modifications

Parallel termination places a resistor at the receiver end of the transmission line to absorb incident energy and prevent reflections. Corrective actions for parallel termination include:

• Adjusting termination resistor values to better match measured characteristic impedance rather than nominal values
• Converting from simple pull-up or pull-down termination to Thevenin termination for better DC and AC matching
• Adding AC coupling capacitors in series with termination resistors to reduce DC power dissipation while maintaining AC termination
• Implementing split termination networks when single-ended termination creates unacceptable power dissipation
• Placing termination resistors closer to receiver pins to reduce stub effects that can cause impedance mismatches

Differential Termination Adjustments

Differential signaling requires both differential and common-mode termination. Corrective actions include:

• Adjusting differential termination resistor values to match the measured differential impedance of the transmission line pair
• Adding common-mode termination to suppress common-mode noise and improve electromagnetic compatibility
• Implementing center-tap termination with bypass capacitors to ground to address both differential and common-mode components
• Replacing single differential resistors with resistor networks that provide tighter matching between positive and negative signals

Termination Placement Optimization

The physical location of termination components significantly affects their effectiveness:

• Moving termination resistors closer to receiver pins to minimize stubs that create secondary reflections
• Placing series termination resistors closer to the source when package parasitics affect source impedance
• Using via-in-pad or minimal via stubs for termination connections to reduce parasitic inductance
• Implementing on-die termination (ODT) when package and board parasitics prevent effective discrete termination

Routing Modifications

PCB routing changes address signal integrity issues related to trace geometry, coupling, and discontinuities. These modifications range from minor length adjustments to complete re-routing of critical nets.

Length Matching and Timing Adjustments

Timing-related issues in buses, differential pairs, and high-speed interfaces often require length modifications:

• Adding serpentine delay sections to lengthen shorter traces and achieve matched propagation delays within bit groups
• Shortening longer traces by finding more direct routing paths while maintaining impedance control
• Adjusting serpentine meander geometry to achieve target delays without creating excessive crosstalk between adjacent segments
• Implementing different length matching strategies for different data rate domains when multiple interfaces share board space
• Correcting phase misalignment between clock and data signals through controlled length adjustments

Differential Pair Routing Corrections

Differential signaling requires careful pairing and matching to maintain signal quality:

• Tightening coupling between differential pair traces to improve common-mode noise rejection and reduce mode conversion
• Increasing separation between loosely coupled pairs when measurements show excessive differential-to-common-mode conversion
• Correcting skew between positive and negative signals through selective length adjustment of one trace
• Maintaining parallel routing through difficult areas such as connector regions and via transitions
• Eliminating asymmetric routing patterns that create impedance imbalances between the two signals

Crosstalk Reduction Through Routing Changes

When crosstalk measurements or simulations reveal excessive coupling, routing modifications can provide significant improvement:

• Increasing spacing between parallel signal traces, particularly for long parallel runs
• Reducing parallel run length by rerouting signals to cross at angles rather than running parallel
• Inserting ground traces or guard traces between aggressor and victim signals to provide shielding
• Moving sensitive signals to different layers with better shielding or isolation
• Implementing orthogonal routing on adjacent layers to minimize broadside coupling
• Rearranging pin assignments to separate high-speed aggressors from sensitive analog or low-speed signals

Return Path Optimization

Return path discontinuities cause impedance variations and increase electromagnetic emissions:

• Adding via stitching to connect adjacent ground planes where signals cross split planes
• Rerouting signals to avoid crossing plane splits entirely, particularly for high-speed differential signals
• Adding return path vias immediately adjacent to signal vias for layer transitions
• Eliminating void areas under high-speed traces that force return currents to take longer paths
• Correcting trace routing that crosses isolated plane islands or narrow plane necks

Stub Length Reduction

Via stubs and component pin stubs create resonances and reflections at high frequencies:

• Using back-drilling to remove unused via stubs extending below the lowest connection layer
• Implementing blind or buried vias to eliminate through-hole via stubs
• Converting from via-to-pad connections to via-in-pad to minimize stub length
• Shortening test point stubs or removing unnecessary test points from high-speed nets
• Minimizing component pad sizes to reduce effective stub lengths while maintaining assembly reliability

Stackup Changes

PCB stackup modifications affect impedance, crosstalk, power delivery, and electromagnetic compatibility. Stackup changes are typically implemented during design phase but can sometimes be incorporated in design revisions.

Impedance Control Adjustments

Stackup modifications to achieve target impedance values include:

• Adjusting dielectric thickness between signal layers and reference planes to correct measured impedance errors
• Modifying trace widths in conjunction with dielectric thickness to achieve impedance targets while meeting minimum feature size requirements
• Changing copper weights when thinner copper allows narrower traces with better impedance control
• Adding or removing prepreg layers to adjust the distance to reference planes
• Implementing differential layer pairs with tighter coupling when measured differential impedance is too high

Layer Assignment Optimization

Reassigning signals to different layers can address multiple signal integrity challenges:

• Moving highest-speed signals to outer layers adjacent to plane layers for better impedance control and lower loss
• Relocating sensitive signals to inner layers surrounded by planes for better electromagnetic shielding
• Separating aggressor and victim signals onto non-adjacent layers to reduce crosstalk
• Implementing orthogonal routing on adjacent signal layers to minimize coupling
• Reserving dedicated layers for specific signal types such as clocks, high-speed serial links, or analog signals

Reference Plane Improvements

Power and ground plane configuration significantly affects signal integrity:

• Adding ground plane layers to provide better return paths and shielding for high-speed signals
• Converting split ground planes to solid planes where measurements show return path problems
• Implementing dedicated power planes for noise-sensitive circuits rather than sharing planes
• Adjusting plane layer positions to be immediately adjacent to critical signal layers
• Adding plane layers to reduce plane-to-plane cavity resonances that affect power integrity

Stackup Symmetry

Symmetric stackups reduce warpage and provide better manufacturing control:

• Balancing copper distribution between top and bottom halves of the stackup to prevent warpage
• Mirroring dielectric layer thicknesses around the board centerline
• Using symmetric prepreg configurations to improve registration between layers

Material Substitution

PCB material selection affects loss, dispersion, impedance stability, and cost. Material changes can resolve signal integrity issues when routing and termination adjustments are insufficient.

Low-Loss Dielectric Materials

High-frequency signal loss often necessitates upgrading to better dielectric materials:

• Replacing standard FR-4 with low-loss laminates for designs operating above 10 Gbps or frequencies above several GHz
• Selecting materials with lower dissipation factor (Df) to reduce dielectric losses at high frequencies
• Using materials with stable dielectric constant (Dk) over frequency and temperature to maintain impedance consistency
• Implementing very low-loss materials such as PTFE-based laminates or specialized hydrocarbon materials for extremely high-speed applications
• Considering spread-weave glass fabrics that reduce variation in effective dielectric constant along trace lengths

Copper Foil Selection

Copper surface roughness significantly affects high-frequency loss:

• Specifying very low-profile (VLP) or reverse-treated foils to reduce skin-effect losses at multi-gigahertz frequencies
• Using smooth foils on high-speed signal layers while using standard foils on power and low-speed layers
• Balancing copper surface selection against adhesion and manufacturing yield requirements
• Considering hybrid stackups with different foil types on different layers based on signal speed requirements

Controlled Dk Materials

Precise impedance control requires materials with tightly controlled dielectric constant:

• Upgrading from standard FR-4 (Dk tolerance typically ±10%) to controlled-Dk materials (tolerance ±2-5%)
• Using consistent material from the same manufacturer and specification across multiple builds
• Implementing materials with lower Dk when narrow trace widths approach manufacturing limits
• Selecting materials with minimal Dk variation across frequency ranges relevant to the signal spectrum

Material Cost-Performance Trade-offs

Material selection involves balancing performance requirements against cost and availability:

• Using premium low-loss materials only on layers with highest-speed signals while using standard materials on other layers
• Implementing hybrid constructions that combine different material types in a single stackup
• Selecting materials that are readily available from multiple board fabricators to ensure supply and competitive pricing
• Evaluating mid-performance materials that provide significantly better signal integrity than FR-4 at moderate cost premiums

Decoupling Improvements

Power delivery network optimization through improved decoupling addresses simultaneous switching noise, power supply noise, and power-induced jitter.

Decoupling Capacitor Placement

Physical placement of decoupling capacitors critically affects their impedance and effectiveness:

• Moving decoupling capacitors closer to IC power pins to reduce loop inductance and improve high-frequency response
• Positioning capacitors to minimize via count and via stub lengths in the connection path
• Implementing via-in-pad connections for smallest capacitor values to minimize parasitic inductance
• Distributing decoupling capacitors around the perimeter of large ICs rather than clustering on one side
• Placing bulk capacitors at power entry points and higher-frequency capacitors progressively closer to active circuits

Capacitor Value Selection

The capacitor value distribution affects PDN impedance across the frequency spectrum:

• Adding smaller-value capacitors to extend PDN impedance control to higher frequencies when measurements show insufficient high-frequency decoupling
• Increasing bulk capacitance values when low-frequency droop or insufficient charge storage is identified
• Selecting capacitor values to avoid anti-resonance peaks in PDN impedance by proper spacing of self-resonant frequencies
• Using multiple capacitors in parallel at the same location to reduce effective series inductance
• Implementing capacitor values that create overlapping parallel resonance zones for smooth impedance transitions

Decoupling Capacitor Types

Different capacitor technologies offer different performance characteristics:

• Using X7R or X5R ceramic capacitors for most high-frequency decoupling due to low ESL and good temperature stability
• Selecting lower-K ceramic dielectrics (C0G/NP0) for critical applications requiring minimal voltage and temperature coefficients
• Implementing reverse-geometry MLCC capacitors to achieve lowest possible ESL for highest-frequency decoupling
• Using polymer capacitors for bulk decoupling where low ESR is critical
• Considering embedded capacitance in PCB substrates for ultra-low impedance at very high frequencies

Voltage Regulator Bypassing

Proper bypassing of voltage regulators prevents oscillation and improves transient response:

• Adding or increasing input bypass capacitance to stabilize regulator operation and reduce conducted noise
• Implementing manufacturer-recommended output capacitor types and values to ensure regulator stability
• Using low-ESR output capacitors when transient response improvements are needed
• Adding feed-forward capacitors when regulator datasheets recommend them for specific operating conditions

Shielding Additions

Electromagnetic shielding reduces coupling between circuits and improves radiated emission compliance. Shielding can be implemented at component, board, and system levels.

PCB-Level Shielding

Shielding structures integrated into the PCB design include:

• Adding ground vias along the edges of high-speed routing channels to create via fences that reduce radiated emissions
• Implementing via stitching around sensitive analog circuits to create electromagnetic barriers
• Creating coplanar waveguide structures with grounded traces on either side of sensitive signals
• Adding ground planes above and below critical signal layers to provide vertical shielding
• Using buried or inner layer routing for highest-speed signals with complete plane enclosure

Component Shielding

Localized shielding around specific components addresses isolation requirements:

• Installing board-level shields over oscillators, synthesizers, or other noise-generating components
• Implementing shields around sensitive receivers or analog circuits to prevent interference
• Using compartmentalized shielding to isolate different circuit blocks on the same PCB
• Ensuring proper grounding of shields through multiple low-impedance connections to board ground planes
• Selecting shield materials and geometries that provide adequate attenuation at problematic frequencies

Cable and Connector Shielding

Shielding at interface points prevents common-mode currents and reduces radiation:

• Upgrading to shielded cables when measurements show excessive common-mode current on interconnects
• Implementing proper shield termination with 360-degree connection at connectors
• Adding ferrite beads or common-mode chokes on cables to suppress common-mode noise
• Using backshell connectors that provide proper shield continuity from cable to chassis
• Bonding cable shields to chassis ground at appropriate points to control shield current paths

Aperture Control

Managing openings in shields prevents electromagnetic leakage:

• Reducing the size of shield apertures to below critical dimensions for frequencies of concern
• Adding conductive gaskets around access panels and seams to maintain shield continuity
• Implementing filtered connectors at shield penetrations to prevent conducted coupling
• Using waveguide-below-cutoff structures for necessary ventilation openings

Equalization Tuning

Equalization compensates for frequency-dependent channel loss by emphasizing high-frequency components and attenuating low-frequency components. Proper tuning optimizes the trade-off between signal recovery and noise amplification.

Transmitter Pre-Emphasis Adjustment

Transmit-side equalization boosts high-frequency content before the signal enters the lossy channel:

• Increasing pre-emphasis tap coefficients when eye measurements show insufficient eye opening due to high-frequency loss
• Decreasing pre-emphasis when excessive high-frequency content causes receiver saturation or increases jitter
• Adjusting the number of pre-emphasis taps to match the channel loss profile and dispersion characteristics
• Using de-emphasis (reducing low-frequency amplitude) rather than emphasis when transmitter output swing is limited
• Tuning pre-emphasis differently for different data patterns when pattern-dependent loss is significant
• Implementing adaptive pre-emphasis that adjusts based on link training or back-channel communication

Receiver Equalization Optimization

Receive-side equalization recovers signal quality after channel attenuation:

• Adjusting continuous-time linear equalizer (CTLE) gain and peaking frequency to compensate for channel loss slope
• Tuning decision feedback equalizer (DFE) tap coefficients to cancel post-cursor intersymbol interference
• Optimizing the balance between CTLE and DFE to minimize noise amplification while achieving adequate equalization
• Adjusting receiver termination impedance in conjunction with equalization settings for optimal performance
• Using adaptive equalization algorithms that train equalizer settings during link initialization
• Fine-tuning equalizer settings for different lanes in multi-lane links when channel variations exist

Clock and Data Recovery Optimization

CDR settings work in conjunction with equalization to recover timing information:

• Adjusting CDR bandwidth to optimize jitter tracking versus jitter transfer characteristics
• Tuning phase interpolator settings to optimize sampling point in the presence of equalization-induced waveform changes
• Optimizing threshold voltage settings for receivers using multi-level signaling
• Adjusting CDR lock range and acquisition characteristics when equalization changes received signal levels

Protocol-Specific Equalization

Different high-speed standards have specific equalization capabilities and tuning procedures:

• Following USB, PCIe, Ethernet, or other protocol-specific equalization training sequences
• Using standardized link training patterns to optimize equalization for specific channels
• Implementing protocol-defined adaptation algorithms rather than manual tuning where possible
• Adjusting link speed or operating mode when equalization cannot achieve required bit error rate

Layout Optimization

Comprehensive layout optimization addresses multiple signal integrity issues through coordinated changes to placement, routing, and design rules.

Component Placement Optimization

Strategic component placement reduces routing complexity and improves signal integrity:

• Relocating high-speed components to minimize critical trace lengths and reduce propagation delay
• Positioning components to enable direct point-to-point routing without vias or layer changes
• Arranging multi-chip interfaces to minimize trace length mismatches and crossings
• Placing decoupling capacitors on the same side of the board as the ICs they support to minimize via inductance
• Orienting connectors to align with natural signal flow and reduce routing congestion
• Separating noise-generating circuits from sensitive circuits with adequate spacing or shielding

Via Optimization

Via structures significantly affect signal integrity at high frequencies:

• Minimizing the number of vias in high-speed signal paths to reduce impedance discontinuities
• Using smaller via pad sizes to reduce capacitive loading while maintaining manufacturing reliability
• Implementing blind and buried vias to reduce stub lengths and allow more direct routing
• Adding ground vias immediately adjacent to signal vias to provide controlled return paths
• Using via-in-pad construction for high-frequency signals to eliminate stubs entirely
• Back-drilling through-hole vias to remove unused stubs that create resonances

Reference Plane Management

Continuous, well-designed reference planes are essential for signal integrity:

• Eliminating or bridging plane splits that disrupt signal return paths
• Ensuring adequate clearance between planes and board edges to prevent fringing effects
• Avoiding narrow plane necks that increase inductance in return current paths
• Providing solid plane regions under and around high-speed components
• Using via stitching to connect planes at layer transitions and around board periphery

Constraint Management

Design constraints ensure consistent application of signal integrity rules:

• Implementing net class-based constraints that automatically apply appropriate rules to signal groups
• Defining length matching requirements at the net, differential pair, and bus levels
• Specifying minimum spacing rules between signal classes to control crosstalk
• Setting impedance control constraints that account for manufacturing variations
• Establishing via count limits and layer transition rules for critical signals
• Using parametric constraints that adapt to local geometry rather than fixed global rules

Manufacturing Tolerances

Accounting for manufacturing variations ensures robust designs:

• Designing for worst-case impedance variations considering dielectric thickness, trace width, and dielectric constant tolerances
• Specifying controlled impedance tolerances based on actual measurement and correction capabilities
• Avoiding minimum feature sizes for critical impedance-controlled traces to reduce sensitivity to process variations
• Implementing impedance test coupons that match actual trace configurations for accurate verification
• Working with fabricators to understand capability limits and establish realistic tolerances

Validation and Verification

After implementing corrective actions, thorough validation confirms that fixes resolve the original problems without introducing new issues.

Measurement-Based Validation

Physical measurements verify that corrective actions achieve desired results:

• Repeating original diagnostic measurements to confirm that problematic signatures have been corrected
• Performing eye diagram analysis to verify that eye opening meets requirements with adequate margin
• Conducting time-domain reflectometry to verify impedance discontinuities have been eliminated
• Using vector network analysis to confirm frequency response improvements
• Running bit error rate tests to validate that signal quality meets error rate requirements
• Comparing measurements before and after modifications to quantify improvements

Simulation Validation

Updated simulations verify that models predict measured improvements:

• Updating simulation models to reflect implemented changes and confirming predicted improvements
• Correlating simulation results with measurements to validate model accuracy
• Running corner-case simulations to verify design margins under worst-case conditions
• Performing sensitivity analysis to understand robustness to remaining variations

System-Level Testing

Complete system validation ensures corrective actions don't compromise overall functionality:

• Running functional tests to verify that signal integrity improvements maintain system operation
• Conducting stress testing under temperature, voltage, and timing extremes
• Performing compatibility testing with different components and configurations
• Validating that fixes don't degrade other signal integrity parameters such as crosstalk or EMI
• Conducting margin testing to verify adequate design headroom

Documentation and Knowledge Transfer

Proper documentation of corrective actions ensures that lessons learned benefit future designs and enable effective communication among team members.

Problem Documentation

• Recording the symptoms, measurements, and analysis that identified the original problem
• Documenting the root cause analysis process and findings
• Noting why particular corrective actions were selected over alternatives
• Capturing constraints that limited available solutions

Solution Documentation

• Providing detailed specifications of all implemented changes including component values, routing modifications, and stackup changes
• Creating before-and-after comparison measurements and simulations
• Documenting validation results that confirm problem resolution
• Recording any remaining limitations or areas requiring further optimization

Design Rule Updates

• Incorporating lessons learned into design guidelines and constraint sets
• Updating simulation models and libraries to reflect new components or techniques
• Creating reusable design blocks that implement proven solutions
• Establishing design review checkpoints to catch similar issues early in future projects

Conclusion

Effective signal integrity corrective actions require systematic problem diagnosis, comprehensive understanding of available solutions, careful implementation, and thorough validation. The choice of corrective strategy depends on the nature of the problem, design phase, manufacturing constraints, and available resources. Successful correction often involves multiple coordinated changes addressing different aspects of the signal path.

Modern high-speed design tools and methodologies enable more sophisticated corrective actions including adaptive equalization, advanced materials, and complex routing optimizations. However, fundamental approaches such as proper termination, controlled impedance, adequate decoupling, and careful layout remain essential. By combining theoretical understanding with practical measurement and simulation, engineers can efficiently resolve signal integrity issues and develop robust, high-performance electronic systems.