In collaboration with the Ministry of Information Industry, the Institute of Telecommunications Science and Technology, Chongqing University of Posts and Telecommunications, Chongqing Mobile Communication Engineering Research Center, China Post, and Siemens, we are conducting research and development on 3G TD-SCDMA communication equipment. This initiative aims to develop a complete set of TD-SCDMA system equipment in the near future and implement it in trial operations. One of the key features of the 3G system is the significant increase in data transmission rates.
In the design of TD-SCDMA terminal systems, the processing rate of the analog baseband section is 1.28 × 12 × 4 × 2 = 122.88 Mb/s. The ADC and DAC designs operate at 12-bit precision with 4 times oversampling, placing this technology in the high-speed data rate domain. As technology advances, digital systems are experiencing higher data rates, clock frequencies, and circuit densities. New serial communication standards have pushed data transfer speeds into the Gb/s range, with rise times reaching sub-nanoseconds. At such speeds, cables, interconnections, printed circuit boards, and silicon substrates exhibit different behaviors compared to low-speed designs, leading to signal integrity challenges that must be addressed during the development of third-generation mobile communication devices.
Signal Integrity Issues
Definition of Signal Integrity
Signal integrity refers to the condition where a signal remains undistorted as it travels through the signal path. It ensures that the signal maintains its correct functional characteristics and can respond with accurate timing and voltage levels within the circuit. If a signal has good integrity, the circuit will maintain proper timing relationships and signal amplitude, ensuring accurate data capture by the receiver. However, if signal integrity issues like false triggering, damped oscillation, overshoot, or undershoot occur, distorted signals may be captured incorrectly, leading to system malfunctions and performance degradation. Figure 2 illustrates the simulation results for signal integrity.
Causes and Effects of Signal Integrity
Signal integrity problems arise from the interconnection of circuits, including wires, substrates, and vias. A wire is not just a simple conductor; it behaves differently depending on the frequency—resistive at low frequencies, capacitive at mid-frequencies, inductive at high frequencies, and even acts as an antenna at very high frequencies. This antenna effect leads to crosstalk and electromagnetic interference (EMI). As feature sizes shrink below 0.5 micrometers, the skin effect causes surface resistance to decrease more slowly than the bulk resistance, further degrading signal integrity. Capacitive effects also increase due to closely spaced wiring, which affects signal transmission. Inductive effects, influenced by trace size and return paths, become significant in package and board design. When ICs are smaller than 0.5 micrometers, inductance becomes a major concern. Mutual inductance between parallel traces can introduce noise into logic circuits, causing signal behavior that differs from low-frequency designs. Digital systems have limited tolerance for signal integrity issues, and even minor problems can lead to performance degradation or system failure. Simulations confirm that excessive switching speed, poor component placement, and improper interconnection can all cause signal integrity problems. Common issues include reflection, crosstalk, oscillation, and ground bounce.
Signal Reflection
Signal reflection occurs when part of a signal is reflected back toward the source rather than being fully transmitted to the load. In high-speed designs, conductors behave like transmission lines, and their impedance at various frequencies determines their transmission characteristics. If the edge rate is as high as 1V/ns, even a short wire can be modeled as a T-type RLC network. Proper impedance matching between the source, transmission line, and load eliminates reflections. Improper termination, signal geometry, connectors, and power plane discontinuities can all cause reflections.
Signal Overshoot and Undershoot
Overshoot refers to the first peak or valley of a signal transition exceeding the specified value, while undershoot is the next valley or peak. These phenomena are caused by high IC switching rates and signal reflections. Multiple reflections between the driver and receiver can create damped oscillations. If the oscillation amplitude exceeds the input threshold of the IC, it may lead to incorrect clocking or data reception. Excessive overshoot can also damage internal components of the IC.
Signal Crosstalk
Crosstalk is an electromagnetic coupling phenomenon between non-connected signal lines. This coupling causes one signal line to act as an antenna, inducing current or voltage on adjacent lines. As clock speeds increase and design sizes decrease, crosstalk becomes more pronounced. On PCBs, crosstalk is modeled using capacitance or LC networks depending on the frequency. Factors such as PCB layer parameters, signal spacing, and termination methods influence the level of crosstalk.
Electromagnetic Interference (EMI)
EMI is similar to crosstalk but involves external radiation sources, such as test probes or other PCBs. EMI modeling often treats wire segments as dipole antennas.
Signal Oscillation and Damping
Signal oscillation and damping refer to repeated overshoot and undershoot, causing the signal to fluctuate around the logic level threshold. Oscillations are underdamped, while damping is overdamped. These issues are typically caused by excessive parasitic inductance and capacitance, leading to mismatched terminal impedance. Proper termination can suppress these effects, especially in periodic pulse signals like clock signals.
Signal Delay
Signal delay occurs when a signal does not reach the destination within the specified time and amplitude. Excessive delay can cause timing violations and data corruption. It is often caused by long transmission lines or overloading. The equivalent capacitance and inductance on the line can delay signal transitions, affecting setup and hold times of the IC.
Ground Bounce and Substrate Coupling
Ground bounce refers to noise generated between the power and ground planes due to large current surges. Synchronous switching of multiple chips can cause transient currents, leading to voltage fluctuations on the power plane. These fluctuations can disrupt other components. Similarly, substrate coupling can affect MOSFET threshold voltages, making the design unreliable. As dimensions shrink, substrate resistance increases, worsening the situation.
Signal Integrity Solutions
For chip design, two main approaches are used: RF solutions focus on impedance matching, while digital solutions emphasize package design, switching control, and bypass capacitors. However, deep sub-micron designs require new strategies, such as silicon-on-insulator (SOI) technology. Modern solutions involve circuit design, layout optimization, and simulation.
Circuit Design
Designers should control the number of simultaneous outputs and limit edge rates to minimize signal integrity issues. Differential signaling is used for high-output blocks. Passive components like resistors and capacitors are used for impedance matching. Termination strategies must balance component count, speed, and power consumption.
Reasonable Wiring
Wiring is critical. Designers should optimize routing based on experience and avoid violating general principles. While some routers support signal integrity analysis, no tool is fully customizable. Accurate parasitic extraction is essential for predicting delays and latency.
Modeling and Simulation
Simulation is crucial for modern high-speed design. It helps identify hidden issues early, reducing design time and cost. For ICs, simulations must be conducted in a packaged environment to match real-world results. Emphasis is placed on synchronous switching, simulation, and packaging to ensure signal integrity before fabrication.
Introduction to Signal Integrity Simulation Models and Tools
Various tools are available for signal integrity analysis. SPICE models are widely used but computationally intensive. IBIS models offer fast and accurate simulations for I/O buffers. VHDL-AMS is a newer standard with limited support. Quantic EMC and XTK provide specialized tools for signal integrity and EMI analysis. HyperLynx’s LineSim and BoardSim help analyze crosstalk and signal integrity issues efficiently.
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