PCB设计中如何消除反射噪声?

2018-12-29 16:09:59 来源:电路设计技能
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Maintain Constant Impedance
The first thing you should consider is how to maintain constant impedance for your traces whenever possible. Remember: reflections occur when impedance changes.
 
Calculate the impedance of your traces - In order to maintain constant impedance, you'll need to be able to calculate the impedance of your traces. Your PCB program should allow you to do this, but there are also online tools available. Once you determine what your trace and space widths are, maintain them along your routes. 
 
Consistency across traces - To maintain constant impedance in your differential pairs or single-ended traces, you must maintain constant trace-width, constant spacing, and constant separation from all other conductors. If you route over your impedance-controlled pairs with a random trace, you will change the impedance and create a point of reflection.
 
 
Use impedance-matching circuitry where impedance changes - When you must change impedance (e.g. from a linear-amplifier to an antenna), use impedance-matching circuitry (calculated with Smith Charts, online tools, etc…).
 
 
Reduce Reflection Points
You can also consider how to reduce the occurrence of reflection points in the first place.
 
Watch your vias at the edge of the board - Vias can be a problem for high-speed circuit designers. If the via extends beyond the signal traces to unused layers, the impedance of the circuit suddenly changes. At the transition at the edge of a board, there is an impedance mismatch as the traces leave the via (~50-150 ohms) and enter air (~377 ohms). This creates a reflection point at that location that can severely degrade a signal.
 
Back-drill your vias - The solution is to have your PCB manufacturer “back-drill” your vias to remove the via from the board on the unused outer layers. Back-drilled vias significantly improve logic transitions.
 
Image of back-drilled vias from Sanmina-SCI
 
Mitigate Existing Reflection Noise
Use damping resistors - Another important technique is to use damping resistors in series near all driving signal sources with fast rise/fall times. This is sometimes referred to as a snubbing resistor. 
 
 
Any signal reflection that occurs will be quickly attenuated by each pass through the resistor. These are typically <100Ω resistors placed close to the driving signal source (e.g., clock source, GPIO, etc…).
 
The general idea is to create a damped circuit—where the signal rises to the appropriate logic level once without excessive overshoot and ringing.
 
Summary
For high-speed signal lines, maintaining constant impedance is critical to designing a working circuit. When moving a signal from one IC to another, include damping resistors to prevent ringing. 
 
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