Length Mismatch

Description

Length Mismatch evaluates the difference in overall trace length between the two either two nets or the trace path between two pins in separate nets

Passing Criteria: This check passes when the absolute length difference between the two sides of the differential pair falls below a specified tolerance.

Reporting Units: Length in CENTIMETERS (cm)

Examples

Passing Message:

Length mismatch of 0.27cm within acceptable range (0cm to 1cm)

Failing Message:

Length mismatch of 2.57cm outside acceptable range (0cm to 1cm)

Physics Justification

Length matching in differential pair traces is crucial for ensuring that the signals arrive at the receiver simultaneously, thus maintaining their phase alignment. In particular:

• Noise Cancellation

  Differential pairs rely on signals being exact opposites. If they’re misaligned, they cannot cancel external noise effectively, which reduces signal integrity.

• Avoiding Mode Conversion

  Length mismatches can convert the differential signal into common-mode noise, leading to interference and degraded performance.

• Signal Timing

  In high-speed circuits, even small timing differences caused by mismatched lengths can introduce errors in data transmission.

Helpful Definitions

• Phase Alignment

  This refers to two signals in a pair (like a differential pair) staying in sync and reaching their destination at the same time. Misalignment can distort the signal and cause errors.

• Mode Conversion

  This occurs when differential signals (opposites) lose their balance because of mismatched traces, causing part of the signal to change into common-mode noise, which is unwanted.

• Common-Mode Noise

  This is noise shared by both signals in a pair, caused by imbalances or interference, disrupting the signal the circuit is trying to read.