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What is the mode conversion in a differential pair and how does it create EMI?

What is mode conversion in a differential pair, and how does it create EMI? Mode conversion occurs when energy transfers between the differential mode (desired signal) and the common mode (undesired noise) in a differential pair, and the common-mode current is the primary source of radiated EMI: (1) Differential vs common mode: differential mode: the two traces carry equal and opposite signals (V+ and V-). The fields cancel in the far field → very low radiation. This is the desired signal mode. Common mode: the two traces carry a signal component that is in-phase. The fields add in the far field → significant radiation. Common-mode current is the primary source of EMI from high-speed digital circuits. (2) What causes mode conversion: intra-pair skew: if the P and N traces have different propagation delays (due to length mismatch, fiber weave, or asymmetric routing): the differential signal develops a common-mode component. Skew of 10 ps at 10 GHz: converts approximately 6% of the differential energy to common mode. Asymmetric impedance: if the two traces have different impedance (due to asymmetric routing near vias, connectors, or BGA breakout): differential-to-common mode conversion occurs. Asymmetric coupling: if the two traces have different coupling to adjacent signals or ground, the mode conversion increases. (3) EMI impact: the common-mode current radiates from the traces and cables like a monopole antenna. Even a small common-mode current (1 mA at 10 GHz) can generate significant far-field radiation. The radiated field strength depends on: the common-mode current magnitude, the effective radiating length (trace length, cable length), and the frequency (radiation efficiency increases with frequency). At 25+ Gbps: the harmonics of the data signal extend to 50+ GHz. Even small mode conversion at these frequencies can cause radiated EMI failures. (4) Measurement: mode conversion is measured as Scd21 (common-to-differential) and Sdc21 (differential-to-common) on a 4-port VNA using mixed-mode S-parameters. A good differential pair: Scd21 and Sdc21 < -30 dB across the signal bandwidth. A problematic pair: Sdc21 > -20 dB at some frequencies (indicates significant mode conversion and EMI risk).
Category: Signal Integrity and High Speed Digital
Updated: April 2026
Product Tie-In: PCB Materials, Connectors, Test Equipment

Differential Mode Conversion

Mode conversion is the hidden link between signal integrity and EMC: a differential pair that looks clean on a differential eye diagram can still fail EMC testing if the mode conversion creates excessive common-mode current.

  1. Performance verification: confirm specifications against the application requirements before finalizing the design
  2. Environmental factors: temperature range, humidity, and vibration affect long-term reliability and parameter drift
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Common Questions

Frequently Asked Questions

How does mode conversion relate to EMC testing?

Radiated EMI (CISPR 32, FCC Part 15 Class B): the radiated emission limits are absolute values (in dBμV/m at a specified distance). If the common-mode current from mode conversion exceeds the limit: the product fails EMC certification. This is particularly common at: cable I/O connectors (USB, Ethernet, HDMI) where the differential pair transitions to a cable. The cable acts as an efficient antenna for common-mode radiation. A common-mode choke at the connector dramatically reduces this emission (often 10-30 dB improvement).

Can the receiver tolerate common-mode noise?

Modern differential receivers (SerDes) have common-mode rejection ratio (CMRR) of 20-30 dB. This means common-mode noise at 20% of the differential signal (-14 dB) is rejected to 0.2-2% of the differential signal. However: at very high frequencies (> 10 GHz): the CMRR of the receiver degrades (the parasitic imbalance in the receiver becomes more significant). And: the EMI from common-mode radiation does not depend on the receiver CMRR, it radiates regardless of whether the receiver rejects it.

How do I measure mode conversion?

Use a 4-port VNA configured for mixed-mode S-parameter measurement. Connect ports 1 and 3 to the differential pair at one end, and ports 2 and 4 at the other end. The VNA calculates: Sdd21 (differential transmission, the desired signal), Scc21 (common-mode transmission), Sdc21 (differential-to-common mode conversion, the EMI source), and Scd21 (common-to-differential conversion, a noise susceptibility metric).

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Glossary

Related Terms

Noise Figure → LNA → Noise Floor → S-Parameters → Insertion Loss → Return Loss →
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