Ethernet Cables & Audio Quality
An Ethernet cable always delivers data 100% correctly — but the RF energy carrying that data can leak off the cable and travel into the DAC. Here is the path, step by step.
Physical structure and components of a LAN cable
A LAN cable is not ordinary wire — it is a transmission line built to carry RF in the 100–800 MHz band, and every layer of its construction directly affects its electromagnetic behaviour. It holds eight copper conductors in four twisted pairs. Permanent runs use solid conductors (lower DC resistance, better skin effect at HF); patch cords use stranded for flexibility but with more total surface.
| Grade | Z₀ tolerance | Conductor | Reflection |
|---|---|---|---|
| Cat 5e | 100 Ω ±15% | solid/stranded | looser geometry |
| Cat 6A | 100 Ω ±~8% | solid | tighter pitch control |
| Cat 7/8 | 100 Ω ±5% | solid, shielded | most uniform |
Skin effect and conductor material
At high frequency the current crowds into the surface of the conductor — the surface matters more than the bulk.
Fig 1. AC-to-DC resistance ratio of an AWG24 conductor — barely above DC below ~50 kHz, but at 125 MHz it climbs to ~22× because the current is squeezed into a 5.9 μm surface layer
OFC copper (ASTM B170, under 10 ppm oxygen) has a smoother surface than ETP (200–400 ppm), so lower surface resistance. At RF the point isn’t bit errors (the margin is huge) but lower RF emission, because a more uniform current distribution radiates less EMI that couples into the DAC chassis.
Twist pitch, symmetry and crosstalk
Twisting makes induced noise equal on both conductors (common-mode) so it cancels at the receiver — but only as far as the symmetry of the twist holds. Asymmetry converts common-mode back into differential.
Each pair uses a different twist pitch to avoid resonance between pairs at the same frequency. Cat 6A controls pitch more tightly, so NEXT is lower.
PAM-5 line coding and the digital domain
Gigabit Ethernet doesn’t send binary; it uses PAM-5 (five levels: +2,+1,0,−1,−2 V) across all four pairs full-duplex, so the symbol rate drops to 125 MBaud/pair instead of running at 1000 MHz.
Before transmission the data passes a scrambler that whitens the spectrum, so the on-cable spectrum looks like broadband noise spanning 0–500+ MHz continuously, not a single spike.
TCP/IP has CRC-32 + retransmit, so the audio data reaching the DAC is 100% correct — but that only guarantees data, not that the RF energy carrying it stays on the cable. Every component in the 100–500 MHz band can couple into a circuit sensitive to that frequency.
Impedance mismatch and signal reflection
When impedance changes abruptly (a kink, a poor RJ45 termination, non-uniform geometry), the signal reflects, creating a brief high voltage on the cable → more radiated EMI.
Common-mode noise and mode conversion
This is the main mechanism by which digital-domain noise leaks into the DAC’s analog domain. Ethernet is differential — noise equal on both wires is subtracted out (CMRR) — but asymmetry converts between differential and common-mode.
Ground loops and shield topology
Two connected devices usually sit at slightly different ground potentials (different SMPS, different outlet circuits). Even a 10–100 mV difference drives current through the conductor that joins them.
| Topology | Ground loop | Note |
|---|---|---|
| UTP (unshielded) | no conductive loop | but radiates RF fully |
| S/FTP, shield both ends | loop forms automatically | shield current may exceed UTP |
| S/FTP, one end (DAC side) | breaks the loop | recommended |
RF radiation and capacitive coupling into the chassis
Even without direct contact, the electric field from the RF cable couples into the DAC chassis through stray capacitance.
PHY chip, Ethernet transformer and interwinding capacitance
The PHY chip (Realtek/Intel/Marvell) is mixed-signal; the current it draws from the supply changes with the data pattern → data-dependent switching current creates supply noise synchronised to the data.
IEEE 802.3 mandates an isolation transformer on every port. The transformer blocks DC and audio frequencies completely, but at RF its weak point is interwinding capacitance.
In other words: above 100 MHz the transformer behaves like a 5–15 pF capacitor that lets common-mode current pass straight into the DAC’s ground plane.
RF rectification in the op-amp and DAC analog stage
Once RF reaches the ground plane it enters the analog stage two ways: through the op-amp input pin and through the supply rail. Every input pin has ESD diodes with a nonlinear I–V curve → they rectify RF into a DC offset.
DC offset → shifted bias point → more even-order harmonic distortion (2nd, 4th), which the ear reads as “warm/smooth” — but it is measurable distortion, not the recording.
Supply-rail modulation and the noise floor
Besides rectifying at the input pin, a dirty ground plane modulates the DAC supply directly (because the decoupling cap ties supply to ground tightly).
Fig 2. Both protective mechanisms — PSRR and CMRR — collapse at high frequency. At 100 MHz only ~15–28 dB remains. This is the quantitative reason RF noise in this band slips into the analog domain even though audio frequencies are buried
The net effect: RF from the cable → ground plane → (1) op-amp input → rectify → harmonic distortion, and (2) supply path → modulate the rail → raise the noise floor across the spectrum → the sound loses “air” and detail sinks.
Mitigation: fixing each noise path
Fix it path by path — not “expensive cable = good.”
| Noise path | Fix | Effect |
|---|---|---|
| common-mode on cable | common-mode choke (CMC) | most cost-effective |
| conducted + ground loop | optical fiber + media converter | cuts all network noise |
| switching noise at source | linear regulated PSU vs SMPS | fixes the source |
| multiple return paths | star ground + single-end shield | reduces ground loop |
Work from source to sink: swap SMPS → linear (fix the source), add a CMC at the interface (block common-mode), break to fiber if possible (cut all conducted noise), star-ground (prevent loops). Every step has a clear, measurable mechanism — not belief.
References
- aesHockanson, D. et al. “Measurements and simulations for ground-to-ground plane noise…,” IEEE Trans. EMC.
- aesArchambeault, B. et al. “Impact of analog/digital ground design on circuit functionality and radiated EMI,” IEEE Trans. EMC, 2005.
- stdIEEE Std 802.3-2022, Ethernet — §40 (1000BASE-T PHY).
- appAnalog Devices “MT-096: RFI Rectification Concepts,” Rev.0.
- appMicrochip “AN1767: EMI Rejection Ratio (EMIRR) of Op Amps.”
- appAbracon LLC “Common Mode Chokes: Basics and Applications,” white paper.
- stdANSI/TIA-568.2-D, Balanced Twisted-Pair Cabling and Components, 2018.
- stdIEC 62153-4-3, Longitudinal conversion loss (LCL); IEC 61156-5; IEC 60958-1.
- stdASTM B170, Oxygen-Free Electrolytic Copper.
- bookOtt, H. W. Electromagnetic Compatibility Engineering, Wiley 2009 (ch.4–5).
- bookPaul, C. R. Introduction to Electromagnetic Compatibility, 2nd ed., Wiley 2006.
- bookWilliams, T. EMC for Product Designers, 5th ed., Newnes 2017.
- patUS 10,026,544 B2 “Common mode noise restrainer applicable to Ethernet,” USPTO 2018.