1CT:1CT Network Transformer: Turns Ratio, Center Tap Functions, and PoE Architecture

The 1CT:1CT notation on every Ethernet transformer
datasheet encodes three distinct electrical features. Here's the complete technical breakdown.
Notation Decoded
1CT : 1CT
│ │
│ └── Secondary winding: 1 turn, with Center Tap
└───────── Primary winding: 1 turn, with Center Tap

Meaning:
Turns ratio: 1:1 (no voltage transformation)
Impedance ratio: 1:1 (Z_sec = Z_pri × (1/1)² = Z_pri)
Center taps: Both primary and secondary — BOTH accessible
Why 1:1 Is Fixed for Ethernet

Transformer impedance transformation

Z_secondary = Z_primary × (N_sec / N_pri)^2

Z_primary = 100 # Ω — Ethernet differential impedance

ratios = [1.0, 1.5, 2.0]
for r in ratios:
Z_sec = Z_primary * (r ** 2)
match = "✅ matched" if Z_sec == 100 else f"❌ mismatch → {Z_sec}Ω"
print(f" Ratio 1:{r} → Z_secondary = {Z_sec}Ω {match}")

Output:

Ratio 1:1.0 → Z_secondary = 100.0Ω ✅ matched

Ratio 1:1.5 → Z_secondary = 225.0Ω ❌ mismatch → 225.0Ω

Ratio 1:2.0 → Z_secondary = 400.0Ω ❌ mismatch → 400.0Ω

Any ratio other than 1:1 breaks 100Ω impedance continuity → return loss degradation → link errors.
Center Tap: Three Functions
Function 1 — Bob Smith Termination
Primary side center tap (PHY side):
CT_PRI ──── 75Ω ──┐
├── CTAP_NODE ──── 1000pF ──── CHASSIS GND
CT_SEC ──── 75Ω ──┘ (cable side)

Effect on signal:
Differential mode: CT is at 0V differential → termination invisible to data
Common mode: CT collects common-mode voltage → discharged to chassis

Missing termination consequences:
❌ Radiated emissions fail at 30m cable test
❌ ESD events reach PHY chip directly
Function 2 — PoE Alternative A Power Injection
Alternative A PoE current path (802.3af/at):

PSE (+) ── cable pair conductor 1 ──┐
│ (DC superimposed on data)
PSE (−) ── cable pair conductor 2 ──┘

DC flows: CT_SEC (cable) → through winding conductors → CT_PRI (PHY side)

Why CT makes this work:
CT at differential zero → DC doesn't modulate differential signal
CT provides the return path for DC current flow

Without CT: Alt A PoE is architecturally impossible
Function 3 — Winding Balance Test Reference
Balance measurement:
Apply common-mode voltage to input pair
Measure differential output voltage at CT reference

Longitudinal Balance (LB) = 20 × log10(Vcm_in / Vdiff_out)

IEEE 802.3 requirement: LB > 40 dB (1–100 MHz)
Good transformers: LB > 50 dB

Imbalance effect:
External common-mode noise → partial conversion to differential noise
→ Degrades receiver SNR without obvious cause
1000BASE-T: Four Independent 1CT:1CT Sets
Package contains: 4 × 1CT:1CT winding pairs (one per Ethernet pair)

Each set:
Primary: TX+, TX−, CT_PRI
Secondary: RX+, RX−, CT_SEC

Total center taps in Gigabit transformer: 8 (4 primary + 4 secondary)
Bob Smith termination networks required: 4 (one per pair CT)

Common mistake:
Using 10/100 reference (2 CTs) for Gigabit PHY (needs 4 CTs)
→ 2 pairs unterminated → EMC failure
Datasheet Red Flag: "1:1" Without CT
"1:1 ratio" (no CT notation)
→ Center tap may NOT be accessible
→ Bob Smith termination impossible
→ PoE Alt A impossible
→ Will fail EMC compliance

Always verify: "1CT:1CT" or explicit statement of center-tapped windings
Source
Voohu Technology (www.voohuele.com) — 1CT:1CT transformers for 10/100 and 1000BASE-T, SMD and THT, standard and PoE-rated. MOQ 50pcs, DHL 3–5 days.