Every 1000BASE-T design needs a properly specified network transformer. Here's the complete technical reference for selecting and integrating Gigabit Ethernet magnetics.
Minimum Electrical Specs
Parameter Specification
───────────────────────────────────────────────────
OCL (Open Circuit Inductance) ≥ 1000µH
Measurement condition: 100kHz, 0.1V RMS
Turns ratio: 1CT:1CT × 4 pairs
Insertion loss: < 1.1 dB (1–100MHz per pair)
Return loss: > 16 dB (1–100MHz per pair)
Pair-to-pair balance: < 0.5 dB amplitude
Isolation voltage: ≥ 1500V AC (IEEE 802.3)
Package: 16-pin SMD (typical)
PHY-Specific Selection Workflow
Step 1: Open PHY datasheet → Application section
Step 2: Find recommended transformer part number(s)
Step 3: Confirm: OCL ≥ 1000µH, 4-pair coverage, 1CT:1CT ratio
Step 4: Verify isolation voltage ≥ 1500V AC
Step 5: Check operating temp range matches your environment
Common PHYs and typical magnetics requirements:
Marvell 88E1111/88E1512 → 1000µH, 16-pin, 1CT:1CT × 4
Realtek RTL8211F → 1000µH, 16-pin, 1CT:1CT × 4
Microchip KSZ9031/KSZ9131 → 1000µH, -40°C rated for industrial
TI DP83867 → Specific to automotive/industrial grade
Bob Smith Termination (All 4 Center Taps Required)
1000BASE-T center taps: 4 (one per pair)
Termination per tap:
75Ω resistor → common node (CTAP)
1000pF cap → chassis ground (NOT signal GND)
Schematic snippet:
PHY_CT1 → 75Ω → CTAP → 1000pF → CHASSIS_GND
PHY_CT2 → 75Ω → CTAP
PHY_CT3 → 75Ω → CTAP
PHY_CT4 → 75Ω → CTAP
⚠️ Missing any center tap = EMC failure at 30m cable test
⚠️ Connecting to signal GND instead of chassis = noise injection into PHY
PCB Layout Checklist
layout_rules = {
"differential_impedance": "100Ω (±10%)",
"trace_length_matching": "±2mm between pairs",
"via_count": "Minimize — each via = impedance discontinuity",
"pair_symmetry": "Equal routing geometry for all 4 pairs",
"bob_smith_placement": "Within 5mm of transformer center tap pins",
"isolation_zone": "6.4mm creepage between primary/secondary copper",
"signal_separation": "Route away from switchers, oscillators, digital buses"
}
common_layout_mistakes = [
"Asymmetric routing between pairs → insertion loss imbalance",
"Via on single-ended trace instead of differential via pair",
"Bob Smith caps to signal GND instead of chassis GND",
"Only 2 center taps terminated (copied from 10/100 reference)",
"Transformer too far from RJ45 → extra trace length adds loss"
]
PoE Compatibility
Standard 1000BASE-T transformer: signal only
→ DC bias from PoE saturates core
→ Effective OCL drops below 1000µH
→ Link unstable under PoE load
PoE-rated 1000BASE-T transformer: modified core (air-gapped)
→ Maintains ≥ 1000µH OCL under DC bias
→ Rate current to your PoE class:
| Class | Standard | Current |
|---|---|---|
| 3 | 802.3af | 350 mA |
| 4 | 802.3at | 600 mA |
| 6 | 802.3bt | 960 mA |
Quick Validation Test
Post-layout checklist before sending to fab:
[ ] All 4 PHY differential pairs connected to transformer
[ ] All 4 center taps have 75Ω + 1000pF to chassis GND
[ ] 100Ω differential impedance verified in stackup calculator
[ ] Trace length match between pairs within 2mm
[ ] Creepage distance ≥ 6.4mm between primary/secondary copper
[ ] PoE-rated magnetics if PoE is enabled
[ ] Operating temp range matches deployment environment
Source
Voohu Technology (www.voohuele.com) — 1000BASE-T transformers, standard and PoE-rated, 50pcs MOQ, DHL 3–5 days. PHY cross-reference support available.
Top comments (0)