Choosing between a 10/100BASE-TX and 1000BASE-T network transformer is not just about picking the "faster" part — the electrical architecture is fundamentally different. Here's the complete technical breakdown.
Core Architecture
10/100BASE-TX:
Wire pairs used: 2 (1 TX + 1 RX)
Signal direction: Unidirectional per pair
Transformer sets: 1 (handles 2 pairs)
Typical package: 8-pin
1000BASE-T:
Wire pairs used: 4 (all pairs active)
Signal direction: Bidirectional per pair (echo cancelled by PHY DSP)
Transformer sets: 4 (one per pair)
Typical package: 16-pin
OCL Requirements
Open Circuit Inductance (OCL) is the critical performance spec — measured at 100kHz, 0.1V RMS:
// Why Gigabit needs higher OCL:
// PAM-5 at 125 Msymbols/s → fundamental at 62.5 MHz
// BUT low-frequency energy extends further down than MLT-3
// Transformer with low OCL rolls off that energy → waveform distortion → link errors
// Minimum OCL check (simplified):
const minOCL = {
'10BASE-T': 350e-6, // 350 µH
'100BASE-TX': 350e-6, // 350 µH
'1000BASE-T': 1000e-6, // 1000 µH
};
Gigabit adds pair-to-pair balance as a specification — amplitude imbalance between the four transformer sections degrades PAM-5 decision thresholds.
Bob Smith Termination
10/100BASE-TX:
Center taps: 2 (1 TX, 1 RX)
Networks: 2× (75Ω + 1000pF to chassis GND)
1000BASE-T:
Center taps: 4 (one per pair)
Networks: 4× (75Ω + 1000pF to chassis GND)
// Missing any center tap termination on Gigabit
// → poor common-mode rejection on that pair
// → EMC failures, especially at longer cable lengths
PoE Compatibility
Standard transformer: Signal only (no DC bias handling)
PoE-rated transformer: Handles DC bias without core saturation
PoE current by class:
802.3af (Class 3): 350 mA max
802.3at (Class 4): 600 mA max
802.3bt (Class 6): 960 mA max
// Always specify PoE-rated magnetics for PSE or PD designs
// Standard Gigabit transformer WILL saturate under PoE DC bias
Backward Compatibility
Gigabit transformer in 10/100 circuit: ✅ Compatible (larger/costlier than needed)
10/100 transformer in Gigabit circuit: ❌ NOT compatible
// Even if PHY autonegotiates down to 100BASE-TX:
// 4-pair Gigabit PHY + 2-pair transformer = 2 pairs unterminated
// → impedance discontinuities → EMC issues → potential link instability
Quick Selection Rule
PHY supports 1000BASE-T?
YES → Use Gigabit magnetics (works at all speeds)
NO → Use 10/100 magnetics (smaller, cheaper)
PoE enabled?
YES → PoE-rated version of the above
NO → Standard magnetics
Source
Voohu Technology (www.voohuele.com) — stocks both 10/100 and Gigabit network transformers, with PoE-rated and industrial-grade options. 50pcs MOQ, DHL 3–5 days. PHY reference schematic matching available on request.
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