Ethernet LAN transformers—also known as Ethernet isolation transformers or LAN magnetics—are critical components in 10/100/1000Base-T and PoE Ethernet interfaces. However, many engineers and buyers struggle to correctly interpret LAN transformer electrical specifications such as OCL, insertion loss, return loss, crosstalk, DCMR, and isolation voltage.
This guide explains what each LAN transformer electrical parameter really means, how it is measured, and why it matters in real Ethernet and PoE designs, helping you select the right magnetics with confidence.
| Parameter | Typical Value | Test Condition | What It Indicates |
|---|---|---|---|
| Turns Ratio | 1CT:1CT (TX/RX) | — | Impedance matching between PHY and twisted-pair cable |
| OCL (Open Circuit Inductance) | ≥ 350 µH | 100 kHz, 100 mV, 8 mA DC bias | Low-frequency signal stability and EMI suppression |
| Insertion Loss | ≤ -1.2 dB | 1–100 MHz | Signal attenuation across Ethernet frequency band |
| Return Loss | ≥ -16 dB @1–30 MHz | Differential mode | Impedance matching quality |
| Crosstalk | ≥ -45 dB @30 MHz | Adjacent pairs | Pair-to-pair interference isolation |
| DCMR | ≥ -43 dB @30 MHz | Differential-to-common mode | Common-mode noise rejection |
| Isolation Voltage | 1500 Vrms | 60 sec | Safety isolation between line and device |
| Operating Temperature | 0°C to 70°C | Ambient |
Environmental reliability
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A LAN transformer provides:
Incorrect interpretation of electrical specifications can lead to:
Understanding these parameters is therefore essential for hardware engineers, system designers, and procurement teams.
The turns ratio defines the voltage relationship between the PHY side and the cable side of the transformer.
Typical examples:
For 10/100Base-T and PoE, a 1:1 turns ratio with center taps is the industry standard and safest choice.
OCL (Open Circuit Inductance) measures the transformer's inductance with the secondary open, typically at:
OCL indicates how well the transformer:
PoE injects DC current through the center taps, which pushes the magnetic core toward saturation.
A PoE-rated LAN transformer must maintain sufficient inductance under DC bias, not just at zero current.
| OCL Value | Interpretation |
|---|---|
| < 200 µH | Risk of low-frequency distortion |
| 250–300 µH | Marginal |
| ≥ 350 µH | PoE-capable, robust design |
Insertion loss measures how much signal power is lost when passing through the transformer, expressed in dB.
High insertion loss results in:
For 10/100Base-T:
Low insertion loss is essential for stable links and margin against poor cabling.
Return loss quantifies signal reflections caused by impedance mismatch.
Higher absolute values (more negative dB) mean less reflection.
Excessive reflections:
Return loss requirements relax slightly at higher frequencies, consistent with IEEE 802.3 templates.
Good return loss indicates:
Crosstalk measures how much signal from one differential pair couples into another.
Ethernet uses multiple differential pairs. High crosstalk leads to:
| Crosstalk @ 100 MHz | Evaluation |
|---|---|
| −30 dB | Marginal |
| −35 dB | Good |
| −40 dB or better | Excellent |
Strong crosstalk isolation is especially important in compact PoE designs.
DCMR measures how effectively the transformer prevents differential signals from converting into common-mode noise (and vice versa).
PoE systems introduce:
Poor DCMR leads to:
Isolation voltage specifies the maximum AC voltage the transformer can withstand between primary and secondary without breakdown.
Typical values:
For most Ethernet and PoE equipment, 1500 Vrms meets IEEE and UL expectations.
Specifies the ambient temperature range where electrical performance is guaranteed.
Typical classes:
Higher temperature ratings generally imply:
When comparing LAN transformers, always evaluate parameters together, not individually:
OCL (Open Circuit Inductance) measures the transformer’s ability to maintain signal integrity at low frequencies. Higher OCL values improve EMI suppression and help meet IEEE 802.3 return loss requirements.
The turns ratio ensures impedance matching between the Ethernet PHY and the twisted-pair cable. A 1:1 ratio is standard for 10/100Base-T Ethernet to minimize signal reflection and distortion.
Insertion loss represents how much signal power is lost when passing through the transformer. Lower insertion loss ensures better signal quality, especially across the 1–100 MHz Ethernet bandwidth.
Return loss indicates impedance mismatch in the transmission path. Poor return loss causes signal reflections, increasing bit error rates and link instability in Ethernet systems.
DCMR (Differential to Common Mode Rejection) measures how well a transformer suppresses common-mode noise. High DCMR is essential for PoE systems where power and data share the same cable.
Most PoE LAN transformers require at least 1500 Vrms isolation to protect equipment and users from surge voltages and comply with safety standards such as UL and IEEE 802.3.
