If you're designing an ethernet port and you've ever wondered whether you actually need that little black part between the PHY chip and the RJ45 jack, you're not alone. The same question shows up on r/AskElectronics in five different forms: "Can I solder Cat5 directly to a PCB?" "Do I need magnetics if my run is only a few inches?" "What's actually inside an RJ45 magjack?"
The short answer: yes, in almost every case you do need an ethernet pulse transformer, and selecting the wrong one is one of the most common reasons designs fail EMC pre-compliance or drop packets in the field.
This post walks through what an ethernet pulse transformer is, what it actually does inside the signal chain, the four specs that decide whether it passes IEEE 802.3 compliance, and the narrow set of cases where you can skip it.
What Is an Ethernet Pulse Transformer?
An ethernet pulse transformer is a small magnetic component that sits between the PHY (the silicon that converts digital data to analog signals) and the RJ45 connector. It transfers the data signal across an isolation barrier using magnetic coupling rather than direct electrical connection.
In a typical schematic, the signal path looks like this:
PHY → common mode choke → pulse transformer → RJ45 → cable
The transformer is the part that creates the galvanic isolation barrier. Everything on the cable side of it is electrically separated from your PCB's ground, even though the data signal still gets through.
It's called a "pulse" transformer because it's optimized for fast-edge digital pulses rather than the 50/60 Hz line frequency a power transformer handles. The same magnetic principle (a changing primary current induces a secondary voltage), tuned for very different signal characteristics.
What Does an Ethernet Pulse Transformer Actually Do?
It performs four jobs at the same time:
- Galvanic isolation. The cable side and the PCB side share no copper path. If lightning hits the cable or a ground loop carries a few hundred volts of common-mode noise, the transformer keeps it off your PHY. IEEE 802.3 mandates a 1500 Vrms isolation barrier (60-second HiPot test, or 2250 Vdc for 60 s per IEC 60950) between the line side and the device side. More on that in the spec section.
- Signal balancing. Ethernet uses differential signaling on twisted pairs at a nominal 100 Ω differential impedance (±15% per IEEE 802.3). The center-tapped winding on the cable side keeps the two conductors balanced relative to each other, which is what gives ethernet its noise immunity in the first place.
- Common-mode noise rejection. Combined with a common mode choke (which usually sits on the PHY side), the transformer rejects noise that appears equally on both wires of a twisted pair (the kind of noise that radios, motors, and switching power supplies generate). Typical CMRR targets for 10/100 magnetics are around 30 dB at 30 MHz, 20 dB at 60 MHz, and 15 dB at 100 MHz.
- DC blocking. The transformer passes the AC component of the data signal but blocks DC. This matters for two reasons: it prevents the PHY from dumping DC current onto the cable, and it makes Power over Ethernet (PoE) possible by keeping the DC power (44 to 57 Vdc nominal) and the AC data on separate paths through the same wires. Modern PoE goes up to 90 W at the powered device under IEEE 802.3bt Type 4, which puts a real thermal load on the magnetics.
If you remove the transformer, you lose all four of these functions. That's why "transformerless ethernet" only works in narrow conditions where each function can be replaced with something else (capacitors for DC blocking, careful layout for noise immunity, no need for galvanic isolation because both ends share a chassis).
The IEEE 802.3 Specs Your Pulse Transformer Must Meet
For 10/100BASE-T, seven numbers determine whether your design will pass compliance testing. For 1000BASE-T (gigabit) the list grows, but these seven are still the foundation.
| Spec | Requirement / typical value | What it controls |
|---|---|---|
| Isolation voltage (HiPot) | 1500 Vrms at 50/60 Hz for 60 s, or 2250 Vdc for 60 s (per IEC 60950) | Survival under lightning, ground loops, line transients |
| Open-circuit inductance (OCL) | ≥ 350 µH minimum, measured at 100 kHz, 100 mVrms, with 8 mA DC bias (per IEEE 802.3 Clause 14.3.1.2.1) | Low-frequency response, baseline wander, droop on long frames |
| Leakage inductance | ≤ 0.4 µH typical (often spec'd 0.5 to 1 µH max) | Rise/fall times, signal integrity, high-frequency response |
| Interwinding capacitance | ≤ 12 pF typical | Common-mode rejection, rise/fall times, EMI coupling |
| Insertion loss | ≤ 1.0 dB from 1 MHz to 60 MHz; ≤ 1.1 dB to 100 MHz typical | Signal level reaching the cable; system-level noise margin |
| Return loss | ≥ 16 dB from 1 to 40 MHz; ≥ 12 dB from 30 to 60 MHz; ≥ 10 dB from 60 to 100 MHz; differential impedance 100 Ω ±15% at MDI | Signal reflections, eye opening at the receiver |
| Common-mode rejection | ≥ 30 dB at 30 MHz, ≥ 20 dB at 60 MHz, ≥ 15 dB at 100 MHz (typical industry targets) | Whether the design passes radiated EMC testing |
The detail behind each number:
- Isolation voltage (HiPot rating). A pulse transformer rated below 1500 Vrms will not meet IEEE 802.3. Most reputable parts (including Allied's AH1601CI) are specified at 1500 Vrms HiPot. For medical applications, see the medical isolation section below.
- Open-circuit inductance (OCL). This is the parameter that ensures the transformer can hold a low-frequency edge without sagging. The 350 µH minimum at 8 mA bias is the IEEE-mandated floor for 10/100. Test conditions vary across vendor datasheets (frequency, drive level, bias current); compare apples to apples before assuming one part has more margin than another.
- Leakage inductance. Determined by winding geometry. Higher leakage rounds off the rising and falling edges of the differential signal, which directly compresses the eye diagram. Look for ≤ 0.4 µH on serious 10/100 designs.
- Interwinding capacitance. Acts as an unintended shortcut for common-mode noise around the isolation barrier. Above roughly 12 pF, you start losing CMRR at the higher frequencies that EMC testing scrutinizes (30 to 100 MHz).
- Insertion loss. This is how much signal the transformer eats. A part with high insertion loss can still pass on its own, but it stacks badly with cable loss (about 24 dB worst case for 100 m of Cat5e) and PHY output tolerance, eating your noise margin at the system level.
- Return loss. Magnetics with at least a 2 dB return loss margin against the spec above are recommended, because the PHY output impedance and the transformer combine in ways that eat margin.
- Common-mode rejection. This is what stops your design from radiating across the EMC test chamber. CMRR is the spec engineers most often skip when reading a datasheet, then run into during pre-compliance. Conducted emissions are tested from 150 kHz to 30 MHz; radiated emissions from 30 MHz to 1 GHz (or higher) under FCC Part 15 and CISPR 32. FCC Class B (residential) limits are roughly 10 dB tighter than Class A (industrial), which is why a part that passes in an industrial design can fail in a consumer design with the same PHY.
If a datasheet gives you only one or two of these seven numbers, that's a sign the part is not characterized for serious design work. A complete LAN magnetics datasheet specifies all seven plus operating temperature range and turns ratio.
"Do I Really Need Ethernet Magnetics?" The Honest Answer
There are three cases where you can skip a pulse transformer. Outside of these, you need one.
| Case | When it applies | What replaces the transformer | Standard / reference |
|---|---|---|---|
| 1. Same PCB, shared power domain | Both PHYs on the same board, same power rail, controlled-impedance differential traces shorter than roughly 30 cm (12 in) | 50 Ω single-ended termination at each device (100 Ω differential), no AC coupling | Not standardized; vendor-characterized only |
| 2. Board-to-board, same chassis | Two PCBs mechanically and electrically tied to the same chassis ground, no exposed cabling | AC coupling capacitors, 0.01 to 0.1 µF (typically 0.022 to 0.1 µF for 10/100; 10 nF for gigabit per Clause 40) | Not standardized; vendor-characterized only |
| 3. Backplane / SFP+ Direct Attach | Fixed, short (typically ≤ 1 m for SFP+ DA), characterized link between known PHYs | Capacitor coupling defined in the standard | IEEE 802.3 Clause 70 (1000BASE-KX); SFF-8431 for SFP+ DA |
If your design doesn't fall into one of these three cases, you need ethernet magnetics. That covers nearly every product that exposes an RJ45 to the outside world.
How to Choose an Ethernet Pulse Transformer (Worked Example: AH1601CI)
To make the spec discussion concrete, here's how Allied's AH1601CI maps to the seven specs above. It's a 16-pin SMD single-port 10/100BASE-T module, the kind of part you'd drop into a typical industrial IoT or networking design.
| Spec | AH1601CI value | What it tells you |
|---|---|---|
| Isolation (HiPot) | 1500 Vrms (60 s) | Meets IEEE 802.3 minimum |
| Turns ratio | 1CT:1CT (both channels) | Matches standard 10/100 PHYs without external matching |
| Open-circuit inductance | ≥ 350 µH at 100 kHz, 8 mA bias | Meets IEEE 802.3 OCL floor with margin |
| Insertion loss | ≤ 1.0 dB, 1 to 100 MHz (typical for the family) | Comfortable system noise margin |
| Return loss | ≥ 16 dB, 1 to 30 MHz; ≥ 12 dB, 30 to 60 MHz; ≥ 10 dB, 60 to 100 MHz | Compliant with IEEE 802.3 MDI return loss mask |
| Operating temperature | −40°C to +85°C | Industrial temperature range |
| Pin count / package | 16-pin SMD, gull-wing leads | Drops into the standard footprint, no layout rework |
| Application | 10/100BASE-T, non-PoE | Use the AHSC- or AGSC- series if you need PoE |
(Always cross-check these against the live datasheet; magnetics vendors update test conditions and limits between revisions.)
When you're evaluating any pulse transformer, the practical workflow is:
- Confirm the speed grade. 10/100BASE-T parts will not work for gigabit, and gigabit parts are over-spec'd (and pricier) for 10/100. Gigabit parts also tend to be larger (often 24-pin packages) and run roughly 30 to 50% higher unit cost at typical OEM volumes.
- Check whether you need PoE support. PoE adds DC bias (350 mA per pair under 802.3at, up to 600 mA per pair under 802.3bt Type 4) and thermal load that a non-PoE part won't survive. If you're doing PoE, look at parts in Allied's PoE collection rather than retrofitting a non-PoE design.
- Match the turns ratio to your PHY. Most PHY vendors publish a recommended magnetics list. The PHY's recommended ratio (1CT:1CT for most 10/100, often 1CT:1CT or 1.41:1 for 1000BASE-T depending on PHY architecture) is the safe starting point.
- Verify the operating temperature. Industrial designs need −40°C to +85°C. Commercial-grade parts (typically 0°C to +70°C) will save a few cents and fail in the field.
- Check the package and pinout. Many "equivalent" parts have subtly different pin assignments. Dropping in a part with the same footprint but a different pin map will brick your design.
Where This Fits on the PCB
A complete ethernet port has four magnetic and connector components, and they have to be selected together to avoid impedance mismatches and EMC failures:
- Common mode choke (PHY side): rejects common-mode noise before the signal reaches the transformer
- Pulse transformer: provides isolation, balancing, and DC blocking
- RJ45 connector: physical interface to the cable, with optional integrated magnetics
- Bob Smith termination network: terminates unused pairs and reduces common-mode currents on the cable
Engineers who pick each component independently from a different vendor end up debugging impedance mismatches at EMC test. The cleaner workflow is to specify the transformer, choke, and connector together. That's one of the reasons Allied stocks all four component types in a single catalog.
Where Allied Fits
Allied Components International has been designing and manufacturing magnetic components since 1992, with headquarters in Foothill Ranch, California, and full product coverage across inductors, transformers, common mode chokes, LAN magnetics, and RJ45 connectors. Compared to the larger magnetics vendors, two things tend to matter to the engineers who choose us:
- Direct engineering support. When you have a question about turns ratio, center-tap configuration, or PCB layout, you reach an engineer who knows the product, not a routed support queue. For mid-tier OEMs that don't have a dedicated FAE relationship with the big magnetics houses, this is usually the deciding factor.
- Full ethernet port catalog from one source. Allied's 10/100 Base-T, 100/1000 Base-T, PoE, common mode choke, and RJ45 connector ranges are designed to work together. You don't have to harmonize specs across three vendors.
If you're working on a 10/100 design and want a starting part, the AH1601CI is the one most engineers begin with. If you need PoE support, gigabit, or a custom turns ratio, the engineering team can point you to a stock part or quote a custom build.
Common Questions
Can I use a 100/1000 Base-T transformer for a 10/100 design?
Technically yes (gigabit parts are backward compatible), but you'll typically pay 30 to 50% more, use a larger 24-pin footprint instead of 16-pin, and accept a slight insertion-loss penalty in the 10/100 frequency range. Spec a 10/100 part for 10/100 designs.
Does the pulse transformer go on the PHY side or the cable side of the common mode choke?
PHY side typically (i.e., the CMC sits closer to the cable). The standard signal chain is PHY → transformer → CMC → RJ45 in some references, or PHY → CMC → transformer → RJ45 in integrated modules like the AH1601CI, where both are in one package per channel. Either way, swapping the order changes the impedance environment and usually makes EMC worse. Follow the PHY vendor reference design.
Are integrated magnetics (magjacks) the same as discrete pulse transformers?
Functionally yes, mechanically different. An integrated RJ45 magjack puts the transformer, common mode choke, and connector in one housing. Discrete designs separate them. Magjacks save board area (typically 30 to 40% versus discrete plus separate RJ45) at the cost of higher unit price and less flexibility in part substitution. That's a deeper topic we'll cover in a later post.
Is a 1500 Vrms isolation rating enough for medical devices?
For most IT/networking equipment, yes. Medical devices governed by IEC 60601-1 use the Means of Patient Protection (MOPP) framework: 1× MOPP requires 1500 Vrms isolation with 4 mm creepage and 2.5 mm clearance (basic insulation, equivalent to standard ethernet magnetics), while 2× MOPP requires 4000 Vrms with 8 mm creepage and 5 mm clearance (reinforced insulation). Type CF applied parts (the most stringent classification, used for direct cardiac contact) require 4000 Vrms. For 2× MOPP or Type CF, you need a medical-grade transformer specifically rated for that voltage.
What does PoE actually do to a pulse transformer?
The transformer's center-tapped winding carries the DC PoE current as a common-mode bias. Current per pair scales with the PoE class: roughly 175 mA per pair under 802.3af (15.4 W PSE, 12.95 W PD), 350 mA under 802.3at (30 W PSE, 25.5 W PD), 600 mA under 802.3bt Type 3 (60 W PSE, 51 W PD), and 960 mA per pair under 802.3bt Type 4 using all four pairs (90 to 100 W PSE, 71 W PD). DC bias current is what causes core saturation in undersized parts; it's the single most common reason a "10/100 magnetic" fails when bolted into a PoE design.
If you're working through ethernet magnetics selection on a current design and want a second pair of eyes on the spec sheet, the engineering team at Allied is happy to walk through it with you. Browse the 10/100 Base-T LAN magnetics catalog or get in touch through the contact page.