HX1188NL Is Our Best-Selling Network Transformer — Here’s the Full Application Guide We Give Every New Customer

We’ve shipped more HX1188NL network transformers than any other single part number in our catalog. Engineers designing 10/100BASE-TX Ethernet interfaces order it for STM32 projects, IoT gateways, industrial I/O modules, and smart home controllers. After explaining the same application circuit and Bob Smith termination mistake to hundreds of customers, I figured it was time to write it all down properly.

What the HX1188NL actually does

The HX1188NL sits between your PHY chip and the RJ45 connector. It provides two critical functions that most engineers understand separately but sometimes miss the interaction between:

First, galvanic isolation — 1500V AC between the board side and the cable side. This protects the PHY chip from voltage differences between connected devices, and is the specification that determines whether the design survives a surge on the cable.

Second, impedance transformation — the 1CT:1CT turns ratio maintains the 100Ω differential impedance of the Ethernet cable across the isolation barrier. A 100Ω cable connected to a 100Ω transformer connected to a 100Ω PHY means reflections are minimized and return loss stays above 16 dB.

The spec that matters most: OCL at 100kHz

Open Circuit Inductance must be ≥ 350µH, tested at 100kHz with a 0.1V RMS test signal. Both conditions matter. Our production tests for the HX1188NL run at these exact conditions. When customers have asked us why their current supplier’s “400µH” part underperforms our “350µH” part, the answer is usually that the competing measurement was at 1kHz or 1V — which gives artificially inflated readings.

Measure your incoming samples yourself. LCR meter, 100kHz, 0.1V RMS. If you get below 350µH, the link will be unreliable, especially at temperature and over long cables.

The application circuit mistake I see most often

The Bob Smith termination. Every time.

The center taps of the HX1188NL — there are two, one on each winding — must connect to chassis ground through 75Ω + 1000pF to chassis ground. Not signal ground. Chassis ground.

I’ve reviewed schematics from customers who were failing radiated emissions testing and in nearly every case the Bob Smith network was either:

1. Connected to signal ground instead of chassis ground
2. Placed too far from the transformer (traces longer than 10mm to the components)
3. Missing entirely, replaced by a direct center-tap-to-ground connection

The 75Ω + 1000pF network is a high-frequency filter. At DC, it presents 75Ω to chassis ground. At 100MHz, the capacitor’s impedance is less than 2Ω, so common-mode noise gets a low-impedance path to chassis ground instead of radiating off the cable. Remove the filter and that noise radiates. That’s the CISPR Class B failure mode.

Which PHY chips pair well with the HX1188NL

I’ve seen good results with: RTL8201 (Realtek, most popular in cost-sensitive designs), LAN8720A (Microchip, dominant in STM32 Ethernet projects), KSZ8081 (Microchip, industrial grade, −40°C), DP83848 (TI, stable, well-documented), and W5500 (WIZnet, for standalone TCP/IP IoT modules).

All of these PHY chips are designed for standard 100Ω differential Ethernet magnetics. The HX1188NL is electrically compatible with all of them. The application circuit is the same; the only variation is whether your PHY needs series resistors on the TX side. LAN8720A, for example, typically doesn’t need them because the integrated output impedance is already matched. RTL8201 sometimes calls for 49.9Ω series resistors — check the PHY evaluation board schematic from the manufacturer.

PCB layout non-negotiables

Transformer within 10mm of RJ45. 4mm clearance moat under the transformer between board-side and RJ45-side copper. Bob Smith components within 5mm of center tap pins. 100Ω differential traces, equal length. Chassis ground and signal ground connected at one point only.

None of these rules are guidelines — all of them affect either EMC or link performance in ways that show up in testing.

For sourcing: we’re Voohu Technology (voohuele.com), and the HX1188NL is our highest-volume product. 50pcs MOQ, DHL 3–5 days to Japan, Korea, and Southeast Asia. We include lot OCL test data with every shipment and can cross-reference against Pulse H1102NL or other equivalents if you’re qualifying a replacement.