Complete application reference for the HX1188NL 10/100BASE-TX network transformer — circuit, Bob Smith termination, PHY compatibility, and layout checklist.
Key Electrical Specs
Parameter Value Test Condition
─────────────────────────────────────────────────────────
Application 10/100BASE-TX IEEE 802.3 Clause 40
OCL (min) 350 µH 100 kHz, 0.1 V RMS
Insertion Loss (max) < 1.0 dB 1–100 MHz, 100Ω diff
Return Loss (min) > 16 dB 1–100 MHz, 100Ω diff
Isolation voltage 1500 V AC 60 seconds
Turns ratio 1CT:1CT Center taps on both windings
DCR (per winding) < 5 Ω 4-wire measurement
Package SMD 8-pin or 10-pin
Typical Application Circuit
PHY SIDE HX1188NL RJ45 SIDE
┌─────────────────┐
TX+ ──[49.9Ω opt]──────►│P_TX+ S_RX+ │◄────────────── RJ45 Pin 1
TX- ──[49.9Ω opt]──────►│P_TX- S_RX- │◄────────────── RJ45 Pin 2
│ CT_P CT_S │
RX+ ◄──────────────────-│P_RX+ S_TX+ │──────────────► RJ45 Pin 3
RX- ◄───────────────────│P_RX- S_TX- │──────────────► RJ45 Pin 6
└────┬────────┬──┘
│ │
[75Ω] [75Ω] ← Bob Smith
│ │
[1nF] [1nF]
│ │
CHASSIS_GND ← NOT signal GND
Bob Smith Termination: Correct vs. Wrong
python# Bob Smith termination requirements
BOB_SMITH_SPEC = {
"resistor": "75Ω",
"capacitor": "1000pF (1nF)",
"connection": "CHASSIS GND", # ← grounded to RJ45 shell
"placement": "≤ 5mm from CT pin",
"networks": 2, # one per winding CT
}
Common mistakes
MISTAKES = {
"wrong_ground": {
"description": "CT connected to signal GND instead of chassis GND",
"symptom": "Radiated EMC failure, CISPR 22 Class B exceeded",
"fix": "Separate chassis GND pour; connect to RJ45 shell metal"
},
"too_far": {
"description": "Bob Smith components placed > 10mm from CT pins",
"symptom": "HF common-mode noise not suppressed; fails at 30-100 MHz",
"fix": "Move components to within 5mm of transformer CT pins"
},
"missing": {
"description": "CT pins left floating or shorted directly to ground",
"symptom": "Emissions failure or degraded return loss",
"fix": "Always use 75Ω + 1000pF — never leave CT floating"
}
}
PHY Chip Compatibility Table
PHY Chip Vendor Temp Range Series R Needed Notes
─────────────────────────────────────────────────────────────────────────────
LAN8720A Microchip 0/+70°C No (usually) Popular with STM32
KSZ8081 Microchip −40/+85°C Check datasheet Industrial grade
DP83848 TI −40/+85°C No Stable, well-documented
RTL8201F Realtek 0/+70°C 49.9Ω recommended Cost-optimized designs
W5500 WIZnet 0/+70°C No Standalone TCP/IP
ENC28J60 Microchip −40/+85°C No SPI interface
IP101G ICPlus 0/+70°C Optional Low cost
PCB Layout Checklist
Placement:
[ ] Transformer within 10mm of RJ45 connector
[ ] Bob Smith components within 5mm of CT pins
[ ] No signal traces routed under transformer body
Isolation moat:
[ ] ≥ 4mm copper clearance between board-side and cable-side pours
[ ] ≥ 6.4mm creepage for 1500V AC isolation (IPC-2221)
[ ] No vias inside isolation moat
Routing:
[ ] TX+/TX- equal length (match within 0.5mm)
[ ] RX+/RX- equal length (match within 0.5mm)
[ ] 100Ω differential impedance (verify with stack-up calculator)
Grounding:
[ ] Signal GND and Chassis GND separated under transformer
[ ] Single connection point between signal GND and chassis GND
[ ] Chassis GND tied to RJ45 shell tabs
Quick Cross-Reference
HX1188NL equivalents (verify pin-out before swapping):
Pulse H1102NL — electrical equivalent, different pin-out
HanRun HR911103A — check OCL test conditions before accepting
Always rebuild footprint from target part's datasheet — never reuse pin assignments across vendors.
Voohu Technology manufactures the HX1188NL with lot OCL test data included.
→ www.voohuele.com | MOQ 50pcs | DHL 3–5 days to Japan / Korea / SE Asia
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