WHDG88409G Gigabit LAN Transformer

The two match on electrical ratings and land pattern, so choose purely by which side the common-mode choke should land on: the WHDG88409G puts it toward the low-numbered pins, the WHDG88403G toward the high-numbered ones.

The deciding factor is the PHY-to-RJ45 arrangement on the board and the element order in the PHY reference design, not performance — insertion loss, return loss, cross talk, common-mode rejection and isolation match item for item.

One group per port, eleven pins on each side: 1–11 with 78–88, 12–22 with 67–77, 23–33 with 56–66, and 34–44 with 45–55.

Each group holds four 1CT:1CT units — the four channels of one Gigabit port. Orient the WHDG88409G with the low-numbered side toward the PHY and the high-numbered side toward the RJ45, and each port's differential pairs run their own path without crossing under the part.

Yes. Every port on the WHDG88409G has four channels: a Gigabit port uses all four, that is two pairsets, while a 100M port uses one pairset and leaves the rest unused.

The transformer does not distinguish speed — the rate is negotiated by the PHYs at each end — so mixing speeds across the four ports needs neither a different part number nor a different termination. Dedicated 10/100 parts are listed in the 10/100 BASE-T range.