explanation for discretes on ethernet PHY interface

[Apparatus]

Hello,

I am looking at pg. 108 of 51471a.pdf (found on
http://www.microchip.com), in particular at the FL1012 ethernet
transformer chip. I see that before the transformers, there is a 200
ohm resistor across the transmit lines. I also notice that on the
primary sides of the transmit and receive transformers, there are
0.1uF capacitors tied to the transformer center taps and ground. Why
the need for these components?

Cheers,
Chandler

 

[Allan Herriman]

Hello,

I am looking at pg. 108 of 51471a.pdf (found on
http://www.microchip.com),

This one?
http://ww1.microchip.com/downloads/en/DeviceDoc/51471a.pdf

Here is the relevant datasheet (for the Ethernet chip):
ftp://152.104.125.40/cn/nic/rtl8019as/spec-8019as.zip

in particular at the FL1012 ethernet
transformer chip. I see that before the transformers, there is a 200
ohm resistor across the transmit lines. I also notice that on the
primary sides of the transmit and receive transformers, there are
0.1uF capacitors tied to the transformer center taps and ground. Why
the need for these components?

This is an Ethernet interface, so it must (should) comply with the
appropriate requirements:
http://standards.ieee.org/getieee802/802.3.html

The 200 ohm resistor is actually on the receive lines, and sets the
input return loss, which is a requirement of 802.3 section 14.3.1.3.4
The output return loss (also defined in 14.3) seems to be determined
by resistors inside the '8019.

The 100nF caps help reduce the common mode signal coupled across the
transformer (by the few pF of stray cap between the windings). This
helps to reduce EMI, and to reduce the rate of bit errors caused by
common mode signals. See e.g. 802.3 14.3.1.3.5

Regards,
Allan.