Hi,
Usually a series termination at the source is used to swallow up reflections if the transmission line is not terminated at the end or is not well terminated. I think it would also make the effective load the driver sees be higher impedance, as the driver would see the source termination in series with the load of the transmission line. So if the coax were 50 ohms and you had a series 50 ohm resistor, then the driver only is trying to drive a 100 ohm load. But then you also get a voltage divider so you lose half your swing in the series termination. But without it, you would suffer a reduced swing anyway if the driver can't well drive a 50 oh load. I don't think the source termination would hurt at all.
The Coilcraft transformers are 1:1 impedance ratio, so for a single 50 ohm coax signal source the proper termination after the transformers would still be 50 ohm differential. Usually this would be a 25 ohm plus a 25 ohm to a center point that has an AC cap to ground, but i think this EVM spread that termination out a bit. 100 ohm differential after the transformers in parallel with a 100 ohm differenital between the transformers (if I remember right). I don't know that spreading out the termination like this really did anything or not. Normally we would just make R47 and R50 be uninstalled and make R48 and R49 = 25 ohms. Different people were doing the EVMs back then.
Regards,
Richard P.