It seems that RF3 type devices are not understood by everyone so this is an attempt to clarify and an appeal for more facts.
The device acts as a mode filter, attenuating interference. It is NOT a frequency selective filter so it can improve S/I ratio even when the frequency bands overlap.
Firstly imagine a single wire which is acting as an aerial and picking up interference. Then inserting a coil in series adds a large impedance and reduces the interference current by producing an out of phase "back emf". (fig 1)
If the "wire" is a pair then similarly, a coil in each wire has the same effect.
A 1:1 transformer in place of the coils has a similar effect to this parallel mode current(red arrows in fig 2).
However if there is a differential mode signal (shown in blue arrows), the transformer has negligible effect on this since the back emf from one current is cancelled out by the opposing back emf by the return current in the opposite wire
fed in via the transformer.
This simple explanation ignores losses, stray inductance and capacitance. The effect of these is a slight loss to the
wanted signal, a fraction of a dB, and restriction of the frequency bandwidth of operation. Thus a design which is OK for ADSL1 may be poor for VDSL2 due to their different max frequencies.
The I plate includes such a device as well as the bell wire choke. Can anyone confirm this is the same component as the in the RF3 ?
I think that RF3 = RF2 but with less inductance, to work on ADSL2/2+ anyone confirm ?
The ideal no-compromise solution would be to build the device into the modem, so the requirements match and also remove any unprotected length of cable between v device and modem, but I'm not holding my breath ! Or do some modems include one
already ?