"Ian White, G3SEK" wrote in message
...

Ferrite isn't magic, so don't expect one small clip-on ferrite core to
solve all your RFI problems.

There are some RFI cases where a single clip-on core is a complete
cure... but there are many more cases where it's not enough.

To have some serious chance of curing RFI, you need to use several turns
on a fairly large lump of ferrite.

Also, note that the simple ferrite fix only works for common mode RF.
Anything that is differential will get rectified and detected. The phone
line could be unbalanced at RF, even though it is balanced at audio. The
only way to supress differential RF is to filter each lead separately.

The reason for the rectification is that they tend to put a bridge rectifier
at the line input. This way the phone still works if the tip/ring polarity
of the 48V has been switched.

Tam/WB2TT