[HomeBrew] winding toroids

[Richard Blackman]

Hello to group and best wishes for a peaceful and healthy 2007.

I am wondering where I can read up on winding trifilar toroids for double 
balanced mixers. I understand the phasing, toroid types and winding, but how 
does one calculate just how many turns are needed through the core? Most 
cookbooks say 15 turns on a type 43 core, but never explain how that figure 
is arrived at. Is it size dependant?
Frequency dependant? Broadband dependant?

Any comments or guidance is welcome.

Richard VA3NDO