For a number of years, I've been using tiny high-mu toroid cores made by Vacuumschmelze for ISDN transformers. These were amorphous metal cores, Vitrovac T60009-E4006-W650. It's a 6.9*3.2*2.3mm core with Al=13uH/t^2. The LF mu_r was in the 100k ballpark. The data sheet is at , if you're interested.
They were little miracles, these cores, and I ended up using them in lots of places: RF transformers, hybrids, low noise amplifiers, etc. I've used thousands just by myself, despite the fact that I never manufacture large numbers of any particular device. It's with these cores that I made these fantastic five decade bandwidth transformers. Bandwidth is obtained with the greatest possible inductance in the tiniest possible space. With ferrites, I usually get only three decades.
The trouble is that Vacuumschmelze stopped making them, and I want more! I've looked at lots of other cores, both ferrite and amorphous or nanocrystalline, but nothing works quite as well. The closest I've found is a Metglas Magnaperm core, type MP0805L4AF. It's close, but still not quite as good. I thought I'd give it a shot, asking if anyone here can point me to a secret stash of these Vitrovac cores?
Jeroen Belleman