Contrary to information which sometimes appears in magazine articles, the choice of material is of prime importance if the expected results are to be realised from any design using ferromagnetic cores. Let us look at the two most common core materials in use; ferrite and iron powder. The choice between the two is made by considering whether the core will be used in a wideband or narrowband application and how much signal power will be handled. For a given size core, ferrite material will saturate at a much lower magnetic flux density than one made from iron powder. Permeability for ferrite materials ranges from 20 to 15000 while for iron powder it is from 2 to 75. As a "rule of thumb", the higher the permeability of the material the greater will be the temperature coefficient. If we are to use the core in a narrowband (tuned) circuit it therefore makes sense to use an iron powder type, which will remain closer to the calculated inductance as the temperature changes. In wideband applications (e.g. a balun) this is not so important.
For wideband circuits a ferrite is commonly used because the higher permeability of ferrite material will provide a higher inductance for a given number of turns and also provide tighter coupling. The type of ferrite chosen must exhibit low loss over the desired range of frequencies. The common rule for design of wideband transformers is that the reactance (XL) of a winding must not be less than four times the source impedance at the lowest frequency. "What about the effects of this at the high frequency end?" you may ask. Well luckily there is no cause for concern, as the effective permeability of the ferrite core material decreases with increasing frequency, thus reducing the inductance of the winding. With the proper selection of core material it is easy to make wideband transformers which cover one decade in frequency: e.g. 3 - 30 MHz.
Ferrite materials can be divided into two groups: those with initial permeabilities below 1000 which are nickel-zinc compounds and those above 1000 which are made from manganese-zinc compounds. The permeability of all ferrite materials is dependant on frequency, so simple calulations using the low frequency (approx. 10kHz) AL quoted values cannot be used where RF is involved. Nickel-zinc ferrites exhibit high volume resistivity, moderate temperature stability and can offer high Q factors for the 0.5 to 100 MHz frequency range. They are well suited for low power, high inductance applications, and their high permeability factors make them very useful for wideband transformer applications. The manganese-zinc group have relatively low volume resisitivity and moderate saturation flux density. They can give high Q factors for the 1 kHz to 1 MHz frequency range, and some are sutiable for switched-mode power conversion transformers operating between 20 and 100 kHz. Incidently, the high permeability iron powder core made from 26 or 52 material is particularly suitable for the filter inductor in switched mode power supplies.
Narrowband applications usually use iron powder cores which can provide good Q values into the VHF frequencies. Iron powder materials have very high resistivity due to the manufacturing process, and inductors made with any of these materials maintain their inductance up to about 500MHz. Wideband circuits by their very nature cannot have a high Q (circuit Q = centre freq/bandwidth). The better temperature coefficient of iron powder materials makes them the usual choice for any tuned circuit application. This is particularly important when using cores in oscillator and filter circuits.
Apart from the physical dimensions of a toroid (outside and inside diameter, thickness) there is a value given for each particular core size and material, which is usually called the AL value, and is the manufacturers inductance index for the core. Unlike ferrite materials the AL value for iron powder materials is not frequency dependant, and the quoted values can be used directly in the formula. Manufacturers data for iron powder and ferrite cores are in the data tables and show all the required information. See the core cross reference tables for some equivalents. The AL figure for iron powder cores and ferrite cores is normally quoted as nH/turn2.28/12/13