Published:2011/7/24 22:52:00 Author:Li xiao na From:SeekIC
By B. Kainka
Inductors with cores
RF inductor forms with threaded ferrite cores are often used in practice. The core increases the inductance, typically by a factor of four or even more. The inductor can be tuned by adjusting how far the threaded core is screwed into the inductor. Ferrite cores are made for specific frequency ranges, within which they exhibit small energy losses.
Significantly higher inductances can be obtained using closed cores with or without air gaps. Although an air gap reduces the inductance, it allows a higher magnetization levels to be used, since it prevents the core from becoming magnetically saturated even at high currents. The types of cores commonly used are toroidal (ring) cores, E-I transformer cores (Figure 2) and closed pot cores.
With such cores, the inductance strongly depends on the material used and the geometry of the core, as well as the number of turns. This means that it is not possible to give a general formula for calculating the inductance, as with air-core inductors. Instead, manufacturers state an AL value’ in nH/n2 for each core, such that
L = AT xn2 {nH}
For example, an Amidon T37-2 ring core has an AL value of 40 nH/n2. If you wind a coil of 10 turns on such a core, you will obtain an inductance of L = 4000 nH = 4 μH.
Ring-core inductors, like air-core inductors, are suitable for building RF resonant circuits. Besides the AL value, the design frequency range of the core is also important. Amidon Type xxx-2 cores (with red marking) are suitable for frequencies up to 30 MHz. The calculation program, which is described in more detail below, can be used to quickly determine the inductance of air-core inductors and inductors with known AL values.
Reprinted Url Of This Article: http://www.seekic.com/blog/project_solutions/2011/07/24/DIY_RF_Inductors__The_ABCs_of_inductors_(2).html
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