Toroidal inductors are often used in passive filter and equalizer circuits in the 1- to 100-MHz range. For appreciable relative permeabilities and moderate circuit selectivities, stray magnetic fields are sparse1. Coils can be adjusted to desired inductance by spreading or squeezing the turns of the magnet wire winding1. The design of toroidal inductors entails tradeoffs between cost and factors such as desired inductance value, physical size, unloaded Q, power handling, adjustability, fragility, and operating temperatures. For small signal levels, small physical size, and reasonable unloaded Q, Micrometals powdered iron toroids with outer diameters of 0.25 in. and 0.375 in. are commonly used2.
The maximum single-layer winding for a given size toroidal inductor is tabulated for various wire sizes2. For convenient adjustability, the number of turns must be somewhat less than the maximum single layer winding. Between 3 and 40 MHz, a suitable toroid is the Micrometals T37-6. This core, which uses Carbonyl SF iron powder, has a relative permeability of 8.5 and a temperature stability of 35 ppm/°C 2. It also has nominal dimensions of 0.375-in. outer diameter, 0.205-in. inner diameter, and 0.128-in. thickness. Wire sizes between 22 and 28 AWG are preferable. Thinner wire is fragile and prone to breakage. Thicker wire, when pulled tight, can fracture the toroidal core.
With 26-AWG wire, the maximum single-layer winding is 29 turns for the T37-6 toroid2. Allowing for adjustability, an inductor of 20 turns was wound and tested. It was connected in parallel with a 200-pf ceramic capacitor with a ±2% tolerance. Shown in the figure is a tank circuit installed in a series transmission path, within a 50-Ω test setup. This circuit acts as a single-resonator band-reject filter. It's tuned to maximum and minimum resonant frequencies by spreading and squeezing the turns. From the measured resonant frequency and known capacitance, the toroidal inductance is calculated as per equation 1:
L = 126.65/f2 (1)
where f is frequency in megahertz and L is inductance in microhenries.
The test setup illustrated in the figure also can be used to perform indirect measurements of the inductor's unloaded Q. Table 1 indicates the measured values of inductance.
It's possible to extend the range of realizable inductance without going to a larger diameter toroid or thinner magnet wire. This is accomplished by pressing two identical T37-6 cores together and winding them simultaneously with 20 turns of 26-AWG wire. The measurements previously made for a single-core inductor are repeated for a two-core inductor (Table 2).
By using two or more of the same toroidal cores wound together, an extended inductance range with acceptable adjustability can be achieved. The composite multicore inductor can be staked using Q-dope or suitable alternative materials. Such a design technique is appropriate for small-quantity applications. Fixed surface-mount inductors are typically employed for large-scale manufacture.
References:
- M.F. "Doug" DeMaw, "Ferromagnetic-Core Design Applications & Handbook," Prentice-Hall, Englewood Cliffs, NJ, 1981.
- "RF Applications, Micrometals Iron Powder Cores," Issue F, September 1996.