Electronic Design

Low-Cost VHF Inductors Use Nylon Toroids

Toroidal inductors can be used in passive-filter and equalizer circuits in the VHF range. Although SAW filters have taken over many bandpass filter applications, LC low-pass and high-pass filters are still viable.

When inductance values are smaller than 1 µH, air cores with unity relative permeability can be used instead of powdered-iron or ferrite cores. For magnet-wire sizes thinner than number 20 AWG, a coil form is often needed for mechanical support. Nylon 6/6 standard flat washers are usable as low-cost coil forms. The nominal electrical parameters of this material are a dielectric constant of 3.6, a dissipation factor of 0.04, and a dielectric strength of 385 V/mil.

Two different-sized nylon washers were used in fabricating these toroidal inductors (see Table 1). Some typical toroidal inductors were then wound and tested as illustrated in Table 2. The maximum inductances shown were obtained with the windings squeezed, and minimum inductances were obtained with the windings spread. Inductance values were calculated from measured resonant frequencies using a known capacitor. As the winding area is filled, the adjustability of toroidal inductors decreases. The nylon toroids have limitations on the number of turns that can be applied as a single-layer winding (see Table 3).

Despite its dielectric constant and dissipation factor, the quality of nylon toroidal inductors is quite good. Typical inductor unloaded Q's are in the 75 to 125 range. Inductor unloaded Q's of 50 are very adequate for most low-pass and high-pass filters. Surface-mount inductors with unloaded Q's of only 10 to 20 can still be used in many of these filters. Nylon toroidal inductors are primarily suitable for small-signal applications.

Low-cost VHF inductors can be realized using readily available standard nylon washers as coil forms. These inductors can provide small size, some adjustability, and efficient unloaded Q's. When operating between 30 and 100 MHz, they are very useful for prototypes and small-quantity runs. For large-volume production, surface-mount inductors or integrated circuits, at the subsystem and system levels, would typically be preferred.

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