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Miniature 30 pF multiturn trimmer capacitors.

Under increasing pressure to reduce the size of their circuitry, designers of personal communications services, paging and specialized mobile radio base stations have been attempting to eliminate 30 pF piston trimmer capacitors because of their prohibitive size and poor tuning linearity. The AT30SD series multiturn, solid-dielectric, 30 pF piston trimmer capacitors solve these problems. Each trimmer capacitor is half the size of conventional air-dielectric types and has extremely linear tuning without reversals. Figure 1 shows the trimmer capacitor's mechanical outline.

By utilizing a solid dielectric instead of air, the narrow 0.003[inch] gaps between the concentric ring capacitor plates of air-dielectric capacitors are eliminated. The trimmer's concentric ring piston is separated from the intermeshing stator plates by a contiguous sheath of solid insulation. This design inherently eliminates any possibility of shorting, voltage breakdown or capacitive microphonics, which are serious potential problems in conventional 30 pF air-dielectric piston trimmer capacitors. High reliability and long operating life are assured even trader severe conditions of shock, vibration and environmental stress.

Capacitor tuning is accomplished with a unique nonrotating torque mechanism that keeps the tuning screw head position fixed in the axial plane, permitting the use of an O-ring to accomplish a 40 psi dynamic environmental seal without requiring separate seal caps that can fall off and cause short circuits. The trimmer's nonrotating piston design typically provides tuning resolution of [+ or -]1 percent, a tenfold improvement over the [+ or -]10 percent performance of conventional air-dielectric piston trimmer capacitors. Since the AT30SD series' tuning screw head does not traverse the axial plane, extended metallic or insulated shafts permit through-panel tuning to be accomplished easily.

The unit's positive piston travel stops at minimum and maximum settings, ensuring the piston cannot exit the trimmer capacitor body as it can in conventional air-dielectric rotating piston-type trimmer capacitors. The AT30SD series is the highest capacitance member of a 0.5-inch-long family of trimmer capacitors [TABULAR DATA FOR TABLE I OMITTED] with capacitance ranges of 4, 10, 16 and 23 pF, as listed in Table 1. When maximum capacitance values greater than 30 pF are required, the models AT40SD and AT55SD offer ranges of 1.5 to 40 pF and 1.5 to 55 pF, respectively. Standard model AT40SD voltage ratings are 250 V DC working voltage and 500 V DC withstanding voltage. A working voltage of 1000 V DC and withstanding voltage of 2000 V DC are standard on the model AT40HV. The high voltage version of the model AT30SD has a working voltage of 750 V DC and a withstanding voltage of 1500 V DC.

Both SD and HV versions of the 40 and 55 pF trimmers are identical in size with an outside diameter of 0.31[inch] and a body length of 1[inch]. Standard mounting configurations are horizontal printed circuit, horizontal surface mount and panel mount. Nonmagnetic and extended-shaft versions are available. Technical information on these trimmer capacitors is available at the company's Web site at

Voltronics Corp., Denville, NJ (973) 586-8585.

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Title Annotation:Product Feature
Publication:Microwave Journal
Date:Sep 1, 1997
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