Sprague-Goodman: PISTONCAP® Precision Trimmer Capacitors

PISTONCAP® PRECISION TRIMMER CAPACITORS
Design Details

All Sprague-Goodman PISTONCAP® Trimmer Capacitors in US Patent No. 3,840,786 utilize a simplified and highly reliable adjust mechanism. There are two slots 180° apart, more than halfway through the bushing threads. Following slotting, the bushing is compressed, causing the slotted end to behave as a spring. The threaded segments of the bushing are biased axially, pressing the bushing threads firmly into flank-to-flank engagement with the screw threads. This eliminates all backlash and provides the required smooth and uniform adjust torque. Note that no extra threaded washers, springs, etc. are required. Fewer parts mean greater reliability.

Form Factor C, Open End
Form Factor P, Open End

SEALED TYPES. Sealed versions are available in panel mount, printed circuit mount, surface mount, and direct traverse. The electrode end is sealed either by closing the end of the glass tube or by soldering on a turret cap. The adjust end is sealed by means of a multilobed seal. This seal provides twice as many lobes as an "O" ring and good sealing is provided with one-quarter of the gasket compression required for an "O" ring.

Form Factor L, Printed Circuit Mount,
Sealed Construction
Form Factor R, Panel Mount,
Sealed Construction

COLLARED BUSHING STYLES (Form Factors N and V) Certain applications require that the head of the adjusting screw remains wholly within the bushing over the entire range of adjustment. To meet this need, collared bushing types, both sealed and unsealed, have been added to the broad line of printed circuit mount styles. All panel mount styles, other than the "W" series, are also designed for blind hole tuning.

Form Factor N, Collared Bushing,
Open End
Form Factor V, Collared Bushing,
Sealed Construction

W STYLES. In some panel mount applications, an extended adjust screw is required. In W series capacitors, the adjust screw protrudes from the mounting bushing for all positions of adjustment.

Form Factor W

THREE DIELECTRICS. Three dielectrics are available. The standard glass has a dielectric constant of about 6.7 and a dissipation factor of about 0.0012. High-range glass achieves the unusual combination of higher dielectric constant but a lower dissipation factor (of about 8.5 and about 0.008, respectively). Quartz dielectric has the much lower dielectric constant of about 3.7, but has extremely low loss, for a dissipation factor of about 0.0001.

Generally, the standard glass, which is least expensive, will suit most applications. For some applications, where higher capacitance and/or greater Q is required for a given physical structure, high-range glass should be specified. For applications involving moderate RF currents, fused quartz is used.

High-range glass trimmers will generally cost about 25% more than standard glass, and quartz trimmers are about three times the price of standard glass trimmers. Expanded range types generally cost about 2.5 times the non-expanded range equivalents.

EXPANDED RANGE models are available in panel mount, printed surface mount, surface mount, and direct traverse styles - sealed and unsealed. The electrode band is embedded within the wall of the glass tube, providing a thinner dielectric gap and, at the same time, keeping the overall wall thick enough for mechanical strength. This generally yields about three times the capacitance range for a given size compared with the standard PISTONCAP® Capacitors. This is accomplished by a laminating technique. An electrode band is metalized on the OD of a thin (0.005") glass tube, and a slightly larger and heavier-walled tube is then telescoped over it. The two tubes are then fused, providing the embedded structure. Additional metalization is then added at both ends to provide electrical and mechanical termination.

Form Factor R, Expanded Range.