Understanding the different options helps simplify the task of selecting a trimmer capacitor for high-frequency
and wireless applications.
Scott Newman
Voltronics Corporation, 100 Ford Road, Denville, NJ 07834; (973) 586-8585, FAX: (973) 586-3405, Internet:
www.voltronicscorp.com.
Trimmer capacitors provide a convenient and reliable means of tuning the latest wireless communications circuits. Although
using a trimmer capacitor is as simple as turning a screw, choosing the right trimmer for a particular application is a
matter of meeting sets of electrical and mechanical requirements.
Trimmer capacitors are still the most cost-effective way to tune a circuit for optimum performance. They provide a range of
adjustable capacitance useful in compensating for circuit variations. For example, the crystal resonators employed in fixed
oscillators can drift in frequency following burn-in and temperature cycling. A properly selected trimmer capacitor can bring
the crystal back to the precise desired frequency.
Not all trimmers are created equal. Conventional trimmer capacitors are not sealed components and can suffer degraded
performance or even failure due to contamination from dirt and other foreign substances. More reliable designs such as
trimmer capacitors from Voltronics Corp. incorporate O-ring seals to allow tuning within a sealed mechanism that maintains
reliable and consistent performance over a wide range of environmental conditions.
Trimmer capacitors come in many package styles and performance ranges. Choosing the right trimmer is a matter of matching
the component to the application. Trimmer types include:
- half-turn trimmers, with film, mica, and paper dielectrics and in ceramic through-hole and chip-sized
surface-mount-technology (SMT) packages; and
- multi-turn trimmers, in tubular style packages with glass, quartz, sapphire, air, and PTFE dielectric materials.
The most common half-turn trimmers are ceramic, with millions in operation. They are small, inexpensive, and can be
supplied in tape-and-reel format for use with automated assembly equipment. In spite of their small size, they can provide a
generous amount of capacitance (up to 100 pF) for high-frequency applications. On the negative side, many half-turn ceramic
trimmers cannot be tuned with a great deal of precision, and they tend to exhibit at best mediocre temperature stability,
which degrades with increasing capacitance.
But for applications that require adjustable capacitance in a miniature, surface-mountable package at low cost, the half-turn
ceramic trimmer provides enough capacitance range to fine-tune filter center frequencies and correct for oscillator drift.
The quality factor (Q) of these trimmers is about 1500 at 1 MHz. The temperature coefficient is typically 0 to -750 ppm/°C
while capacitance drift ranges from ± 1% to ±5%. These trimmers are generally rated for maximum DC withstanding
voltage of about 220 V.
Multi-turn tubular trimmers are more suited for applications requiring higher levels of performance. Based on glass, quartz,
sapphire, air, or polytetrafluoroethylene (PTFE) dielectric materials, these trimmers can be made with extremely linear tuning
characteristics that suffer no reversals in the tuning range. They can be produced with high Qs, low RF loss, low series
inductance, and self-resonant frequencies (SRFs) as high as 10 GHz.
Mult-turn glass, quartz, and sapphire trimmers change capacitance by moving a piston inside a dielectric tube, which is
metalized on the outside. As the piston overlaps with more of the stator plates, the capacitance increases. The dielectric
tubes feature inner diameters manufactured to ±0.0002 in. precision with matching pistons. They are available in
numerous capacitance ranges, depending on the dielectric, with maximum values of 16 pF for quartz, 250 pF for glass, 8 pF for
sapphire, and 55 pF for PTFE .
Two types of tuning mechanisms are used in tubular piston-type trimmers: rotating and non-rotating. In a rotating tubular-type
trimmer, the piston is attached to the adjustment screw as a single assembly. As the assembly is turned into a threaded
bushing, the piston moves within the metalized part of the dielectric tube. This type of trimmer is relatively easy to
assembly and machine, resulting in a lower-cost capacitor. Unfortunately, when the piston rotates, the air gap between the
piston and the glass can vary due to irregularities in the machining of mating parts, resulting in capacitance tuning
nonlinearities or tuning reversals. The tuning linearity of a rotating tubular trimmer capacitor is on the order of ±10%.
In non-rotating tubular trimmers, the piston is placed on bushing rails and is driven by a screw that’s captured in the
bushing and does not move axially. As the screw is turned, the piston slides along the rails and moves into the metalized
area of the dielectric tube. Because the piston does not rotate, the air gap is constant and tuning is linear within ±1%.
It also assures better stability under shock and vibration. Because the screw slot remains fixed in the axial plane, tuning
adjustments are easier to make than for rotating tubular-type trimmers. Because the current runs along the bushing rails
rather than along the screw, non-rotating tubular trimmers have lower inductance and higher SRFs.
For higher-frequency applications in which high Qs and high SRFs are essential, multi-turn trimmer capacitors based on air,
sapphire, or PTFE dielectric materials provide the lowest loss and best overall performance. In an air-dielectric tubular
trimmer, capacitance is varied by a movable set of concentric metal rings fitted into a fixed set of parallel rings. As the
rings mesh, the capacitance increases. By using a screw with fine threads, multiple turns of capacitance adjustment can be
provided with extremely high adjustment resolution. In a PTFE trimmer, the air cavity is filled with solid PTFE dielectric
material The PTFE material exhibits a higher dielectric constant than air and much higher voltage rating (to 2 kV and higher).
