Designing trimmer capacitors into high frequency circuits may sometimes make more sense than using lower-cost, fixed-value capacitors.

Scott Newman
Voltronics Corporation, 100 Ford Road, Denville, NJ 07834; (973) 586-8585, FAX: (973) 586-3405, Internet: www.voltronicscorp.com.

Design engineers are often faced with the cost-versus-performance tradeoff. At first, the use of fixed-value capacitors rather than variable-value trimmer capacitors may seem more practical in new designs. But, in addition to high levels of performance and stability, trimmer capacitors are now approaching the size of fixed capacitors, with several advantages in terms of flexibility.

For example, a general assumption is that the best performance can be achieved by using fixed-value circuit elements, such as capacitors and resistors, with the tightest tolerances possible. Of course, tighter tolerances mean higher prices. And it is simply not possible to specify and assemble fixed capacitors on a printed-circuit board (PCB) while maintaining frequency tolerance of 0.01% or even 0.1%. These tolerances cannot be maintained due to component drift, manufacturing variability (such as component placement on a circuit trace), and even stray capacitances will increase the effective tolerance. Changing a capacitor to change the capacitance value usually means reworking or destroying the PCB. And laser trimming of components, although capable of providing extremely tight tolerances, is a one-shot deal that cannot be readjusted to account for frequency drift during burn-in or during aging and temperature cycling.

A practical alternative to fixed-value capacitors is to specify a small, low-cost trimmer capacitor set to a specific value (equal to the fixed-value component) at the factory. Then, if a change in capacitance is required in the design, the capacitor can be adjusted without reworking or destroying the PCB. For example, in place of a 6.4-pF fixed-value capacitor, Voltronics Corp. can supply a model A1M8 variable capacitor set within 1% of the 6.4-pF value and the equivalent stability of a fixed-value capacitor. The latest solid-dielectric (glass, sapphire, PTFE) trimmer capacitors from Voltronics Corp. are as stable as fixed-value capacitors, and won’t suffer degraded performance or short out due to moisture, dust, shock, vibration or space environments.

The A1 through A4 series trimmer capacitors, for example, are based on a patented solid dielectric for low-loss, high-frequency applications. The low-cost trimmers provide high quality factors (Qs) and can be supplied from the factory set to values that exactly match those of popular fixed-value capacitors, with the added capability of capacitance tenability in the field. The trimmer families include the following models:

A1-0.4 in. long by 0.155 in. diameter, maximum values of 4, 8, and 12 pF;
A2-0.225 in. long by 0.094 in. diameter, tuning range of 0.3 to 1.2 pF;
A3-0.5 in. long by 0.3 in. diameter, tuning range of 1 to 10 pF; and
A4-0.225 to 0.325 in. long by 0.16 in. diameter, maximum values of 2, 3, and 5 pF.

Designed for low-cost wireless communications and instrumentation applications, all four product families can be surface-mounted or PCB mounted. High-voltage versions are available with ratings from 500 to 1500 WVDC. In addition, non-magnetic versions are available for magnetic-resonance-imaging (MRI) and nuclear-magnetic-resonance (NMR) applications. Larger styles, measuring 0.5 or 1.0 in. long by 0.3 in. diameter, are also available with maximum capacitance values of 10, 16, 23, 30, 40, and 55 pF, also in non-magnetic versions.

Although the trimmer capacitors already mentioned are standard parts, the company also offers quick-turnaround on special customer requirements. For example, one customer recently saved expense and assembly time by ordering a 10-pF trimmer with a special hex bushing. This eliminated two small nuts and allowed immediate assembly without adjusting the smaller nuts. To increase reliability the capacitor used solid dielectric instead of air in the same size body (model V3600). Also, a customer with a requirement for a split stator trimmer was supplied with a design featuring 1.5- and 2.0-pF trimmers housed in a 0.53-in.-long sapphire NMR non-magnetic trimmer package (model NMKZ3S). Finally, a customer with a difficult accessibility problem found the solution in the V2592 trimmer with thumb-wheel side tuning and maximum ratings of 40 pF and 3 kV.

The Voltronics Corp. catalog (available for download from this website) represents just a small sampling of the company’s available trimmer capacitor technology. The company has grown by being responsive to our customers’ needs. For a special requirement, please contact our engineering department for a quick turnaround of your ideal trimmer.