How mechanical filters work

Seven-resonator torsional mechanical filter

The principles of operation of a mechanical filter are quite simple. By analogy, electrical tuned circuits are replaced by mechanical metal alloy resonators, and the coupling inductors are replaced by stiff coupling wires, as shown in the filter drawing on this page. An electrical signal applied to one of the filter's ports will produce an electrical field across the piezoelectric transducer. This causes the transducer resonator, to vibrate in torsion. The mechanical vibration is coupled from resonator to resonator by means of the coupling wires. Vibration of the output transducer causes a filtered voltage to be generated at the filter's output port.

Types of mechanical filters

Rockwell produces three types of mechanical filter. These are a torsional mode, a bar flexural mode and a disk flexure mode filter.

The torsional mode filter uses rods that vibrate in torsion. Electrical energy is coupled in by means of a piezoelectric ceramic transducer into torsional motion. We build these filters with center frequencies from below 100 kHz to above 700 kHz.  Bandwidths range from .05 to 5 percent of center frequency. Designs can have as many as twelve poles.

Bar Flexural Mode Filter

The bar flexural mode mechanical filter is used for low frequency designs. The available center frequency range is from 5 to 100 kHz and bandwidths of .2 to 1.5 percent.

 
Disk Wire Filter

Disk wire filters use a drumhead mode of vibration of a disk. While these are no longer designed, we still produce many of the 2000 designs created over the years.