April 1969 Electronics World
Table of Contents
Wax nostalgic about and learn from the history of early electronics. See articles
Electronics World, published May 1959
- December 1971. All copyrights hereby acknowledged.
Mechanical filters of the type
described in this 1969 Electronics World article are yet another example of
the genius of some people. They are actually a form of electromechanical device in that
the applied electrical signals are first converted into mechanical signals, followed
by resonant mechanical elements that discriminate according to frequency, and finally
a conversion back to an electrical signal is made. It is fundamentally the same principal
as a crystal, SAW, or BAW filter, albeit each with distinctly different methods and topologies.
Mr. Donovan Southworth, of Collins Radio, presents the basics of mechanical filters in
this brief write-up. There is an excellent article on
filters on Wikipedia.
The author has experience in all phases of mechanical filter research
and development. He joined Collins in 1961 after receiving his BSEE degree from Washington
State University. His initial work was in the field of synthesis and fabrication of crystal
filter networks. Since 1963 he has participated in studies of higher order vibration
modes for extension of the mechanical filter operating frequency range, and in the development
of filter design by computer programs. He was project engineer of the mechanical Minifiiter
and is currently involved in the design and fabrication of mechanical filter networks.
By Donovan A. Southworth / Collins Radio Company
Mechanical filters are not new, but new manufacturing techniques and new filter configurations
have made them "tops" in sophisticated transceivers.
Mechanical filters were conceived and designed to provide a unique combination of
high selectivity and stability in a compact package at a low cost. Many of these filters
use a disk-wire construction (Fig. 1.) which has become popular in our industry.
The disk-wire filter is a mechanically resonant device which receives an electrical
signal, converts this signal into mechanical vibrations, rejects unwanted frequencies
within the mechanical structure, and then converts the mechanical vibrations back into
electrical energy. The filter consists of three basic elements: input and output transducers
which convert electrical signals into mechanical vibrations and vice versa; high-"Q"
mechanically resonant metal disks; and coupling wires which acoustically transmit energy
between the disk resonators.
In Fig. 3, if an electrical signal is applied to the input coil, it produces an alternating
magnetic field that passes through the magnetostrictive transducer attached to the end
disk. The transducer, when magnetically biased and tuned to vibrate at the impressed
signal frequency, drives the first disk. The short coupling wires connecting the disks
drive the next disk, and so on until the signal reaches the output transducer. The output
transducer converts the mechanical vibrations into an induced voltage across the output
coil and creates an electrical output.
Engineers are busy in Europe and Japan, as well as in the United States, designing
mechanical filters and resonators. One popular i.f. filter design utilizes a wire-coupled
torsional rod. But much of the engineering activity involves units operating below 50 kHz,
where physical configurations other than the disk-wire or rod-wire arrangements are more
suitable for production. In this frequency range, filters/resonators utilize a tuning
fork or flexure mode bar. The devices commercially available in this range are often
single resonator types rather than multiple-coupled resonators.
Disk-wire type filters are manufactured in the 60- to 600- kHz frequency range
with pass bandwidths ranging from 0.06% to 10% (see Fig. 2). The shape factor (60 dB
to 3 dB bandwidth ratio) is typically from 2:1 to 1.5:1 although in certain critical
applications where even better selectivity is required, shape factors as low as 1.2:1
are being built. Frequency stability with temperature change can be made equal to the
stability of a DT-cut crystal. This is one of the most widely used crystal cuts in the
200- to 500-kHz frequency range. Modern network design techniques have resulted in passband
response variation of less than 0.5 dB; and insertion loss values as low as 2 dB can
be realized, but a more typical value is 6 dB. Prices start around $7.00 and are related
to performance requirements.
Mechanical filters were originally designed in the U.S. for use in single-sideband
radios. They contributed to the success of such radios in the early 1950's and are still
being used extensively in single-sideband equipment. The small size and weight of a mechanical
filter make it very desirable for use in equipment such as manpack radios. These characteristics,
in addition to high reliability, excellent frequency stability with temperature, and
good cut-off characteristics, combine to satisfy the stringent requirements of military
or commercial aircraft communications and navigation equipment. Generally, mechanical
filters should be considered for use anywhere that high performance, small size, and
reasonable cost are required.
Fig. 1 - Varying the coupling of a multi-element mechanical filter
changes its bandwidth. Bandwidths range from 350 Hz - 50 kHz.
Fig. 2 - Available percent bandwidth versus center frequency for a
typical filter unit.
Fig. 3 - An electrical analogy of a typical multi-element mechanical
filter. The mechanical vibration of the input transducer is coupled by successive disks
to the output magnetostrictive transducer where the vibration is converted into electrical
energy. The equivalent circuit is also shown.
Mechanical filters made in the U.S. employ two basic transducer types: a nickel-iron
alloy wire and a nickel-ferrite rod. The filters which use the nickel-iron wire transducers
are essentially self-terminated and have a relatively high insertion loss. They may be
driven from any source impedance greater than 50k ohms by parallel tuning the transducer
coil, or they may be driven from any impedance lower than 200 ohms by series-tuning the
transducer coil. The same conditions apply for the filter output. If the circuit designer
finds it to his advantage, a combination of series- and parallel-tuning capacitors may
Filters using ferrite transducers have low insertion loss and are designed to work
with a specific terminating resistance. For standard filters (either wire or ferrite
transducer types), the terminating resistance can be modified using a transformer or
capacitor dividing network to match some other value of termination. For special filter
designs, the terminating resistance may be specified in the range from 50 to 100,000
ohms. Essentially, the value of resistance determines whether the filter is parallel-
or series-tuned. For low-impedance applications, such as conventional transistor circuits,
the filter is tuned to series resonance. For high-impedance requirements - FET's, vacuum
tubes, etc., parallel tuning is used.
All standard mechanical filters are designed with both input and output terminals
isolated from ground. This eliminates the need for isolation transformers in applications
using balanced loads. However, it is necessary to ground the filter case (either a terminal
or mounting studs are provided for the ground connection). In any case, optimum selectivity
is achieved when the coupling or "feed-through" between input and output terminals is
minimized. If proper care is exercised, 120-dB discrimination is attainable.
Picking the Right Filter
Some of the characteristics to be considered when specifying a filter are: 1. center
frequency or carrier frequency; 2. required passband width; 3. selectivity or skirt cut-off;
4. maximum passband ripple or response variation; 5. maximum insertion loss; 6. source
and load impedances; 7. operating temperature range; 8. other environmental requirements,
e.g., shock and vibration; 9. package configuration; 10. special requirements, if any,
such as particular phase shift or envelope delay requirements.
Confusion frequently exists when talking about "passband ripple." Passband ripple
is sometimes interpreted to mean the ratio, in dB, between the maximum and minimum amplitude
of immediately adjacent peaks and valleys, and does not define amplitudes relative to
other peaks and valleys in the passband. A more meaningful interpretation is to use the
term "response variation since it always describes the worst-case condition. This term
means the ratio in dB between the maximum amplitude and the minimum amplitude occurring
anywhere across the entire passband whether these points are adjacent or not. It is important
that the equipment designer realize how a particular manufacturer defines this characteristic
since it may affect the performance or his equipment.
When specifying a filter, the circuit designer should remember that the more stringent
his requirements, the higher the cost of the filter. It is usually worthwhile to analyze
circuit performance so that the filter will not be "over-specified." Conservative design
is always good engineering practice, as long as the designer recognizes that this might
increase his costs.
The general outlook for the future of mechanical filters is excellent. New filter
configurations are being investigated which will result in further advances in the state-of-the-art.
For example, lattice configurations, which give the filter designer another degree of
freedom, are being utilized. Filters with built-in delay equalization are being realized,
resulting in characteristics that heretofore could be achieved only with an expensive
filter and a separate expensive equalizer. Piezoelectric ceramic transducers are being
used to give another design approach for filters with requirements that were previously
unobtainable. Techniques for achieving better selectivity by means of bridged coupling
wires have been developed and metallurgical techniques are being expanded to give even
better operating temperature characteristics. In addition, advances in manufacturing
processes have made it possible to miniaturize and build highly selective filters in
less than a 0.07-cubic-inch package.
Mechanical filters have far exceeded the original requirements for which they were
conceived. Future developments in mechanical filter technology will continue to place
emphasis on high quality and sophisticated filter requirements in minimum size and at
Hathaway, J. C. and Babcock, D. F.: "Survey of Mechanical Filters and Their Applications,"
Proceedings of the IRE, January 1957
Borner, M.: "Progress in Electromechanical Filters;" The Radio and Electronic Engineer,
Konno M., Kusakabe, C., and Tomikawa, Y.: "Electromechanical Filter Composed of Transversely
Vibrating Resonators for Low Frequencies," Jour. of Acoustical Society of America, April
Johnson, R.A. and Teske. R. J.: "A Mechanical Filter Having General Stopband Characteristics,"
IEEE Transactions on Sonics and Ultrasonics, July 1966.
Posted December 21, 2017