February 1953 QST
Wax nostalgic about and learn from the history of early electronics. See articles
QST, published December 1915 - present (visit ARRL
for info). All copyrights hereby acknowledged.
Magnetostriction is a term not seen very often these days. It
describes the physical shape change that takes place in certain
ferrous materials when subject to a magnetic field, and is responsible
for most of the familiar 'hum' that comes from transformers
(the other part comes from attracting
and opposing fields rattling windings). The effect is
used in mechanical filters as transducers between the electronic
circuit and the mechanically resonant disks that define filter
bandpass characteristics. Elemental cobalt exhibits the highest
room temperature magnetostriction (units
are 'microstrains'). Nickel, with about half the value
as cobalt, is cheaper and more abundant and is therefor more
commonly used in modern magnetorestrictive transducers. Way
back in the 1980s while working at the Oceanic Division of Westinghouse
Electric, in Annapolis, MD, I built sonar transducers that used
an array of ceramic elements having a layer of nickel deposited
on opposing surfaces (which, conveniently,
made for good soldering of interconnect wires). Those
ceramic elements were physically sized to transmits and receive
specific frequencies used by the sonar.
Mechanical Bandpass Filters for I.F. Ranges
An Approach to the Ideal Selectivity Curve
By Ben Roberts,* W0IEU
For a good solid QSO in a crowded amateur band, we like to
hear just one signal and nothing but that signal. The receiver
that will come closest to meeting this requirement must have
a selectivity curve with a flat top, straight vertical sides,
and a bandwidth only wide enough to pass the desired signal.
This is the "ideal" selectivity curve.
The mechanical filter is shown here removed
from its 2 13/16 X 1 X 15/16·inch case. The filter proper consists
of the small disks in the center - each one is carefully machined
to the correct dimensions to give the proper mechanical resonance.
Receiver selectivity is usually increased by adding tuned
i.f. stages. However, when this method is used to increase selectivity,
even to the point where so much sideband power is lost that
'phone signals become unintelligible, the skirts of the selectivity
curve may still be broad enough to pass interference from strong
signals a few kilocycles away. Crystal-lattice filters1
offer one approach to the ideal selectivity characteristic,
but they are usually expensive and their commercial use has
been confined to telephone-company applications.
The Mechanical Filter
An entirely new approach to high selectivity is available
through the use of the "mechanical filter," a resonant mechanical
device. Shown in the photograph, it consists of three sections:
an input transducer, a mechanically-resonant filter section,
and an output transducer. The input and output transducers are
identical and use magneto-striction to convert the electrical
signal to mechanical energy and vice versa. Three small metal
rods are used to connect together the resonant disks of the
filter section. The second disk from each end connects to a
transducer by means of a small metal rod, and the two end disks
are secured to the transducer housings to serve as supports
for the filter section.
If you have been waiting for "something new" in the selectivity
department, here it is. By cascading accurately-machined bits
of metal, bandpass filters can be built in the i.f. range that
approach in performance the straight-sided "ideal," and this
article describes their operation and their advantages. Unless
you are a genius who has a more practical solution to the problem
of crowded bands than the use of higher selectivity, we're sure
you will want to know something about this latest development.
Magnetostriction, which makes possible the electrical-to-mechanical
and mechanical-to-electrical transformation of energy in the
transducers, is a fairly well-known but rarely used phenomenon.
When a highly magnetic substance such as nickel is subjected
to magnetic flux, the shape and volume of the magnetic substance
change. The metal will elongate, twist, or bend. The magnetostrictive
transducer used at each end of the mechanical filter consists
of a small coil of wire surrounding a nickel core. Application
of a 455-kc. signal (or whatever other frequency the filter
is designed for) to the input coil causes a magnetostrictive
action resulting in a mechanical vibration of the nickel core.
This 455-kc. mechanical vibration is transmitted through the
interconnecting metal rods to the mechanically-resonant disks
of the filter proper. Each disk is mechanically driven by the
preceding disk, so that all of the disks vibrate at 455 kc.
The last resonant disk drives the core of the output transducer.
Here the vibrations of the nickel core are changed by magnetostriction
into a varying magnetic field. The output coil intercepts this
field and supplies a 455-kc. output voltage.
Fig. 1 - Selectivity at 455 kc. of two mechanical
filters of different bandwidths, shown for comparison with the
selectivity obtainable with nine tuned circuits at the same
frequency. The narrower "skirts" and the flat "top" of the mechanical
filters account for their superior performance in crowded amateur
In order to avoid a frequency-doubling action that would generate
a mechanical cycle for each electrical half-cycle, a small magnet
in the mounting above each transducer applies a magnetic bias
to the nickel transducer core. The electrical pulses then add
to or subtract from the magnetism that already exists, causing
the filter elements to reproduce the input cycle. There is no
movement in the mechanical filter except for the imperceptible
vibration of the internal filter elements.
The mechanically-resonant disks of the filter proper have
extremely low losses at their resonant frequencies. Each disk
has a Q greater than 2000. Q These high-Q components exhibit
characteristics that make possible application of the theory
of lossless elements to filter design. A mechanical filter can
be constructed for either narrow or broad bandpass without sacrificing
its nearly rectangular selectivity curve. The relatively low
Q of electrical elements does not permit the design of equivalent
electrical filters. Typical characteristics obtainable with
mechanical filters are shown in Fig. 1, with the selectivity
curve of an i.f, amplifier using nine tuned circuits (electrical
elements) shown for comparison. The transmission loss of 23
db. or so through the 3-kc. mechanical filter is made up easily
by subsequent amplification by vacuum tubes.
Once the mechanical filter has been constructed, it is enclosed
in a hermetically-sealed case and requires no further adjustment.
Connections to the input and output transducer coils are brought
out of the unit on feed-through insulators whose edges are sealed
to the case.
Using the Filter
A receiver using the 3-kc. mechanical filter in its i.f.
amplifier handles differently than one with a conventional selectivity
characteristic. As the receiver is tuned across the band, signals
appear and disappear with more than usual suddenness. The straight-sided
selectivity curve makes the band appear less crowded - such
a curve is easier to interpose between two signals without responding
to either of them.
Using a steep-sided 3-kc. bandwidth for 'phone reception
is by no means standard procedure, and it requires a bit of
explanation. With a good conventional i.f. curve, like the one
obtained from nine turned circuits and shown in Fig. 1, the
carrier frequency must be tuned very close to the center of
the selectivity curve. This is because the carrier level decreases
as the receiver is tuned off. As the signal is moved off the
center of the selectivity curve, the carrier level decreases
but one of the sidebands does not. This results in too much
sideband for the available carrier amplitude and causes the
distortion (overmodulation at the detector) that always results
when a receiver with a rounded selectivity curve is not tuned
"on the nose." The only way to avoid this distortion is to tune
the receiver to the carrier rather than to a sideband. This
is the conventional way to tune a receiver; however, when we
do this, we are splitting our available bandwidth between two
sidebands, although we need to receive only one. Therefore,
if a receiver with a conventional i.f. curve has a 3-kc. bandwidth
and is tuned to the carrier, as it must be for distortionless
reception, it will respond to audio frequencies up to about
1500 cycles - not to 3000 cycles as is sometimes assumed. To
accept side frequencies up to 3000 cycles, the carrier must
be set off to one side, and distortion will result, except at
very low percentages of modulation. But the curve of the mechanical
filter has a flat top, and setting the carrier off to one side
does not substantially reduce its amplitude with respect to
the sideband. By keeping the carrier inside of the flat-topped
selectivity curve, we can tune to either sideband without introducing
the "overmodulation" type of distortion. Since it is no longer
necessary to split the available bandwidth between two sidebands,
we can pass a given range of audio frequencies with a passband
only half as wide as would be required with a conventional i.f.
selectivity curve. Since only one sideband is needed for reception
of a signal, setting the carrier at one edge of the passband
will still permit us to hear all of the audio frequencies up
to 3000 cycles, when the 3-kc. mechanical filter is used.
The 3-kc. filter is excellent for use in reception of s.s.b.
signals, when used with a receiver of good oscillator stability.
It has the correct bandwidth for s.s.b. reception, although
the 1-kc. filter can be used when conditions require extreme
selectivity. Under such conditions, the reinserted carrier (b.f.o.)
is set about 300 cycles outside the i.f. passband, allowing
an audio range of 300 to 1300 cycles to be received.
A top view of part of the Collins 75A-3 receiver,
showing the mechanical filter mounted to the right of the tuning
unit. Sockets are provided for an additional filter section
if desired, The shaft along the right-hand side of the filter
platform is a double shaft - the outer shaft permits switching
between the two filter units, and the inner shaft is the b.f.o.
The good skirt selectivity of the mechanical filter allows
it to reject the carrier and one sideband of an ordinary a.m.
signal, thus converting it to a s.s.b. signal in the receiver
past the filter. The b.f.o. is then set just outside the filter
passband, to coincide with the frequency of the eliminated carrier,
for proper detection. This type of s.s.b.-plus-exalted-carrier
reception minimizes the effects of selective fading and of certain
types of noise. Either the 3- or the 1-kc. filter can be used
in this application.
The Filter Applied to the Collins 75A-3 Receiver
The amount of selectivity that is desirable in a communications
receiver is a good subject for debate. Most of us like a very
selective receiver, but we also want faithful reproduction when
'phone signals are in the clear. Development of the mechanical
filter has solved this problem, since to change the selectivity
of a receiver using the filter it is only necessary to cut in
a filter of different bandwidth. The new Collins 75A-3 receiver
is supplied with a 3-kc. mechanical filter and has plug-in provisions
for the optional installation of a 1-kc. filter. The 3-kc. filter
is ideally suited for all types of 'phone reception including
s.s.b., and for exalted-carrier reception of regular a.m. signals.
Even the 1-kc. c.w. filter can be used for 'phone reception,
as described above. The crystal filter is retained for phasing
The mechanical filter is not just an accessory, but is an
entirely new development in communications. It shows promise
for use in many applications, including the simplification of
single-sideband transmitter circuits. Presently, development
work is proceeding toward the production of mechanical filters
with higher and lower operating frequencies and bandwidths as
required for special applications.
* c/o Collins Radio Co., Cedar Rapids, Iowa.
1 Weaver and Brown, "Crystal Lattice Filters for Transmitting
and Receiving," Part I, June, 1951, QST.
Posted May 18, 2015