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Description  |
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BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a carrier frequency communication
transmission system having a premodulation band lying above the
transmission position of the primary group band of 60-108 kHz, and more
particularly to such a system in which mechanical filters are utilized as
channel filters and also a common group filter in order to filter out the
primary group band.
2. Description of the Prior Art
In order to transpose 12 voice channels into the base primary group band
position of a TF system, the premodulation technique has been proven
advantageous. The frequency spectrum of an input signal in the
premodulation technique is displaced into the premodulation position in a
first transposition step. A channel filter connected to the output of the
modulator in this step limits the transmitted frequency band and
suppresses undesirable modulation products. In a second conversion step,
the frequency spectrum is displaced into the frequency position of the
primary group band, i.e. 60-108 kHz. The undesirable modulation parts of
the second conversion step are suppressed in the group filter.
It is already known in the art to establish the premodulation band in the
frequency range of 48-52 kHz and to arrange the channel carrier
frequencies above the primary group band. In this connection, for example,
see the German Allowed Application No. 1,274,663. In this system, a common
group filter is utilized for filtering out the primary group band arranged
in inverted frequency, the group filter being designed as a band pass
filter. Mechanical filters, designed as longitudinally coupled bending
resonators, are utilized as channel filters for this purpose.
Such a system has the advantage that no multiples of the input signal or
undesired summation of differential products of the input signal with the
carrier frequencies fall into the higher transmission positions during
additional conversions. The bending resonators in this frequency range are
both small in dimension and can be produced economically.
The article "Mechanical Filters for the Carrier Frequency Techique", by M.
Boerner, published in the periodical "NTF", 19, 1960, pp. 34-37, describes
a premodulation system having mechanical filters in which the
premodulation band lies at 200 kHz, i.e. above the primary group band,
whereby torsion resonators having longitudinal coupling are utilized as
mechanical filters. On the basis of the essentially greater dimensions of
mechanical filters with torsion resonators in contrast to mechanical
filters with bending resonators, their utilization in a frequency band
below the primary group band is not possible if one is bound to prescribed
dimensions, particularly provided by the standardized printed circuit
plates. This known premodulation system also utilizes a common group
filter in order to filter out primary group.
Furthermore, a mechanical filter having bending resonators is known from
the German Letters Pat. No. 1,541,975, these resonators being coupled
together by coupling wires attached to the center, or close to the center
whose support is low in feedback via respectively two holding wires which
are attached to the bending node and which, for example, are driven by
piezo electric transducers.
SUMMARY OF THE INVENTION
It is the object of the present invention to provide a carrier frequency
communication transmission system having premodulation and having an
improved spurious response behavior with respect to the mechanical filters
utilized in the system.
In order to achieve the foregoing object, the carrier frequency
communication transmission system is designed such that bending resonators
having longitudinal coupling are utilized as mechanical filters, and that
the frequency range of the premodulation band is determined at its lower
limit by means of spacing of at least 4 kHz from the upper limit frequency
of the primary group band, and the upper limit of the frequency range is
determined in that the spurious responses, as far as they occur in the
lower stop band of the channel filter, lie at least 12 kHz below the lower
limit of the primary group band, and that highly stabilized piezo ceramics
is used as the electromechanical transducer material.
A premodulation system is obtained by the aforementioned measures and can
be constructed with a group filter designed as a low pass filter. The
group filter is thereby considerably simplified in production and design.
Simultaneously, the advantage in contrast to the known premodulation
system having torsion filters is obtained in that spurious responses in
the low stop band of the channel filter no longer have an interfering
influence upon the primary group band, for example, even at the higher
frequency positions.
By means of the advantageous spurious response of the circuit arrangement
constructed in accordance with the present invention it is also possible
to construct the electric terminal circuits as simply tuned transformers
which are then primarily used for optimum matching of the filters to the
modulators.
The property of the bending resonators can simultaneously be utilized
advantageously in that the lower resonator pole of the mechanical
resonators is higher than in torsion resonators, whereby its steepening
effect toward the lower filter pass band edge is particularly great, which
effect must always be very steep in order to suppress the carrier and the
lower side band. The mechanical stability of the bending resonator is
considerably greater than in the torsion resonator by means of the
possiblity provided in the bending resonator of attaching supporting wires
to two spatially separated bending oscillation nodes.
The mounting of the coupling wire in the center of the bending resonstor is
possible in the range of the flattest portion of the amplitude curve, in
contrast to the torsion resonator, whereby a considerably smaller
tolerance sensitivity with smaller dimensions is obtained to the torsion
resonator.
An optimum solution is obtained when the premodulation carrier lies at 132
kHz.
A filter with bending resonators can advantageously be utilized as the
mechanical filter in which the bending resonators are held to a low amount
of feedback by at least, respectively, two supporting wires which are
mounted to the bending node and driven by piezo electric transducers.
A bending resonator filter with a steepened characteristic is thereby
advantageous when particularly high requirements are set.
BRIEF DESCRIPTION OF THE DRAWINGS
Other objects, features and advantages of the invention, its organization,
construction and mode of operation will be best understood from the
following detailed description, taken in conjunction with the accompanying
drawings, on which:
FIG. 1 is a block circuit diagram of a premodulation system;
FIG. 2 is a graphic illustration of a modulation scheme;
FIG. 3 is a longitudinal sectional view of a mechanical channel filter
constructed with bending resonators;
FIG. 4 is a sectional view taken substantially along the line A--A of FIG.
3; and
FIG. 5 is a pictorial representation of a filter constructed in accordance
with the invention and having steepened filter characteristic edges.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring to FIG. 1, in a block circuit diagram a premodulation system is
illustrated in which a voice signal in the audio-position (low frequency)
is fed to an input low pass filter 1 which is connected to a premodulator
2. The voice signal is converted into the premodulation position between
132 and 136 kHz. A channel filter 3 is connected to the output of the
premodulator 2 and is constructed with bending resonators. The channel
filter 3 filters out the upper side band produced by the premodulator 2
and feeds the remainder to a respective channel modulator 4, whereby a
transposition is made into the primary group band of 60-108 kHz with the
aid of one of a plurality of carriers of 196-240 kHz.
FIG. 2 illustrates the modulation scheme. The twelve telephone channels
lying in the audio positions of 0-4 kHz are converted into the
premodulation position of 132-136 kHz with the aid of a 132 kHz carrier.
The second modulation position lies within the primary group band in the
frequency range of 60-108 kHz, whereas the appertaining carriers lie above
the premodulation band in the frequency range of 196-240 kHz.
FIG. 3 illustrates a mechanical channel filter constructed with bending
resonators and in which no particular measures were undertaken in order to
steepen the filter edge characteristic. The mechanical filter constructed
in accordance with FIGS. 3 and 4 comprises several mechanical resonators 6
designed in the exemplary embodiment as resonators which carry out bending
oscillators. In order to excite or take off, respectively, the mechanical
oscillations, i.e. in order to transform electric oscillations into
mechanical oscillators and vice-versa, terminal resonators 6' are
provided. The resonators 6 and the terminal resonators 6' are arranged in
a plane such that their coupling can be carried out by way of a continuous
coupling wire 8, preferably attaching the resonators in ranges of great
motion amplitude. For a bending resonator operated in the base oscillation
this is, for example, the resonator center as is readily apparent from the
cross-sectional view of FIG. 4. The resonators 6, 6' comprise rods having
a circular cross section which are provided with a flattened surface 9
along one lateral surface area of the resonator. The plane of this
flattening in the actual filter resonators lies parallel to the base
plates provided for supporting the total system, whereas the terminal
resonators 6' are arranged such that the plane formed by the flattening
is perpendicular to the general plane formed by the base plate 1. In this
manner, the coupling element 8 can be mounted at such locations of the
filter resonators 6 and also at the terminal resonators 6' at which the
peripheral limitation of the resonators has a circular course, whereby
practically a point-shaped supporting surface is provided at the
resonators 6 and 6', whose longitudinal axis extends perpendicular to the
direction of the coupling element 8. This almost point-shaped connection
between the coupling element 8 and the individual resonators 6, 6' again
contributes to the fact that dependable and reproducible conditions result
with respect to the required coupling effect, which conditions are
essential for modern series production when the tuning operation is to be
limited to a minimum after assembly of the filter.
The oscillation excitement or the oscillation output is provided by plates
10 which comprise electrostrictive material and which are attached to the
flattened surfaces of the resonators 6' in accordance with well-known
methods. The plates 10 are provided with a metallization at the side
facing away from the resonator and a respective electrical terminal 11, 12
is connected to the metallization. An additional connecting wire, 11' and
12', respectively, is connected to a support element 4. If the connecting
terminals 11 and 11' are connected to an electrical AC voltage, the plate
10 of electrostrictive material is subjected to expansions and
contractions in the direction of the effective electric field. Due to
those longitudinal alternations, expansions and contractions are also
produced perpendicularly in relation to the direction of the electric
field via the known cross-contraction effect, i.e. in the direction of the
longitudinal axes of the resonators 6'. On the basis of these motions, for
example, the terminal resonator 6' connected to the terminals 11 and 11'
is then excited into bending oscillations when its bending characteristic
frequency is precisely, or at least approximately, identical with the
frequency of the applied voltage. This bending motion is also transmitted
to the remaining resonators 6 and the second terminal resonator 6' by way
of the coupling element 8, the resonators then also carrying out bending
oscillations in parallel to the base plate 1. Due to these bending
oscillations of the second terminal resonator 6', which is connected to
the terminals 12, 12', an AC voltage is produced between the metallization
carried thereby and the resonator 6' due to the piezo electric effect of
the plate 10. This AC voltage can be taken off at the terminals 12, 12' as
an output AC voltage. The electric AC voltage can readily, and very
accessibly, be fed to the terminal 11, or can be tapped from the terminal
12, respectively, when the flexible wires extending from the
metallizations of the electrostrictive plates 10 are extended through an
insulating leadthrough attached to the base plate 1. The base plate 1 and
the resonators 6, 6' effect the continuous second line required for the
formation of a ground non-symmetrical quadripole (four-terminal network).
In this manner the filter can be completely assembled and tuned, and
subsequently a housing 7 can be rigidly connected with the base plate 1.
Relatively short metallic pins 4 are provided for the support of the
system, the pins 4 being attached to at least one part of the respective
resonators. The pins 4, functioning as support elements, are attached to
the resonators 6, 6' at oscillation nodes, whereby natural ranges are to
be understood by oscillation nodes, in which practically no amplitude
occurs in the oscillation process. At least one notch-shaped indentation 2
is provided in the base plate 1, fashioned such that a casting compound 3
is held therein, even in a liquid condition.
FIG. 5 illustrates a filter having steepened characteristic edges with
increased filter requirements. The filter also contains resonators BR
which are connected to one another by the main coupler HK. The steepening
in this filter is provided by means of an additional coupler UK2 which
connects the resonators AR1, HR and AR2. The phase reversal necessary in
order to produce the pole with the aid of the coupler UK2 is effected by
the particular design of the resonator HR as a dumbbell resonator. The
motion in this resonator is at that point at which it is connected to the
bridging coupler UK2, directed opposite to the motion to the connecting
point with the main coupler HK, in contrast to the motions in the bending
resonators, which always have the same direction at the sides opposing one
another.
Although we have described our invention by reference to particularly
illustrative embodiments thereof, many changes and modifications of the
invention may become apparent to those skilled in the art without
departing from the spirit and scope of the invention. We therefore to
intend to include within the patent warrented hereon all such changes and
modifications as may reasonably and properly be included within the scope
of our contribution to the art.
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