Surface-wave filter with selectively connectable tracks to provide a variable transmission band

A surface-wave filter that has a variable transmission band is formed of at least two parallel tracks having input and output transducers whereby the bus-bars of one transducer of one track lie at two different potentials and one bus-bar of one transducer of a further track lies at one of the two potentials. The other bus-bar is selectively connectable to the other of these potentials so that the transducer of the further track is selectively enabled or disabled. Since the tracks have different transmissions bands, the transmission band of the filter is thus, selectable by connecting and disconnecting the further track.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates generally to a surface-wave filter, and more 
particularly, to a surface-wave filter which can be switched to different 
transmission bands. 
2. Description of the Related Art 
Surface-wave filters are known, for example, from the data book, 
"Oberflaechenwellenfilter fuer Fernsehanwendungen", Datenbuch 1987/1988, 
published by Siemens AG. Surface-wave filters generally are integrated, 
passive components having band-pass filter characteristics. Surface-wave 
filters function based on the interference of mechanical surface waves 
that propagate along the surface of a piezo-electric material. The 
structure of surface-wave filters is characterized by a single-crystal, 
piezo-electric substrate, preferably a lithium niobate substrate, onto 
which a metal layer, normally an aluminum layer, is applied, for example, 
by vapor deposition. Piezo-electric input and output transducers, also 
referred to as interdigital transducers, are formed from this metal layer 
by, for example, by photo etching techniques. 
The interdigital transducers are composed of a plurality of metallic, 
comb-like electrodes. More specifically, the electrodes each have a 
bus-bar as its base with fingers extending perpendicularly from the 
bus-bar and overlapping with like fingers of another comb-like electrode 
of a different polarity in an active region of each of the transducers. 
The substrate per se is fastened on a metal carrier. Bond wires provide 
the contact for the input and output transducers to terminals or pins 
leading to the outside. 
During operation of surface-wave filters, an electrical signal which is 
input to the transducer is converted into a mechanical surface wave that 
runs along the surface of, or in a surface-proximate layer of, the 
substrate to an output transducer. The output transducer, in turn, 
converts the surface wave into an electrical signal. Standard transducers 
have interdigital structures with transit time effects and are 
distinguished by highly frequency-dependent properties so that, by 
appropriate formation of the finger structures of the transducers, a 
filter effect (i.e. the transmission band) for electrical signals is 
achieved. 
The filter effect of the surface-wave filters is thereby essentially 
determined by the transmission loss, or transmission curve, which defines 
the transmission band. The transmission loss is the attenuation of the 
transmitted signal over the frequency and is at a minimum in the filter 
transmission band. Given a known single-crystal substrate, and assuming 
that the fundamental structure of the filter, such as the type and 
position of the transducers as well as the utilization of further 
components suitable for signal processing of surface waves, is fixed, then 
the transmission band of the filter can be varied only by modifying the 
internal structure of one or both transducers. For example, the finger 
spacing, the finger aperture or the weight of the transducer fingers can 
be varied to achieve variation of the transmission band. Of course, it may 
also be possible to vary the transmission band on the basis of external 
wiring of the filter. 
In certain instances, for example, in multi-standard television receivers 
in which, for example, television transmissions that are based on some 
other standard, such as the I-standard, the L-standard or the M-standard, 
can be received in addition to television transmissions, an IF filter 
having a transmission band corresponding to the standard is required. This 
also applies to NTSC transmissions. The user, or possibly even the 
manufacturer of the television receiver, switches to the corresponding IF 
filter, or selects from the possible standards, depending upon the 
standard being received. Included among the possible filters are what are 
known as double video filters having two video channels with different 
video bandwidths and each having two inputs and two outputs. The user 
alternately switches to one or the other of these two channels. 
SUMMARY OF THE INVENTION 
An object of the present invention is to provide a single, switchable 
surface-wave filter in place of the relatively large number of filters 
having different transmission bandwidths which have previously been 
required at a considerable expense, and thereby to considerably reduce the 
expense of providing filters. 
These and other objects and advantages of the invention are achieved with a 
surface-wave filter, also known as a surface acoustic wave filter, having 
at least two parallel tracks with input and output transducers, whereby 
bus-bars of an input or output transducer of one track lie at different 
potentials and a bus-bar also of a transducer of another track is 
optionally connectable to one of these potentials, so that the resulting 
transmission band of the filter is selectable by connecting and 
disconnecting the individual tracks, each of which has a different 
transmission band. 
For example, when switching is to be undertaken between two transmission 
bands, then two parallel tracks of different transmission characteristics 
are adequate. The first of these two tracks has the bus-bars of either the 
input transducer or the output transducer lying at different potentials 
and thus being operable. One bus-bar of either the input or the output 
transducer of the second track lies at one of the two potentials while the 
other bus-bar is selectively connectable to and disconnectable from the 
other potential. This enables the second track to be made operable or 
inoperable, as desired, thus changing the transmission band 
characteristics of the filter. This principle is also applicable to 
filters having more than two tracks. 
Also according to the invention, the output transducers of the tracks may 
be either connected in parallel or in series.

DETAILED DESCRIPTION OF THE RELATED ART 
A surface-wave filter, also known as a surface acoustic wave filter, is 
shown in FIG. 1 formed on a single crystal, piezo-electric substrate 1, 
which is preferably a lithium niobate substrate. On the surface of the 
substrate is formed two parallel tracks A and B having different signal 
transmission characteristics. Each track A and B has a respective input 
transducer EW1 and EW2 of an interdigital structure of comb-like 
electrodes overlapping one another to produce mechanical surface waves in 
the substrate 1. The illustrated input transducers obtain their frequency 
characteristics from a technique known as apodization in which the overlap 
between adjacent electrode fingers is varied. The envelope defined by the 
apodized transducer electrodes is shown schematically in FIG. 1. The track 
A also includes bus-bars 2 and 3, while track B has bus-bars 4 and 5 for 
the interdigital electrodes. The bus-bars, 3 and 4, are electrically 
connected to one another by a linkage 10. 
Input terminals are provided, including an input side terminal 12 connected 
to the bus-bar 3 by a bond wire 13 and a second input side terminal 14 
connected to the bus-bar 2 by a bond wire 15. The bus-bar 5 is connected 
via a bond wire 17 to a terminal 16 which serves as a switchable circuit 
input. 
Output transducers AW1 and AW2 are also of an interdigital configuration 
and are shown in FIG. 1 formed of overlapped fingers of comb-like 
electrodes, the output transducers AW1 and AW2 being connected in parallel 
in the illustrated exemplary embodiment. To this end, bus-bars 7 and 8 of 
the output transducers AW1, and AW2 are connected to one another via a 
linkage contact 11 which is connected to a terminal 18 via a bondwire 19. 
The output transducers AW1 and AW2 also have bus-bars 6 and 9, which are 
connected to one another and connected to a terminal 20 via a bond wire 
21. 
The filter of FIG. 1 is a switchable video filter which may be used as an 
IF-filter in a multi-standard television receiver. This filter is 
switchable to the transmission band which corresponds to the desired 
operating standard. The operation of the filter of FIG. 1 shall be set 
forth below with reference to FIGS. 2a, 2b and 2c. In the filter of FIG. 
1, the first track A is designed for a transmission band (shown in FIG. 
2a) that is greater than the transmission band of the second track B 
(shown in FIG. 2b). When a signal with a greater bandwidth is transmitted 
in track A, such as, for example, a signal having a bandwidth of 5 MHz 
which corresponds to the video bandwidth of the European B/G-standard, and 
a difference signal having a bandwidth of 1 MHz relative to the 4 MHz 
bandwidth standard of the M-standard is transmitted in the second track B, 
then a corresponding signal that is 4 MHz wide (shown in FIG. 2c) is 
obtained at the filter output. A prerequisite for this to happen is that 
the tracks A and B are electrically active, in other words, the bus-bars 3 
and 4 of the input transducers EW1 and EW2 lie at a higher potential and 
the bus-bars 2 and 5 lie at a lower potential, usually at ground. Due to 
the parallel connection of the two output transducers AW1 and AW2, the 
signals of both tracks A and B add up in this fashion. 
The presence of the additional, external terminal 16 for the bus-bar 5 
permits the bus-bar 5 to be selectively disconnected from the lower 
potential, so that the second track B no longer is operative. Thus, such 
as by a switch means S, when only the first input transducer EW1 is 
activated, in other words when the bus-bar 5 does not lie at the same 
potential as the bus-bar 2, only the signal having the greater bandwidth, 
for example 5 MHz., is transmitted. The present filter is thereby 
switchable between different transmission bandwidths. 
The same effect, namely selectively disabling one of the tracks, can also 
be accomplished by disabling the output transducer AW2 for the 
corresponding track. For example, the arrangement of FIG. 1 has the 
bus-bars 2 and 5 connected to one another so that both are at the same 
potential and both input transducers EW1 and EW2 are always electrically 
active. 
At the output side, however, the bus-bars 6 and 9 are each connected to 
their own respective external terminals instead of being connected in 
common. This permits the bus-bar 9, for example, to be selectively 
connected and disconnected to its operating potential, for example ground. 
It is within the scope of the present invention to provide additional 
bandwidth selection possibilities by adding further tracks to the present 
filter. It is also within the realm of possibilities for the present 
invention to connect the output transducers of the two tracks in series 
instead of in parallel. One contemplated application for the present 
filter is for use as an IF-filter for multi-standard television receivers 
such that the IF-filter is switchable into the corresponding transmission 
band depending upon the desired standard. 
Although other modifications and changes may be suggested by those skilled 
in the art, it is the intention of the inventors to embody within the 
patent warranted hereon all changes and modifications as reasonably and 
properly come within the scope of their contribution to the art.