Note: Descriptions are shown in the official language in which they were submitted.
2~6367
P93904 CA
A Filter
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The present invention relates to a filter for providing an
output signal from an input signal input thereto,the output
signal having a frequency in a predetermined frequency
range,and the filter comprising at least one pair of
intercoupled resonators. The invention also relates to a
radio transceiver comprising such a filter.
In virtually all communications systems, for example,in
radio transmitter - receivers (transceivers),for example in
radio telephones,one or more filters are used for the
purposes of separating wanted signals e.g information -
bearing signals from unwanted signals such as
interference,noise and distorting products.
A simplified block diagram of part of a radio telephone is
shown in Figure 1 of the accompanying drawings.
As is well known to persons skilled in the art,a radio
transceiver,for example,as used in a radio telephone
includes a receiver section and a transmitter section for
receiving and transmitting radio frequency ~RF)
communication signals. When transmitting,audio signals are
input via a microphone to a modulator where a carrier signal
is modulated with the input audio signal. The carrier signal
is provided by a synthesizer or,for example,a crystal
oscillator/for sufficient frequency stability,and has a
frequency of between 6 and 25 MHz. The modulated signal is
amplified by a power amplifier 1 and then coupled to a band
pass filter 3 and,then to an antenna 4 for onward
transmission. The power ampli~ier 1 of the receiver section
of the radio telephone usually comprises several stages.
When receiving,a received signal is detected by the antenna
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4 and coupled via the filter 3 to a radio frequency
amplifier 2 and then to a mixer where the signal is mixed -
with a signal from a local oscillator thereby translating
the output from the RF amplifier 2 to an intermediate
frequency signal which is subsequently further amplified and
demodulated. For sufficient selection ability and gain the
receivers are usually constructed on the so-called double
superhet principle in which the RF signal is translated
twice before demodulation. The gain of the RF amplifier 2
is,in general low,and the bandwidth large.
When a single antenna 4 is used for both transmission and
reception,a duplex filter 3 is used to prevent the
transmission signals from travelling to the receiver
section,and the received signals from travelling to the ~i
transmitter section.As with all the above, this is well
known to persons skilled in the art,as is the construction
and operation of duplex filters.
As is well known to persons skilled in the art,filters
having the desired properties can be realised by the
appropriate interconnection of a number of resonators. The
individual resonators are in the form of a transmission line
resonator corresponding to a parallel connection of an
inductance and a capacitance. It is well known in high
frequency technology to employ different types of resonators
for di~ferent applications depending on the conditions of
use and the desired characteristics. Known resonator types
include dielectric, helical, strip line (including
microstrip) and air isolated rod resonators. These various
resonator types each have a relevant range of uses.For
example,dielectric resonators,and filters construc-ted
therefrom are commonly used, e.g in radio telephone
applications because of their relatively small size and
weight, stability and power endurance. For instance, a
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duplex filter such as the one described above with reference
to Figure 1 may be constructed from discrete dielectric
blocks, wherein an individual resonator is formed in each
block.However, instead of dielectric resonators,other
resonator types may be used,for example,helical,strip line
or coaxial resonators. All of these filter designs are well
known to persons skilled in the art and need not be
described in any further detail except in the context of the
present invention.
Figure 2 illustrates,schematically, a bandstop filter
composed of two resonators RESl,RES2.Coupled galvanically,at
an appropriate point A,B,to each resonator RESl,RES2 is a
transmission line TL1 and TL2 respectively. By selecting the
coupling points A,B appropriately, the impedance level of
each resonator RESl,RES2 can be defined and the resonator
RESl,RES2 matched to the rest of the circuit. This matching
is called tapping and the coupling point A,B the tapping
point. This is well known to persons skill~d in the art.When
using helical resonators they are matched respectively by
tiapping,whereby, for example, a transmission line is
soldered, at a given point, to the helical resonator coil,
usually in the first round of the coil. The filter is
constructed by inductively or capacitively coupling the
resonators R~Sl,RES2 together depending on what kind of
filter is desired. In the example described herein,the
bandstop filter is constructed by coupling the two
resonators together RESl,RES2 using an inductance L as
illustrated in Figure 2. In this example, the filter is a
low pass filter. If the ~wo transmission lines TLl,TL2 are
replaced by capacitances,then a high pass filter is
produced. Alternatively,if the inductance L is replaced by
a capacitance, then a band pass filter is obtained. An input
IN and an output OUT are provided at the ends of the
coupling of the resonators. Filters can also be constructed
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with more than two resonators.
Usually,amplifiers such as the two amplifiers described
above are located before or after a filter, for example,the
duplex filter described above. The filters and the
amplifiers are separate components and, as such, take up a
relatively large amount of space on the printed circuit
boards upon which they are placed. This, in turn, puts
constraints on the design and manufacture of the circuits
and circuit boards and, particularly, limits the amount of
space that can be saved,and,consequently, the amount of
miniaturization that can be achieved.
According to an aspect of the present invention there is
provided an amplifier coupled between the at least one pair
of intercoupled resonators such that the input signal is
coupled through the amplifier to provide an amplified output
signal is amplified. This has the advantage that by
incorporating an amplifier in the filter, the amplifier is
no longer a separate component which needs to be placed on
the circuit board,and, as such,space can be saved.
The filter may comprise three or more resonators
and,may,therefore,incorporate more than one amplifier. The
-filter may be part of a duplex filter of a radio transceiver
in which the integrated amplifier or amplifiers may be part
of the power amplifier of the receiver section and/or the
multistage power amplifier of the transmitter section. This
has the advantage of providing for significant space sa~ing
and,therefore,reduction in size of,for example,a radio
transceiver using such a filter.
The invention will now be described,by way of example
only,with reference to the accompanying drawings of which:
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Figure 1 is a schematic block diagram of part of a radio
telephone;
Figure 2 is a schematic circuit diagram of a stop filter of
the prior art;
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Figure 3 is a schematic circuit diagram of a filter ln
accordance with the invention;and
Figure 4 is a schematic circuit diagram of a second
embodiment of the invention.
As with the filter of the prior art described with reference
to Figure 2,a filter in accordance with the invention
llustrated schematically in Figure 3 - comprises two
resonators,also designated by the same references RESl and
RES2 for ease of understanding. Those components which
correspond to those in;the prior art have been designated
with the same references. The filter of Figure 3 is what is
known as a notch filter serving as a low pass filter.It is
almost identical~to the prlor art~filter of Figure 2,except
that the inductance L~has been;replaced by an amplifier AMP
i.e. the amplifie~r~AMP is placed in the coupling path of the
fil~ter.The filter operates~at the~resonance frequency of the
resonators RESl,RES2 a~s a bandstop fllter,filtering out
higher frequencles from an input signal input at IN. Such a
filter could be used for~example in the transmitter branch
of a duplex filter in~a radio telephone where the reception
frequency band is above the transmission frequency band. The
resonators RESl,RES2 present high impedances at points E~and
F of Figure 3 to signals at the reception frequencies,while
at the transmission frequencies, the signal is input at IN
and coupled through the amplifier AMP to the output OUT ~i.e
the RF power passes from the input IN thxough the amplifier
AMP to the output OUT. While the filter also carries out its
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usual function of filtering out the undesired
frequencies,the incorporation of the amplifier AMP in the
filter serves to provide a filtered and amplified output
signal at the output OUT and,as such,the amplifier AMP can
therefore act as one of the stages of a multistage amplifier
of the transmission section of the radio telephone,with the
other amplifier stages being coupled separately to the input
of the amplifier AMP. Because one stage of the amplifier is
now integrated in the filter,space can be saved in the rest
of the transceiver of the radio telephone e.g on the printed
circuit board on to which the components are mounted.
The filter could also be used in the receiver branch of a
duplex filter,the integrated amplifier AMP serving as the
power amplifier of the receiver branch of a transceiver.
Figure 4 represents schematically, another embodiment of the
invention. In this example, a filter comprises three
resonators RESl~RES2,RES3,to which respective transmission
lines TLl,TL2,TL3 have been galvanically coupled as in the
previous example.Coupled between the other ends of
transmission lines TL1 and TL2 is a first amplifier AMP1 and
between transmission lines TL2 and TL3 a second amplifier
AMP2. This filter also functions as a filter and also serves
to provide an amplified, filtered output, but in this case
two stages of a multistage amplifier can now be integrated
into the filter itself.
As will be understood to a person skilled in the art,various
modifications are possible within the scope of the present
invention. For example,where there are three or more
resonators, only one amplifier can be integrated and an
inductance (or capacitance)replaces the other of the
amplifiers. Similarly, if there are sufficient resonators
coupled in the same manner,more stages of a multistage
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7 ~ -
amplifier can be integrated into the filter. Other filter~ ;
arrangements are possible,with one or more amplifiers being
integrated therein. For example, capacitances can be used -~
rather than the transmission lines for coupling the
amplifiers to the resonators i.e the amplifiers are
capacitively rather than inductively coupled to the ;~
resonators.
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