Note: Descriptions are shown in the official language in which they were submitted.
A Filter 2~1089
The present invention relates to a filter.
In the radio telephone industry research and
development has for a long time been aimed at the
production of smaller, less expensive and more
efficient units and components. This direction has lead
to efficient exploitation of the available space on
circuit board surfaces, to an increase in the degree of
component integration, and towards rationalization of
functions.
High frequency components of radio telephones
constitute an exception to this trend. High frequency
components have to date been extremely complex in
construction, comprising discrete components and strip
constructions.
It is known in the art to employ in the transmitter
part of a radio telephone an output control circuit for
controlling the output level of the transmission. The
operation of the output control circuit is based on
indications of the output level of the transmitter and
comparison with a reference value. An essential part
of the output control circuit is formed by a
directional coupler, placed immediately adjacent the
separation filter of the reception circuit, with which
a voltage comparable with the output of the transmitter
is produced. The directional coupler is therefore used
for measuring the transmission output power, whereby
the output level is controlled with the output control
circuit. The directional coupler is usually located
after the transmitter or before the antenna.
The directional coupler is conventionally constructed
using various coupled transmission line constructions
' -~
- :
.
-- 2
and transformers.
A widely used directional coupler, having low
manufacturing costs, can be constructed using
microstrips or strip lines mounted directly on a
telephone circuit board. Drawbacks of the structure
include the relatively large circuit board area re-
quired and the high dependence of the operation on the
properties of the circuit board substrate material and
variations therein. The substrate material affects the
dielectric insulation material losses of the microstrip
line, these being greatly dependent on the so-called
loss tangent of the material, and it affects the line
losses, which are affected by the smoothness of the
surface of the material.
In order to reduce the costs, an advantageous general
purpose material is frequently selected for the
substrate material in radio telephones. The use of
such a material causes deterioration in the functioning
of the directional coupler constructed thereon. The
increased material losses of the substrate material and
the tolerances of the properties can be directly
detected in the insertion attenuation of the
directional coupler and in the variation of said
properties.
By using a directional coupler design which is based on
using separate transmission lines (e.g. coaxial
cables), on a transformer, or by constructing the
switch on a separate, stable or low loss substrate,
these losses can be reduced and the operation
stabilized. However, a plate area of at least the same
size is needed, and in addition, the use of a separate
substrate increases manufacturing costs.
~ 3 ~ 2~ 9
According to the present invention there is provided a
filter comprising filter means coupled to impedance
matching means and coupling means for coupling the
filter to external components, characterized by the
filter comprising a directional coupler.
An advantage of the present invention is the avoidance
of the above mentioned drawbacks by removing the
directional coupler from the substrate of the
transmitter as a discrete component or strip structure
and by forming the directional coupler as an integral
part of the filter.
: .
A filter in accordance with the present invention makes
use of transmission lines in the couplings of the
filter, such as a receiver (RX) or a transmitter (TX)
separation filter, or in the matching circuits for
implementing the directional coupler. With the aid of
the invention the directional coupler can be
transferred from the circuit board substrate,
frequently being a high-loss substrate and afflicted by
environmental disturbances, into a high frequency
filter. The high frequency filter offers an
encapsulated environment protected against interference
with a low-loss and stable substrate as required by a
filter. Furthermore, since the directional coupler in
accordance with the invention comprises a transmission
line of a filter and a switch transmission line coupled
thereto, the only additional losses caused by the
coupler are of the magnitude of the output sample
sampled by the transmitter system, which is a
significant improvement in comparison with the state of
the art.
_ 4 _ ~91~89
The directional coupler in accordance with the
invention is relatively easy to implement by placing
the coupler in conjunction with the filter or by making
use of the components existing in the filter, such as
coupling lines or impedance matching transmission
lines. The directional coupler can be produced by
adding another microstrip e.g. in parallel with a
microstrip. The directional coupler may, in fact, be
implemented in a great number of ways using many types
of transmission lines, preferably microstrips, or sus-
pended substrate lines. The manner of implementation
may vary in accordance with the invention, but the
essential feature is that a directional coupler is
included in the filter. Preferably, the directional
coupler is implemented with the existing filter
constructions.
A radio apparatus which is provided with both a
transmitter and a receiver and in which the same
antenna is used needs a transmission network for
directing the signal to be transmitted and signal
received appropriately.
The signal from the antenna must be directed to the
receiver so that no significant interference is caused .
by the transmitter. Similarly, a signal from the
transmitter must be transmitted to the antenna without
causing any interference from the receiver.
As is well known in the art, duplex filters composed of
two separation filters and based on resonators are used
to separate the signals to be transmitted and received.
The duplex filter is therefore usually composed of two
separate bandpass filters, one of them being connected
to the receiver branch, its mean frequency and
:
~91~g~
-- 5 --
bandwidth corresponding to the receiving band, and the
other filter being connected to the transmitter branch,
its mean frequency and bandwidth corresponding to the
transmission band. The other ends of the filters have
frequently been connected to a common antenna line via
a transmission line matching the impedance. Even
though the duplex filter has frequently been
accommodated within a single housing provided with
terminals for transmitter, receiver and antenna, in
practice it is composed of two separate bandpass
filters because the isolation between the filters must
be made as great as possible so that the
electromagnetic leakages therebetween should not impede
the functioning of the filter.
The filters constructed using the helix technique are
provided with a metal partition between both of the
filters, with which the required isolation is
efficiently implemented. The couplings in duplex
filters to the resonators and the couplings between the
resonators have commonly been implemented by means of
matching couplings formed by various transmission line
constructions and discrete components.
The invention is described below in more detail, by way
of example with reference to the accompanying figure,
which is a schematic diagram of a filter comprising a
filter-directional coupler block in accordance with the
invention.
The figure shows schematically the coupling of a duplex
filter 1 of a radio telephone to a transmitter block 4
and a receiver block 5. The duplex filter 1 comprises
four ports: one port 8 for a transmission signal
entering the filter, a port 9 for the antenna, a port
~ :
:
2~ 089
-- 6 --
(not shown) for the receiver 5, and a directional
coupler port 7. A signal from the antenna 6 to be
received propagates via the reception filter block 3 of
the duplex filter 1 to the receiver 5. Respectively, a
signal from the transmitter 4 propagates through the
receiver (RX) separation filter block 2 of the duplex
filter 1 to antenna 6. From said signal proceeding to
the antenna the directional coupler 10 takes a sample
of a given level, comparable with the output level of
the transmitter, and transmits it to the directional
coupler port 7.
The figure illustrates the integration of a directional
coupler 10 in the matching circuit at the transmitter
end of the RX separation filter 2 of the duplex filter
1. The directional coupler 10 has preferably been ~ -
implemented on a circuit board using coupled
microstrips. One of the strips, strip A, serves as a
coupling strip for the filter input to the helix HX1,
the other one of the strips, strip B, serves as a
coupling strip to the directional coupler port 7. The
coupler strip B ends in a resistor R. In the filter of
the figure, the directional coupler could be positioned
adjacent the antenna port 9, whereby it could be easily
implemented by adding another branch next to strip C
which carries out the impedance matching. The
additional branch would comprise a directional coupler
port 7, a coupler transmission line, comparable to line
B, and an end resistor R.
The directional coupler 10 is preferably composed of
two adjacent microstrips A and B provided with common
ground planes. Electromagnetic coupling exists between
the strips: a voltage in one strip producing an
electric field which induces a voltage also in the
,
_ 7 - 2~089
other strip. Magnetic coupling is likewise caused by a
magnetic field provided by the current propagating in
the strip, said field inducing a current also into the
other strip. When power is transferred from the
transmitter block 4 to port 8, the desired part of the
power, which is determined by the coupling, is coupled
to the directional coupler port 7, another part thereof
passes through the coupler 10 towards a first resonator
HXl of the filter, and a very small leakage part caused
by the non-homogeneity and quasi transverse electric
and magnetic (TEM) waveform of the switch is short
circuited via the end resistor R to the ground. The
microstrip coupler 10 described above is bilateral,
i.e. any power carried to any port will be coupled in
the same way, because of the symmetry.
The proportions of the quasi TEM directional waveform ` ;-
in the coupler is generally used as a quality measure.
In other words comparison of the power at directional
coupler port 7 and the power at the insulated leakage
port leading to ground via resistor R provides a
measure of quality.
By using an integrated filter and directional coupler
design in accordance wiih the invention in the duplex
filter of a radio telephone, losses caused in the
transmitter part of the radio telephone by the
directional coupler can be reduced significantly, the
operation of the radio telephone stabilized, and the
use of the telephone substrate area enhanced.
In view of the foregoing it will be clear to a person
skilled in the art that modifications may be
incorporated without departing from the scope of the
present invention.
