Telecommunications – Transmitter and receiver at same station – Radiotelephone equipment detail
Reexamination Certificate
1999-05-21
2002-06-04
Maung, Nay (Department: 2744)
Telecommunications
Transmitter and receiver at same station
Radiotelephone equipment detail
C455S082000, C455S083000, C455S552100, C333S104000
Reexamination Certificate
active
06400963
ABSTRACT:
FIELD OF INVENTION
The present invention relates to harmonic suppression in dual band mobile phones
BACKGROUND OF INVENTION
Currently mobile Phones are predominantly operated with only a single operating frequency.
FIG. 10
shows one realization of the transmitting/receiving operation in a mobile phone being operated with a transmitting frequency, e.g., of approximately 900 MHz for GSM, approximately 1800 MHz for DCS or approximately 1900 MHz for PCS. An antenna
100
is provided to transmit or receive signals and is connected to a transmitter/receiver change over unit or in other words transmitter/receiver switch
102
. A transmitter/receiver change over unit
102
comprises a transmitter switch TX and a receiver switch RX. In the receiving mode the receiver switch TX is opened and the receiver switch RX is closed. To the contrary, in the transmitting mode the transmitter switch TX is closed and the receiver switch RX is opened.
As shown in
FIG. 10
, in the transmitting mode the transmitting signal is outputted from a power amplifier
102
which—due to its operation near saturation—exhibits a non-linear behaviour such that besides the desired transmitting signal it also outputs harmonics of the transmitting signals. To meet predefined specifications, e.g., the ETSI-GSM-Standard, after the amplification through the power amplifier
104
there is carried out the matching of the output impedance of the power amplifier
104
in a further circuit unit
106
onto a line impedance of typically 50 &OHgr; and in addition a low pass filtering of harmonics.
However, the circuit design shown in
FIG. 10
with the increase of the digital mobile telephony is more and more unable to meet the requirements, in particular in rural areas, as an ever increasing number of subscriber faces only a limited number of transmitting frequencies and related transmission channels. Although an increased transmitting frequency, e.g., approximately 1800 MHz for DCS or approximately 1900 MHz for PCS compared to approximately 900 MHz for GSM allows for an increase in the number of transmission channels due to the increased total transmission bandwidth, this is only possible at the expense of a reduced transmission range at the base stations. As a result the number of base stations necessary to completely cover the prespecified area increases. From a practical viewpoint this results in investment costs which are beyond realistic limits.
To the contrary, a combination of technical advantages of the different approaches outlined above seems to be promising, in particular through the provision of cellular dual band networks and dual band mobile phones adapted thereto, i.e. through a combination of the GSM-frequency band with the DCS-and/or PCS-frequency band.
FIG. 11
shows a possible circuit design adapted to the related transmission/receiving operation in a dual band mobile phone which is directly based on the approach shown in FIG.
10
.
Here, an antenna
200
is connected to two transmitter/receiver change over units
202
and
204
. The transmitter/receiver change over unit
202
comprises a transmitter switch TXa and a receiver switch RXa for a carrier frequency lying in the first frequency band. Further, the transmitter/receiver change over unit
204
comprises a transmitter switch TXb and a receiver switch RXb for a carrier frequency lying in a second frequency band. The switches TXa, RXa, TXb and RXb are operated according to the chosen operating frequency, respectively, as explained with reference to FIG.
10
. Further, there is provided a diplexer
206
to join the transmission paths to the antenna. Further, there are provided two power amplifiers
210
and
212
as well as related circuit units
214
and
216
adapted to carry the impedance matching and low pass filtering. Alternatively, the two power amplifiers
210
and
212
for the two transmitting frequencies may be equivalently substituted through a single power amplifier with a plurality of output terminals.
The advantage of this direct generalization of the single band transmitter/receiver output circuit shown in
FIG. 10
is that the transmission branches for both frequency bands are fully decoupled. Although the power amplifier is operated near saturation such that harmonics of both transmitting signals are generated these may be damped through appropriate dimensioning of the low pass filters TPa and TPb. Nevertheless, these advantages require additional circuit complexity. The additional circuit units not only increase the fabrication costs significantly but also the space requirements for this dual band transmitting/receiving circuit constitute an argument against its realization.
One approach to reduce the fabrication costs and the space requirements is the use of power amplifiers which output transmitting signals in a plurality of frequency bands via a single output terminal or equivalently of power amplifiers having different operation modes. For a combination of, e.g., GSM with a transmitting frequency of approximately 900 MHz and DCS with a transmitting frequency of approximately 1800 MHz the respective output power amounts to approximately 3 W and 1.5 W.
However, as explained with reference to
FIGS. 10 and 11
the operation of the power amplifier is near saturation and thus in the first operation mode harmonics are generated at approximately 1800 MHz, approximately 2700 MHz, . . . and further in the second operation mode harmonics are generated at approximately 3600 MHz, etc. Here, regularly harmonics of first and second order are the dominating harmonics.
The result for the operation mode in the first transmitting frequency of, e.g., 900 MHz is that the first harmonic of the transmitting signal at 1800 MHz is not suppressed in the low pass filter TPb in the second transmission branch. Further, also with an opened transmitting switch TXb in the second transmission branch a full decoupling to the antenna is not achieved and therefore, not only the actual transmitting signal is radiated by the antenna but also the first harmonic thereof via the deactivated transmission branch for the second higher transmitting frequency. This occurs to an extent exceeding the limits set by predefined standards. Generally, this problem occurs for power amplifier outputting transmitting signals in a plurality of frequency bands in case the harmonic of the First, lower transmitting frequency is lower than the second, higher transmitting frequency or is identical thereto.
Therefore, the object of the present invention is to effectively suppress harmonics in a dual band mobile phone with a power amplifier outputting transmitting signals in different frequency bands via one output terminal.
According to the invention this object is achieved through a power amplifier output circuit for a dual band mobile radio unit, comprising a first transmitter/receiver change over means to transmit/receive a first transmitting/receiving signal in a first frequency band via an antenna of the mobile radio unit, a second transmitter/receiver change over means to transmit/receive a second transmitting/receiving signal in a second frequency band above the first frequency band via the antenna of the mobile radio unit, wherein a change over means selectively connects a power amplifier to amplify the transmitting signals with the first and second transmitter/receiver change over means, respectively, and an impedance transformation means is provided to transform a turn off impedance of the second transmitter/receiver change over means during the transmission of the first transmitting signal into a band stop characteristic tuned to harmonics of the center frequency f
1
of the first frequency band.
According to the present invention the switchable band stop comprises three parts: the change over unit to selectively connect the power amplifier to the transmission branch for the second frequency band, the second transmitter/receiver change over unit and finally an impedance transformation means connected therebetween, respectively. All three components constitute a switchable band
Glöckler Roman
Herold Alexander
Johansson Paul
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