Multiplex communications – Communication over free space – Having a plurality of contiguous regions served by...
Reexamination Certificate
1998-03-13
2001-04-24
Ton, Dang (Department: 2661)
Multiplex communications
Communication over free space
Having a plurality of contiguous regions served by...
C370S342000, C370S252000, C370S343000, C455S522000
Reexamination Certificate
active
06222831
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a receiving circuit of a cellular telephone set operating on both the code division multiple access method and the frequency division multiple access method. More particularly, the invention relates to an intermediate frequency circuit in the receiving circuit.
2. Description of the Related Art
Described below with reference to
FIG. 2
is part of a conventional receiving circuit used by what is known as a dual mode cellular telephone set operating on both the code division multiple access method and the frequency division multiple access method.
In a cellular telephone set
31
shown in
FIG. 2
, a signal from a transmitting circuit
32
is fed through an antenna sharing device
33
to an antenna
34
from which the signal is transmitted to a base station, not shown. A signal sent from the base station is received by the antenna
34
and input to a receiving circuit
35
via the antenna sharing device
33
. This type of cellular telephone set
31
is capable of sending and receiving signals to and from the base station in either a code division multiple access mode (called the CDMA mode hereunder) or a frequency division multiple access mode (called the FM mode hereunder).
In the receiving circuit
35
, a received signal on a bandwidth of about 880 MHz passes through the antenna sharing device
33
to reach a low-noise amplifier
36
for amplification. The amplified signal is input to a mixing circuit
37
whereby the signal is mixed with an oscillation signal from a local oscillator
38
. The mixing circuit
37
outputs a signal with an intermediate frequency of about 85 MHz to an intermediate frequency amplifier
39
. The amplifier
39
converts the frequency of the received signal to a bandwidth per channel of about 1.23 MHz for the CDMA mode or about 30 kHz for the FM mode. The received signal of either the CDMA or FM mode alone is converted in terms of frequency.
The intermediate frequency amplifier
39
uses a transistor
42
whose base is supplied with a fixed bias voltage from bias resistors
40
and
41
. An intermediate frequency signal, after being amplified to the appropriate level by the intermediate frequency amplifier
39
, is output from the collector of the transistor
42
.
Following amplification by the intermediate frequency amplifier
39
, the intermediate frequency signal of the CDMA mode is sent through a switching diode
43
to a filter
45
dedicated to filtering CDMA mode intermediate frequency signals; the intermediate frequency signal of the FM mode is forwarded through a switching diode
44
to a filter
46
that filters FM mode intermediate frequency signals.
Where the cellular telephone set
31
is used in the CDMA mode, a mode switching voltage applied to a mode switching terminal X
1
causes a current to flow through a resistor
47
, the switching diode
43
and a resistor
48
. This causes the switching diode
43
to conduct, allowing the CDMA mode intermediate frequency signal to reach the filter
45
. Where the cellular telephone set
31
is operated in the FM mode, a mode switching voltage applied to a mode switching terminal X
2
causes a current to flow through a resistor
49
, the switching diode
44
and the resistor
48
. This causes the switching diode
44
to conduct, carrying the FM mode intermediate frequency signal to the filter
46
.
Different levels of electrical performance are required in each of the CDMA mode and the FM mode. The operation in the CDMA mode, in particular, requires that any intermodulation distortion caused by the intermediate frequency amplifier
39
be minimized. That requirement is met typically by establishing values of the bias resistors
40
and
41
such that a large operating current (i.e., collector bias current) flows continuously through the transistor
42
of the intermediate frequency amplifier
39
. That is, the intermediate frequency amplifier
39
is made to operate from the same operating current regardless of the CDMA mode or FM mode being in effect.
In the conventional receiving circuit of the conventional cellular telephone set, as described, a large operating current flows through the transistor
42
of the intermediate frequency amplifier
39
. It should be noted here that characteristics required of the intermediate frequency amplifier
39
in the FM mode in connection with intermodulation distortion are not as strict as in the CDMA mode, since the FM mode involves frequency modulation. That is, the conventional receiving circuit dissipates more power than is necessary while the FM mode is being selected. This tends to promote dissipation of cells and render the cellular telephone set less convenient than it should be.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide a receiving circuit of a cellular telephone set wherein an intermediate frequency amplifier in the FM mode operates from a reduced operating current, whereby premature dissipation of cells is forestalled and the cellular telephone set is made more convenient than before.
In carrying out the invention and according to one aspect thereof, there is provided a receiving circuit of a cellular telephone set, comprising an intermediate frequency amplifier including an amplifying transistor shared for use in both code division multiple access mode and frequency division multiple access mode; wherein a collector bias current of the transistor is switched to a large current in the code division multiple access mode and to a small current in the frequency division multiple access mode.
In a preferred structure according to the invention, the intermediate frequency amplifier is connected to a switching diode arrangement for deriving one of two types of intermediate frequency signals, one type of intermediate frequency signal coming from the intermediate frequency amplifier in the code division multiple access mode, the other type of intermediate frequency signal coming from the intermediate frequency amplifier in the frequency division multiple access mode; wherein the switching of the collector bias current of the transistor is interlocked to the switching of the switching diode arrangement.
In another preferred structure according to the invention, the switching diode arrangement comprises a first and a second switching diodes, the first switching diode deriving intermediate frequency signals of the code division multiple access mode when driven to conduct, the second switching diode deriving intermediate frequency signals of the frequency division multiple access mode when brought to conduct; wherein the conduction of one of the first and the second switching diodes is accomplished by application of a switching voltage, one of the first and the second switching diodes further acting as an intermediary to supply a base bias voltage to a base of the transistor and for switching the collector bias current.
In a further preferred structure according to the invention, the first and the second switching diodes are each connected to a feeding resistor feeding the switching voltage to cause the appropriate switching diode to conduct; wherein the feeding resistor connected to the first switching diode has a low resistance value and the feeding resistor connected to the second switching diode has a high resistance value, whereby the base bias voltage of the transistor is switched.
In an even further preferred structure according to the invention, the first and the second switching diodes are connected respectively to a first and a second intermediate frequency filters, the first switching diode acting as an intermediary to lead intermediate frequency signals of the code division multiple access mode from the intermediate frequency amplifier to the first intermediate frequency filter, the second switching diode serving as an intermediary to lead intermediate frequency signals of the frequency division multiple access mode from the intermediate frequency amplifier to the second intermediate frequency filter.
Other objects
Alps Electric Co. ,Ltd.
Brinks Hofer Gilson & Lione
Ton Anthony
Ton Dang
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