Pulse or digital communications – Transmitters – Antinoise or distortion
Reexamination Certificate
1999-10-26
2003-11-11
Tran, Khai (Department: 2631)
Pulse or digital communications
Transmitters
Antinoise or distortion
C455S127500
Reexamination Certificate
active
06647072
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a transmitting circuit for use in a portable telephone of the CDMA system or the like.
2. Description of the Related Art
The portable telephone having a transmitting circuit and receiving circuit performs transmission and reception with a base station, whereby the subscribers communicate each other through the base station. Here, since the transmission signal level that the base station transmits to the portable telephones is always constant, the reception signal level that a portable telephone receives varies depending on the distance between the base station and the portable telephone. Further, the base station is designed to receive a signal outputted by a portable telephone always at a constant level.
Therefore, both the receiving circuit and the transmitting circuit of a portable telephone require wide dynamic ranges. Further, in the receiving circuit, the gains of the internal amplifiers are designed to be controlled by the automatic gain control voltage based on the power control signal from the base station, and based on the automatic gain control voltage, the transmission signal level transmitted to the base station from the transmitting circuit is designed to be varied.
The conventional transmitting circuit will be explained with reference to FIG.
4
through FIG.
6
. First, in
FIG. 4
, a QPSK modulated intermediate frequency signal IF of the 200 MHz band is outputted from a modulator not illustrated, and inputted to an intermediate frequency amplifier
21
. The intermediate frequency amplifier
21
is configured with a gain controllable variable gain amplifier, and the gain is controlled by an automatic gain control voltage V based on the power control signal from the base station. The automatic gain control voltage V varies from 3.0 volts to 0 volt, and as shown by the curve A in
FIG. 5
, the gain is controlled to be attenuated from the maximum gain to about −60 dB. The intermediate frequency signal IF is amplified by the intermediate frequency amplifier
21
, and thereafter inputted to a frequency converter
22
configured with a mixer
22
a
and a local oscillator
22
b
. The intermediate frequency signal IF is mixed in the mixer
22
a
with a local oscillation signal outputted from the local oscillator
22
b
, whereby it is converted into a transmission signal RF of about 1.1 GHz band.
The transmission signal RF is first amplified by a high frequency amplifier (called as RF amplifier)
23
. The RF amplifier
23
is also configured with a gain controllable variable gain amplifier, whose gain is controlled by the automatic gain control voltage V, and as shown by the curve B in
FIG. 5
, the gain is made to be attenuated from the maximum gain to about −30 dB. Accordingly, the total attenuation of gain by the intermediate frequency amplifier
21
and the RF amplifier
23
can be secured for 90 dB, as shown by the curve C in FIG.
5
. The RF signal amplified by the RF amplifier
23
is further amplified by a driver amplifier
24
. The driver amplifier
24
is to amplify the RF signal to such a level as to sufficiently drive a power amplifier
25
at the next stage. The RF signal amplified by the driver amplifier
24
is amplified to a specific transmission level by the power amplifier
25
, which is transmitted toward the base station from an antenna
26
.
The driver amplifier
24
is configured with a differential amplifier
7
including two transistors
7
a
,
7
b
. Both the emitters of the transistors
7
a
,
7
b
are grounded through a resistor
7
c
, and the collectors are supplied with a voltage B through load resistors
7
d
,
7
e
. Further, the transmission signal RF amplified by the RF amplifier
23
is inputted to both the bases, and the amplified transmission signal RF is outputted from both the collectors.
In the conventional transmitting circuit thus constructed, the gain of the intermediate frequency amplifier
21
that amplifies the intermediate frequency signal IF of a lower frequency is higher (about double) than the gain of the RF amplifier
23
that amplifies the transmission signal RF of a higher frequency; accordingly to secure the total attenuation of gain, the rate of attenuation gain shared between the intermediate frequency amplifier
21
and the RF amplifier
23
is about two to one at average within the variation range (3.0 volts to 0 volt) of the automatic gain control voltage V. In addition, since the RF amplifier
23
amplifies the higher frequency signal, the gain thereof is saturated as the automatic gain control voltage V becomes high. As a result, in the higher range of the automatic gain control voltage V (for example, from 3.0 volts to 1.5 volts), the rate of the attenuation gain shared with the intermediate frequency amplifier
21
in the total attenuation gain becomes higher (more than 2/3), which deteriorates the SN ratio. Also, in the lower range of the automatic gain control voltage V (for example, from 1.5 volts to 0 volt) that controls the output power into an intermediate power or a low power, the rate of the attenuation gain shared with the intermediate frequency amplifier
21
in the total attenuation gain becomes higher (virtually 2/3), which deteriorates the CN ratio.
On the other hand, the characteristic of the attenuation gain in the receiving circuit is designed to be linear against the automatic gain control voltage. However, as mentioned above, since the RF amplifier
23
is saturated in the higher range of the automatic gain control voltage V, the relation (attenuation gain characteristic) of the automatic gain control voltage V against the attenuation gain does not become linear. In consequence, the characteristic of the total attenuation gain (curve C in
FIG. 5
) does not become linear, and thereby the matching with the characteristic of the attenuation gain in the receiving circuit cannot be achieved, which is a problem to be solved.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the present invention to provide a transmitting circuit that improves the SN ratio in the lower power output (in the higher range of the automatic gain control voltage V (for example, from 3.0 volts to 1.5 volts)) and the CN ration in the lower to intermediate power output (in the lower range of the automatic gain control voltage V (for example, from 1.5 volts to 0 volt)), and makes the attenuation gain characteristic linear.
In order to solve the foregoing problems, the transmitting circuit of the present invention contains an intermediate frequency amplifier that amplifies an intermediate frequency signal, a frequency converter that applies frequency conversion to the intermediate frequency signal into a transmission signal of a higher frequency than the intermediate frequency, a high frequency amplifier that amplifies the transmission signal, and a driver amplifier that further amplifies the transmission signal amplified by the high frequency amplifier and inputs it the result to a power amplifier. And in this construction, the intermediate frequency amplifier, the high frequency amplifier, and the driver amplifier are configured using variable gain amplifiers, and the gain of the intermediate frequency amplifier, the gain of the high frequency amplifier, and the gain of the driver amplifier are made to be varied by an automatic gain control voltage.
Further, in the transmitting circuit of the present invention, the automatic control voltage varies from a first voltage to a second voltage, the gain of the intermediate frequency amplifier and the gain of the high frequency amplifier vary between the first voltage and the second voltage, each of the gains becomes maximum at the first voltage and each becomes minimum at the second voltage, the gain of the driver amplifier is made to be attenuated gradually from the maximum gain to a specific gain, as the automatic gain control voltage varies from the first voltage to a third voltage intervening between the first voltage and the second voltage, and the specific gain is made
Alps Electric Co. ,Ltd.
Brinks Hofer Gilson & Lione
Tran Khai
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