Telecommunications – Receiver or analog modulated signal frequency converter – Frequency modifying or conversion
Reexamination Certificate
1999-05-17
2002-03-12
Bost, Dwayne (Department: 2681)
Telecommunications
Receiver or analog modulated signal frequency converter
Frequency modifying or conversion
C455S323000, C455S209000, C455S214000, C455S161300
Reexamination Certificate
active
06356746
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a mobile communication device such as a personal handy phone, a mobile telephone, a pager, etc. and, more particularly, a direct converting receiver to which a direct conversion system is applied.
2. Description of the Prior Art
In recent years, radio communication systems such as a personal handy phone, a mobile telephone, a radio paging device (pager), a cordless telephone, etc., which are employed mainly in mobile communication, have been drastically spread. As a receiver system of these radio communication systems, a direct converting receiver whose configuration is ready to be integrated and is suitable for miniaturization and light weight has been known.
As the direct converting receiver in the prior art, a configuration as disclosed in Unexamined Patent Publication Hei 7-135514, for example, has been known. An example of a receiver circuit of the direct converting receiver in the prior art is shown in FIG.
17
.
FIG. 17
shows a configuration of the receiver circuit to which frequency shift keying (abbreviated as “FSK” hereinafter) is applied. The configuration and an operation of the receiver circuit will be explained hereinafter.
The direct converting receiver comprises a receiving antenna
101
for receiving a radio signal, a signal attenuator (ATT)
102
for attenuating a received signal, a received signal amplifier
104
for amplifying the received signal supplied from the signal attenuator
102
and outputting it as a modulated signal
103
, a local oscillator
106
for generating a local signal
105
, a 90-degree phase shifter
107
for shifting a phase of the local signal
105
by 90 degree, a first signal mixer
108
for mixing the modulated signal
103
with the local signal
105
, and a second signal mixer
109
for mixing the modulated signal
103
with the local signal
105
which has been subjected to 90 degree phase shift.
Further, an I low-pass filter
111
and a Q low-pass filter
113
are provided in the direct converting receiver. The I low-pass filter
111
can pass only a baseband signal which is common in phase with the received signal, which is contained in an output signal of the first signal mixer
108
, (i.e., an I baseband signal from which harmonic components are removed) to then output an I baseband signal
110
. The Q low-pass filter
113
can pass only a baseband signal which is obtained by phase-shifting the received signal, which is contained in an output signal of the second signal mixer
109
, by 90 degree (i.e., a Q baseband signal from which harmonic components are removed) to then output a Q baseband signal
112
which is intersected orthogonally with the I baseband signal
110
. These low-pass filters
111
,
113
are composed of filters which have a fixed cut-off frequency, i.e., whose cut-off frequency is not varied, respectively.
Furthermore, there are provided a demodulating means
115
and an electric field strength detecting means
116
. The demodulating means
115
can receive the I baseband signal
110
and the Q baseband signal
112
, then demodulate them, and then output a demodulated signal
114
. The electric field strength detecting means
116
can receive the I baseband signal
110
to detect a level of the received signal, and then output a control signal to the signal attenuator
102
.
Next, a receiving operation of the direct converting receiver constructed as above in the prior art will be explained hereunder. First, the radio signal being received by the receiving antenna
101
is input into the signal attenuator
102
which can control the level of the received signal in compliance with the control signal supplied from the electric field strength detecting means
116
. The output signal of the signal attenuator
102
is amplified by the received signal amplifier
104
, and then output to the first signal mixer
108
and the second signal mixer
109
as the modulated signal
103
respectively. In the meanwhile, the local signal
105
being generated from the local oscillator
106
is mixed with the modulated signal
103
in the first signal mixer
108
and then output to the I low-pass filter
111
. At the same time, a phase of the local signal
105
is shifted by 90 degree by the 90-degree phase shifter
107
, then the local signal
105
is mixed with the modulated signal
103
in the second signal mixer
109
and then output to the Q low-pass filter
113
.
Then, the I baseband signal
110
and the Q baseband signal
112
derived after they have been passed through the I low-pass filter
111
and the Q low-pass filter
113
respectively are output to the demodulating means
115
and then demodulated there to produce the demodulated signal
114
. Also, the electric field strength detecting means.
116
, when receives the I baseband signal
110
, detects a level of the received signal and then outputs the control signal, which is in proportion to the level of the received signal, to the signal attenuator
102
.
In the radio receiver, if other radio waves in addition to the desired received wave to be received are caught as the interference waves by the receiving antenna simultaneously, particularly if a plurality of interference waves whose signal levels are high to such extent that they exert an influence upon the receiving bandwidth are caught, there has been such a problem that distortion due to the intermodulation is caused between these received radio wave signals and thus receiving sensitivity of the desired received waves to be received (desired signal) is degraded.
In order to improve the above problem, the technology has been proposed wherein generation of the distortion in the amplifier or the signal mixer can be suppressed by controlling a gain of a receiver system in response to the level of the received signal, which is generally called AGC (Automatic Gain Control), to thus improve the receiving sensitivity of the desired signal to be received. If this technology is applied to the configuration in
FIG. 17
, an AGC means can be so constructed that the electric field strength detecting means
116
outputs the control signal to the signal attenuator
102
, which is positioned immediately after the antenna, in response to the level of the received signal, and then the signal attenuator
102
controls the level of the received signal received by the receiving antenna
101
based on this control signal. According to such AGC means, it is possible to suppress the distortion due to the intermodulation caused in the received signal amplifier
104
, the first signal mixer
108
, and the second signal mixer
109
.
In recent, the demand for the direct converting receiver which is suitable for IC integration has been enhanced with the drastic spread of the radio communication systems such as the mobile telephone, etc., and a communication capacity for mobile communication has been increased abruptly with increase of the user of the mobile communication. In answer to the increase in the demand for such mobile communication, there has been a need to enhance a transmission speed in the radio communication and to proceed a narrower bandwidth of the communication frequency.
In the event that low-pass cut-off frequencies of the I low-pass filter and the Q low-pass filter are set lower so as to take preference of reduction in the influence of the interference wave by the radio waves in adjacent channels (i.e., adjacent waves) when the bandwidth of the direct converting receiver is made narrower, such a problem has arisen that, if the oscillation frequency of the local oscillator has been widely offset and deviated from the carrier frequency of the received waves, desired components of the received signal are out of the passbands of the I low-pass filter and the Q low-pass filter and thus such desired components are eliminated and not to be demodulated.
In the direct converting receiver in the prior art, since normally respective cut-off frequencies of the I low-pass filter and the Q low-pass filter are fixed, there has been such a n
Bost Dwayne
Matsushita Electric - Industrial Co., Ltd.
Pearne & Gordon LLP
West Lewis
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