Telecommunications – Receiver or analog modulated signal frequency converter – Plural receivers
Reexamination Certificate
1999-08-11
2002-02-12
Legree, Tracy (Department: 2681)
Telecommunications
Receiver or analog modulated signal frequency converter
Plural receivers
C455S112000, C455S118000, C455S147000, C455S209000, C455S216000
Reexamination Certificate
active
06347219
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a transmission system and a transmission/reception system to be used for radio communications, and a local oscillator to be used in the same.
2. Description of the Related Art
The system to be used in the field of radio communications is generally equipped with a reception circuit, in which the frequency of a received signal is lowered into an intermediate frequency signal by frequency converting means and is then demodulated, and a transmission circuit in which the intermediate frequency signal is modulated and then converted into a transmission frequency by frequency converting means and is transmitted.
The two above frequency converting means to be used respectively in the reception circuit and the transmission circuit converts an input signal of a frequency fr into an output signal of a frequency fi by mixing it with a local oscillator output signal of a frequency f
1
. At this time, the difference between the frequency fr of the input signal and the frequency f
1
of the local oscillator output signal is the frequency fi of the output signal. Here, the input signal is the received signal in the reception circuit and the intermediate frequency signal in the transmission circuit. On the other hand, the output signal is the intermediate frequency signal in the reception circuit and the transmitted signal in the transmission circuit.
Now when the received signal of the frequency fr is converted in the reception circuit into the intermediate frequency signal of the frequency fi, a signal component of a frequency fg, located on the opposite side of the frequency spectrum from the received signal with respect to the frequency f
1
of the local oscillator output signal, is also converted likewise as the intermediate frequency signal. This signal component is called “image frequency”, and interferes with the desired signal.
The image frequency fg or the frequency component signal, located on the opposite side from the received signal with respect to f
1
, will be described in the following. When the frequency fr of the received signal is higher than the frequency f
1
of the local oscillator output signal (that is, fr>f
1
), more specifically, the frequency fg of the image frequency is expressed by fg=f
1
−(fr−f
1
). When the frequency fr of the received signal is lower than the frequency f
1
of the local oscillator output signal (that is, fr<f
1
), on the contrary, the frequency fg of the image frequency is expressed by fg=f
1
+(f
1
−fr)
In order to avoid the interference caused by such image frequency, it is necessary for the system to filter the image frequency component signal out of the received signal at a stage before the received signal is converted into the intermediate frequency signal.
When the frequency f
1
of the local oscillator output signal and the frequency fr of the received signal are extremely close to each other, however, a filter having minute passing functionality is required, with such strict design specification, that it becomes difficult to realize. In order to eliminate this disadvantage, there has been known in the prior art the double conversion type system in which the passing functionality requirment of the filter is loosened by performing the frequency conversion two times. The system is advantageous not only in that the filter can be realized relatively easily but also in that the output frequency of the local oscillator and the frequency of the transmitted signal do not come close to each other so that the influences of the transmitted signal upon the local oscillator are reduced, thus stabilizing the characteristics of the local oscillator.
FIG. 8
shows one example of the system which was conceived by us but not realized by us to be already well known in the art. This system is constructed to include a reception circuit
102
, a transmission circuit
103
, a first local oscillator
130
and a second local oscillator
131
.
The reception circuit
102
is equipped with band-pass filters
101
and
106
, an amplifier
104
for amplifying a received signal, frequency converters
105
and
107
.
The transmission circuit
103
is equipped with a modulator
110
, a frequency converter
109
and an amplifier
108
for amplifying a transmitted signal.
The local oscillators
130
and
131
are each constructed of a frequency synthesizer.
The local oscillator
130
is equipped with a voltage-controlled oscillator
111
, a frequency divider
112
, a variable frequency divider
113
, a phase comparator
115
and a loop filter
116
. The local oscillator
131
is equipped with a voltage-controlled oscillator
117
, a frequency divider
118
, a variable frequency divider
119
, a phase comparator
121
and a loop filter
122
.
The local oscillator
130
stabilizes the frequency by dividing the oscillatory frequency of the oscillator
111
with the dividers
112
and
113
, by making comparison with the phase of a reference signal using the phase comparator
115
, and by feeding the comparison result to the oscillator
111
through a phase-locked loop [which is composed of the frequency divider
112
, the variable frequency divider
113
, the phase comparator
115
and the loop filter
116
in that order].
The local oscillator
131
also functions to stabilize the frequency by dividing the oscillatory frequency of the oscillator
117
by the frequency divider
118
and the variable frequency divider
119
, by making a comparison with the phase of a reference signal by the phase comparator
121
, and by feeding the comparison result to the oscillator
117
through a phase-locked loop [which is composed of the frequency divider
118
, the variable frequency divider
119
, the phase comparator
121
and the loop filter
122
in that order].
Here will be described the actions of this system.
At time of reseption, the received signal, as received by an antenna
99
, is sent to the receiving circuit
102
through a transmit-receive switch
100
. This received signal is filtered to remove unnecessary frequency component signals by the band-pass filter
101
of the reception circuit
102
and is amplified by the amplifier
104
and converted into a first intermediate frequency signal by the frequency converter
105
. In this case, the frequency converter
105
converts the received signal into the first intermediate frequency signal by mixing it with the output signal of the local oscillator
130
.
The first intermediate frequency signal is filtered to remove the frequency component signals (the noise components) other than the desired intermediate frequency component signal by the filter
106
and is frequency-converted into a second intermediate frequency signal by the frequency converter
107
. In this case, the frequency converter
107
converts the first intermediate frequency signal into the second intermediate frequency signal by mixing it with the output signal of the local oscillator
131
. Finally, the second intermediate frequency signal is outputted to the demodulator (not shown).
At time of transmitting, the output signal of the second local oscillator
131
is inputted to and modulated by the modulator
110
and is then converted into a transmission frequency signal by the frequency converter
109
. In this case, the frequency converter
109
converts the output signal of the modulator
110
into the transmission frequency signal by mixing it with the output signal of the local oscillator
130
. This transmission frequency signal is amplified by the amplifier
108
and is fed to the antenna
99
through the switch
100
.
Such a system is required to have two local oscillators such as the local oscillators
130
and
131
, so that its construction is complicated raising the cost.
Here, the prior art that has succeeded in eliminating the defects of the system is exemplified firstly as disclosed in Unexamined Published Japanese Patent Application Nos. 4-87424 and 4-53302.
The first prior art example is equip
Adachi Hisashi
Kosugi Hiroaki
Nakamura Toshiaki
Ohta Ikuo
Takinami Koji
Davis Temica M.
Jacobson & Holman PLLC
Legree Tracy
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