Pulse or digital communications – Transmitters – Amplitude modulation
Reexamination Certificate
1998-12-16
2003-03-11
Phu, Phuong (Department: 2631)
Pulse or digital communications
Transmitters
Amplitude modulation
C332S170000, C455S109000
Reexamination Certificate
active
06532270
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates generally to communication systems and, more particularly, to time division multiplexed communication systems. This invention has applicability to communication systems operating with bandwidth-limited broadcast spectrums.
2. Description of the Related Art
A typical communication system
10
using time division multiplex techniques is shown in FIG.
1
A. An antenna
101
is coupled to a two position switch
102
. When the switch
102
is coupled to a first terminal, the signal received by the antenna
101
is applied to low noise amplifier
103
. The output signal from the low noise amplifier
103
is applied to mixer unit
104
. The mixer unit
104
also receives a signal from the local oscillator unit
105
. The signals from the low noise amplifier
103
and the local oscillator unit
105
are combined in mixer unit
104
and applied to filter unit
106
. The filtered signal from filter unit
106
is applied to an IF (intermediate frequency) unit
107
. The output signal from the IF unit
107
is applied to demodulation unit
108
. The demodulation unit
108
recoveries the data (information) that has been encoded in the transmitted signal and applies the data to an output terminal. When the switch unit
102
is in the second position, the output signal from power amplifier is applied to the antenna for transmission of information. The power amplifier
109
receives signals from mixer unit
110
. The mixer unit
110
combines signals from the local oscillator unit and from a unit (not shown) that has encoded an intermediate signal with data to be transmitted. Typically, the intermediate frequency is fixed. However, the local oscillator unit
105
has a controllable voltage and can be implemented, for example, by a voltage controlled oscillator. The communication system
10
is multiplexed in that the system is adapted to alternatively receive and transmit signals.
Referring to
FIG. 1B
, the operation of the duplex mode of the time division multiplexed communication system
10
is illustrated. For purposes of the operation of the time division multiplexed system, time is divided into a series of equal intervals. During alternate intervals, the system is transmitting, i.e., switch
102
has coupled the antenna
101
to the power amplifier
109
. During the alternate intervals when the system
10
is not transmitting, the switch
102
couples the antenna
101
to the low noise amplifier
103
. During these intervals, the system is in a receiving mode. Thus, the system alternately transmits data and receives data.
The broadcast frequency spectrum is divided into a plurality of a multiplicity of broadcast bands, each band being reserved for a particular purpose. Referring to
FIG. 2
, a broadcast band ranging from 902 MHz to 928 MHz is used as an example. The broadcast band is divided into channels, 10 channels are shown by way of illustration in FIG.
2
. As will be seen from
FIG. 2
, the transmitted signal will be spread and in fact can overlap the signals of a neighboring channel. Because of this overlap, the signals associated with each channel are offset for purposes of illustration and clarity. The signals in each channel have a central frequency that is a combination of the local oscillator frequency (LO) and the intermediate frequency (IF). However, a noise signal having a frequency of the local oscillator LO will also be broadcast. The noise signal for channel
3
, i.e., LO
3
, is shown in FIG.
2
. As can be seen from this illustration, this noise signal can be positioned in the broadcast band and, therefore, can interfere with other information signals being broadcast in the band.
A need has been felt for apparatus and an associated technique that features the reduction of noise signals in the broadcast band resulting from the transmission of a signal having the frequency of the local oscillator. It is another feature of the present technique that it be applicable to all the channels of broadcast band.
SUMMARY OF THE INVENTION
The aforementioned and other features are provided, according to the present invention, by selecting the local oscillator frequency such that the difference between the local oscillator frequency causes a signal with the local oscillator frequency to fall outside of the broadcast band while the carrier signal having a combination of the local oscillator frequency and the intermediate frequency be the designated frequency of the associated channel. In addition to the selection of frequency of the local oscillator, the low-side injection and high-side injection, i.e., the selection of the upper or lower sideband, can be used to determine whether the carrier signal frequency is below or above the local oscillator frequency. By selecting both the local oscillator frequency and the sideband used as the carrier signal, the noise signal generated by the communication within the broadcast band can be reduced.
REFERENCES:
patent: 4213152 (1980-07-01), Kakinuma
patent: 4310920 (1982-01-01), Hayes
patent: 4994769 (1991-02-01), Kishi
patent: 5548825 (1996-08-01), Maemura et al.
patent: 5758266 (1998-05-01), Kornfeld et al.
Simon Haykin, Analog and Digital Communications, copy right 1989, pp. 370-371.
Bell Eddy Kent
Janesch Stephen T.
Magaña Javier V.
Legerity Inc.
Phu Phuong
Zagorin O'Brien & Graham LLP
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