Modulators – Phase shift keying modulator or quadrature amplitude modulator
Reexamination Certificate
1999-12-22
2001-07-24
Grimm, Siegfried H. (Department: 2817)
Modulators
Phase shift keying modulator or quadrature amplitude modulator
C348S724000, C375S298000, C375S308000
Reexamination Certificate
active
06265949
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a digital television repeater, and more particularly, to a phase compensation apparatus and method for a digital modulator.
2. Background of the Related Art
Generally, a digital television repeater is provided in a digital broadcasting system. The digital repeater performs the function of repeating a digital broadcasting signal to be transmitted to subscribers located in a radio wave shade area that the broadcasting signal does not reach or in a poor reception area. In other words, the digital television repeater receives a weak broadcasting signal transmitted from a digital broadcast transmitter, amplifies the broadcasting signal, and then transmits the amplified broadcasting signal through a desired channel frequency so that subscribers in the radio wave shade area or in the poor reception area can receive the broadcasting signal.
FIG. 1
is a block diagram schematically illustrating a partial construction of a conventional digital television broadcasting system. The conventional digital television broadcasting system includes a television broadcasting station
100
, a digital broadcast transmitter
101
for receiving a digital broadcasting signal from the broadcasting station
100
and transmitting the digital broadcasting signal to subscribers, and digital television repeaters
102
and
103
for amplifying and repeating the weak digital broadcasting signal transmitted from the digital broadcast transmitter
101
to serve an radio wave shade area or areas.
If it is assumed that the digital broadcast transmitter
101
transmits the digital broadcasting signal through a frequency F
1
, the digital television repeaters
102
and
103
located adjacent to the digital broadcast transmitter
101
can repeat the digital broadcasting signal with a predetermined quality even though they use channels different from the channel used by the digital broadcast transmitter
101
. Thus, they repeat the digital broadcasting signal using frequencies F
2
and F
3
, respectively. Also, they may repeat a broadcasting signal from another digital television repeater (not illustrated) located adjacent to the digital television repeaters
102
and
103
.
FIG. 2
is a block diagram schematically illustrating one of the digital television repeater shown in FIG.
1
. The digital television repeater includes a receiving antenna
200
for receiving a weak digital broadcasting signal, a receiver
201
for converting the weak digital broadcasting signal received through the receiving antenna
200
to an intermediate frequency (IF) band signal, a transmitter
202
for modulating the IF digital broadcasting signal converted by the receiver
201
to a required frequency band signal, and a transmission antenna
203
for transmitting the digital broadcasting signal modulated by the transmitter
202
to the subscribers.
FIG. 3
is a block diagram schematically illustrating the transmitter shown in FIG.
2
. The transmitter includes a modulator
300
for converting the digital broadcasting signal, which is composed of audio and video signals and transmitted from the receiver as an MPEG-II transport stream signal, to a required frequency band signal, a filter
301
for converting the digital broadcasting signal converted by the modulator
300
to a radio frequency (RF) signal, and a high-output amplifier
302
for amplifying the RF digital broadcasting signal outputted from the filter
301
. A power supply
303
supplies a required power to the modulator
300
, filter
301
, and high-output amplifier
302
.
FIG. 4
is a block diagram schematically illustrating the modulator shown in FIG.
3
. The modulator includes a signal forming circuit
400
for changing an MPEG-II transport stream signal inputted from the receiver to a quadrature phase signal (hereinafter referred to as a Q signal), a first digital-to-analog (D/A) converter
401
for converting an in-phase signal (hereinafter referred to as an I signal) of the MPEG-II signal inputted from the receiver into an analog signal, a second digital-to-analog (D/A) converter
402
for converting the Q signal into an analog signal, and a quadrature modulation circuit
403
for modulating the I and Q signals to required band signals using two quadrature carriers.
The quadrature modulation circuit
403
comprises a first mixer
404
for multiplying the I signal, which is outputted from the first D/A converter
401
as an analog signal, by a sine wave carrier SinW
c
t, a second mixer
406
for multiplying the Q signal, which is outputted from the second D/A converter
402
as an analog signal, by a cosine wave carrier CosW
c
t, and a summer
407
for summing the I and Q signals modulated by and outputted from the first and second mixers
404
and
406
.
The MPEG-II transport stream signal transmitted from the receiver is inputted to the first D/A converter
401
and the signal forming circuit
400
, and the signal forming circuit
400
produces the Q signal by delaying the phase of the inputted MPEG-II transport stream signal by 90 degrees, and inputs the phase-delayed signal to the second D/A converter
402
. The MPEG-II transport stream signal inputted to the first D/A converter
401
will be the I signal.
The digital I and Q signals inputted to the first and second D/A converters
401
and
402
are then converted to analog signals, respectively, and the converted analog signals are inputted to the quadrature modulation circuit
403
. The first mixer
404
in the quadrature modulation circuit
403
modulates the input I signal by multiplying the I signal by the the sine wave carrier SinW
c
t having a frequency of a required band, and the second mixer
406
modulates the input Q signal by multiplying the Q signal by the cosine wave carrier CosW
c
t having a frequency of a required band. Thereafter, the modulated I and Q signals outputted from the first and the second mixers
404
and
406
are summed by the summer
407
.
The conventional digital television repeater as described above, is commonly located on a high mountain or in a desolate place to serve the radio wave shade area or the poor reception area, and thus suffers from at least the following problem.
First, since the high mountain or the desolate place where the digital television repeater is located undergoes a great change in environment, such as temperature and humidity, respective elements constituting the transmitter deteriorate. Distortion may occur in the I and Q signals. The modulator then outputs the digital broadcasting signal including a phase error so that the subscribers cannot enjoy digital broadcasting service of a predetermined quality.
Second, since the digital television repeater transmits the digital broadcasting signal with a high output of more that 1 KW, the phase distortion in the modulator exerts a serious influence upon other adjacent channels.
Third, in the event that the modulator of the transmitter in the digital television repeater operates abnormally, the system operator should perform a manual compensation work using an instrument. However, considering that the digital television repeater is located on a high mountain or desolate place, a great loss of time and manpower results.
The above references are incorporated by reference herein where appropriate for appropriate teachings of additional or alternative details, features and/or technical background.
SUMMARY OF THE INVENTION
An object of the invention is to solve at least the above problems and/or disadvantages and to provide at least the advantages described hereinafter.
The invention is directed to a phase compensation apparatus and method for a digital modulator that substantially obviate at least one or more of the above-described problems and disadvantages.
An object of the invention is to maintain the optimum operating state of the modulator, preferably, provided in a digital television repeater.
It is another object of the invention to reduce the phase distortion produced from the modulator.
It is another object of the invention to maintai
Fleshner & Kim LLP
Grimm Siegfried H.
LG Information & Communications Ltd.
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