Pulse or digital communications – Receivers – Angle modulation
Reexamination Certificate
1998-01-08
2001-10-30
Pham, Chi (Department: 2631)
Pulse or digital communications
Receivers
Angle modulation
C375S340000, C375S339000
Reexamination Certificate
active
06310924
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a digital demodulator such as a grand alliance (GA) type HDTV (High Definition Television) receiving unit, and in particular, to an improved digital demodulator which is capable of simplifying an application specific integrated circuit (ASIC) of a demodulator using a combined sampling technique.
2. Description of the Conventional Art
As shown in
FIG. 1
, the GA type HDTV receiving unit includes an IF output unit
10
for outputting an IF (Intermediate Frequency) signal based on a signal received from an antenna
1
, a carrier wave demodulator
20
for performing a carrier wave demodulation with respect to an output signal from the IF output unit, and an output unit
30
for performing a segment/field synchronization, a timing recovery, and a channel equalization with respect to an output signal from the carrier wave demodulator.
The operation of the GA type HDTV receiving unit will now be explained with reference to the accompanying drawings.
First, the signal received through the antenna
1
is converted into an IF signal by a first local oscillator
12
of a tuner
11
, and a frequency drifter or variation are compensated by a second local oscillator
28
which is a voltage controlled oscillator. The second local oscillator
28
is controlled by a frequency and phase compensation loop.
In addition, the output signal from the tuner
11
passes through a SAW (Surface ascoutic wave) filter
13
, thus minimizing the noise effect of a surrounding channel, and the output signal from the SAW filter
13
is amplified by a low noise intermediate frequency amplifier
14
and is inputted into mixers
21
and
22
, respectively.
A mixer
21
detects an I-signal component by multiplying the output signal from the low noise intermediate frequency amplifier
14
and a cosine value of the oscillating frequency of a third local oscillator
23
of the fixed frequency, and a mixer
22
detects a Q-signal component by multiplying the output signal from the low noise intermediate frequency amplifier
14
and a sine value of the oscillating frequency of the third local oscillator
23
.
Since the GA type HDTV receiving unit adapts a VSB (Vestigial Sideband) modulation method, an I-signal component is sued for processing data. However, for obtaining a frequency, the I-signal component and Q-signal component are all needed.
The AFC low pass filter
24
operates by the frequency difference between a pilot signal and the second local oscillator
28
before the synchronization of the phase is implemented like the channel of the television receiving unit is changed.
Here, the high frequency components such as a noise or an interference excluding the pilot signal are eliminated.
The pilot signal is limited by the limiter
25
having a value of ±1 and is multiplied by the Q-signal by the mixer
26
. Therefore, it is possible to obtain an AFC characteristic curve having a typical S-curve.
The polarity of the S-curve error signal is determined depending whether it is higher or lower than the frequency input IF signal from the second local oscillator
28
.
The signal DCed by the limiter
25
is rectified by the APC low band pass filter
27
and controls the second local oscillator
28
for reducing the frequency error.
When the frequency error becomes closer to the value of 0, the input IF signal from the carrier wave demodulator
20
and the third local oscillator
23
are phase-synchronized.
When the phases are synchronized, the pilot signal having a value of ±1 is inputted into the mixer
26
.
In addition, the I-signal in which the pilot signal is detected is converted into the digital data by the A/D (Analog/digital) converter
31
.
A segment synchronous detector
32
obtains a synchronous value based on the repeated segment data existing in the receiving data, and obtains a symbol clock of 10.76 MHz which is properly synchronized by the PLL
33
.
An automatic gain controller
34
outputs an AGC signal which is capable of controlling the size using the low noise intermediate frequency amplifier
14
and the tuner
11
and maintains a predetermined level of the input signal.
The output data from the A/D converter
31
is outputted to the segment synchronous detector
32
and a rear portion field synchronous signal detection terminal (not shown), thus detecting the field synchronous value, and determines whether an NTSC interference eliminating filter in accordance with the level of the interference with the NTSC co-existing channel.
In addition,
FIG. 2
is a block diagram illustrating an analog type circuit when converting into a base band width signal of an IF signal in the conventional art. Namely,
FIG. 2
illustrates an analog type conversion circuit converting into a base band width signal with respect to the IF signal.
In order to detect the I-signal component, the IF signal inputted from the mixer
41
a
is multiplied by a cos&ohgr;t, and the Q-signal is multiplied by sin&ohgr;t by the mixer
41
b.
Namely, in order to obtain optimum I and Q signals, the input IF signal should be multiplied by cos&ohgr;t and sin&ohgr;t, respectively, at a corresponding sampling time.
The output signals from the mixers
41
a
and
41
b
are converted into the digital signals by the A/D converters
43
a
and
43
b
through the low pass filters
42
a
and
42
b
and then are outputted as I- and Q-signals.
Namely, the carrier wave demodulator
20
of the GA type HDTV receiving unit performs an analog signal processing step, and the rear portion of the carrier wave demodulator
20
performs an A/D conversion step for a digital processing. Therefore, when implementing the ASIC, since the ASIC in which the digital and analog circuits are mixed is implemented, the implementation of the ASIC is difficult.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the present invention to provide an improved digital demodulator which overcomes the aforementioned problems encountered in the conventional art.
It is another object of the present invention to provide an improved digital demodulator which is capable of simplifying an application specific integrated circuit (ASIC) of a demodulator using a combined sampling technique.
It is another object of the present invention to provide an improved digital demodulator which is capable of implementing an ASIC with a rear portion of the carrier demodulator in which a digital signal processing is performed when a carrier wave demodulator performs a digital signal processing operation.
In order to achieve the above objects, there is provided a digital demodulator which includes a mixer for multiplying an IF signal from the IF output unit and a local oscillating signal from the local oscillator, a low pass filter for eliminating an image frequency component generated during an operation of the mixer, an A/D converter for converting an output signal from the low pass filter into a digital signal, a multiplier for multiplying an output data from the A/D converter and cos&ohgr;t and sin&ohgr;t which are combined signal values and converting into a base band width signal frequency, a demultiplexer for separating a base band width digital data into an I-signal component and Q-signal component, a frequency/phase difference detector for performing the signal processing and compensating the detected frequency and phase difference, an analog low pass filter for filtering an output signal from the frequency/phase difference detector, and a local oscillator for compensating the frequency/phase difference filtered by the analog low pass filter and providing the oscillating frequency which is synchronized with the IF signal inputted from the IF output unit to the mixer.
Additional advantages, objects and other features of the invention will be set forth in part in the description which follows and in part will become apparent to those having ordinary skill in the art upon examination of the following or may be learned from practice of the invention. The objects and advantages of the invention ma
Cheun Kyung-Hoon
Choi Kwon-Hyu
Kang Byung-Joo
Park Kyu-Hyoun
Bayard Emmanue
Hyundai Electronics Industries Co,. Ltd.
Lackenbach Siegel
Nissen J. Harold
Pham Chi
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