Television – Image signal processing circuitry specific to television – Noise or undesired signal reduction
Reexamination Certificate
1997-09-29
2003-09-09
Srivastava, Vivek (Department: 2611)
Television
Image signal processing circuitry specific to television
Noise or undesired signal reduction
C348S697000, C348S622000, C332S170000
Reexamination Certificate
active
06618096
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Technical Field
The present invention generally relates to video digital signal processing (DSP) systems for generating vestigial-sideband (VSB) signals and, more particularly, to a system and method for adaptively balancing quadrature modulators in a video digital signal processing system for generating vestigial-sideband signals.
2. Description of Related Art
Frequency translation is the basic idea behind radio communications. That is, frequency translation allows the generation of signals in radio communications systems with desirable transmission characteristics, such as antenna size, freedom of interference from similar information sources, line-of-sight to long-range propagation, and freedom of interference from noise sources. Moreover, frequency translation permits the efficient utilization of open and closed propagation media by many simultaneous users and/or signals.
One of the most used forms of frequency translation is linear modulation, the most common of which is amplitude modulation. In general, amplitude modulation consists of varying the magnitude of the carrier signal in direct correspondence to the instantaneous fluctuations of a modulating signal source, as described in Electronics Engineers' Handbook, 3rd Edition, Donald G. Fink and Donald Christiansen, Eds., 1989, Section 14
, Modulators, Demodulators, and Converters
, hereby incorporated by reference.
Variations of the basic amplitude modulation process have been developed to achieve more efficient spectrum utilization and to reduce transmitter power requirements. An example is a vestigial-sideband modulation system. In a vestigial-sideband modulation system, information transmitted by an amplitude modulated carrier is wholly contained in the modulation sidebands. The transmission of the carrier energy adds no information and, moreover, each sideband contains the same information, and only one is required to transmit the “intelligence”. As such, elimination of one sideband can effect a substantial transmitter power saving. Elimination of one sideband is typically known as vestigial-sideband transmission modulation. The unwanted sideband is generally filtered out at a transmitter or receiver and is known as transmitter attenuation or receiver attenuation, respectively. The primary objective of vestigial-sideband transmission modulation systems is to conserve spectrum in the transmission medium.
Presently, vestigial-sideband signals, for example, television VSB signals, are transmitted using analog modulation techniques. For example, a typical television VSB signal transmission system uses traditional surface acoustic wave (SAW) filters for suppressing the unwanted sideband. SAW filters have errant imperfections, however, because they are temperature and power sensitive.
The present systems do not use digital signal processing techniques, which are less temperature and power sensitive than analog processing techniques, to generate television VSB signals because DSP processors have only recently achieved the speed, complexity, and price requirements to perform video digital signal processing. With the advent of digital signal processing, however, there are new demands on quadrature modulators performance (balance).
Using DSP techniques to generate video VSB signals has created a new need for balancing quadrature modulators. That is, imbalances in the quadrature modulators typically result in the lower sideband being −40 dB below the desired signal. It is desirable, however, that spurious products, such as the lower sidebands, be −60 dB below the desired signal in a video VSB system. Quadrature modulator errors include gain imbalance, phase imbalance, and DC offset errors.
Referring to
FIG. 1
, there is shown a conventional quadrature modulator
100
. Quadrature modulator
100
comprises two bi-phase modulators
102
and
104
, a phase shifter
106
, and a combiner
108
. The quadrature modulator
100
generally operates as follows. The modulator
102
modulates a first carrier signal
110
using an in-phase (I) baseband signal
112
to produce a modulated in-phase signal
114
. The phase shifter
106
receives the first carrier signal
110
and generates a second carrier signal
116
, wherein the second carrier signal
116
is in quadrature with the first carrier signal
110
(that is, the second carrier signal
116
is 90° out of phase with the first carrier signal
110
).
The modulator
104
modulates the second carrier signal
116
with a quadrature (Q) baseband signal
118
to produce a modulated quadrature signal
120
, wherein the modulated quadrature signal
120
is in quadrature with the modulated in-phase signal
114
. The modulated in-phase signal
114
and the modulated quadrature signal
120
are then combined in-phase by the combiner
108
to produce a radio frequency (RF) output signal
122
.
The in-phase baseband signal
112
and the quadrature baseband signal
118
typically range in frequency from approximately 0 Hz to 2.5 MHz. The frequency of the first carrier signal
110
is approximately between 100 MHz and 1000 MHz. These frequencies are provided for illustrative purposes only, and may be other values. As will be appreciated, the transmit frequency of the RF output signal
122
is substantially equal to the frequency of the first carrier signal
110
.
Modulators
102
and
104
include imperfections that may result in the generation of undesired mixing products. That is, because of the imperfections in the modulators
102
and
104
, the RF output signal
122
may include undesired signal components having frequencies equal to the frequency of the in-phase baseband signal
112
, the frequency of the quadrature baseband signal
118
, the frequencies of the first and/or second carrier signals
110
and
116
, and/or combinations of these frequencies.
The signal components having frequencies equal to the frequencies of the first and second carrier signals
110
and
116
are difficult to eliminate from the RF output signal
122
because their frequencies are so close to the transmit frequency of the RF output signal
122
. These undesired signal components are commonly known as carrier leakage signals, or simply carrier leakage, because they originate from the first and second carrier signals
110
and
116
, respectively.
Referring to
FIG. 2
, there is shown a graph
200
in the frequency domain of the energy output of the circuit of FIG.
1
. The RF output signal
122
has a carrier leakage having a frequency of, for example, w
c
202
, an undesired sideband having a frequency of+for example, w
c
−w
m
204
, and a desired sideband having a frequency of, for example, w
c
+w
m
206
. The carrier leakage frequency w
c
202
and undesired sideband w
c
−w
m
204
represent wasted power and interference, and produce other undesirable effects such as jitter. As such, it is desirable to suppress the carrier leakage frequency w
c
202
and the undesired sideband frequency w
c
−w
m
204
.
It is possible to eliminate the carrier leakage and undesired sideband by applying appropriate DC offsets, amplitude and phase balancing to the quadrature modulator. Also, carrier leakage and undesired sidebands can be eliminated by appropriately selecting component values to thereby balance the bi-phase modulators contained in quadrature modulators.
The application of DC offsets to the quadrature modulator and the selection of component values to balance the bi-phase modulators, however, do not represent complete solutions since carrier leakage varies with many factors, such as temperature, frequency, load impedance, and carrier power.
Some radio communications systems attempt to resolve the above problem of suppressing carrier leakage and undesired sidebands. One such system is disclosed in U.S. Pat. No. 5,162,763 to Morris. Morris is directed to a single sideband modulator for translating low frequency baseband signals directly to radio frequency in a single stage. Morris discloses monitoring amplitude of the RF output of the single
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