Pulse or digital communications – Transmitters – Quadrature amplitude modulation
Reexamination Certificate
2000-02-15
2004-02-03
Tse, Young T. (Department: 2634)
Pulse or digital communications
Transmitters
Quadrature amplitude modulation
C375S296000, C375S308000, C332S103000
Reexamination Certificate
active
06687311
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a direct Quadrature Amplitude Modulation (QAM) modulator that uses advanced digital signal processing techniques and feedback control. The invention is particularly suitable for generating 64/256-QAM signals in the 50 MHz-880 MHz range without an Intermediate Frequency (IF) stage and double-conversion, and may be used, e.g., for communicating digital television data via a cable network.
The following acronyms and terms are used:
ASIC—Application-Specific Integrated Circuits
D/A—Digital-to-Analog
DSP—Digital Signal Processor
EM—Electro-Magnetic
EMC—Electro-Magnetic Compatibility
EMI—Electro-Magnetic Interference
I/Q—In-phase/Quadrature-phase.
IF—Intermediate Frequency
LO—Local Oscillator
MPS—Modular Processing System
PLL—Phase-Locked Loop
QAM—Quadrature Amplitude Modulation
QPSK—Quadrature Phase-Shift Keying
RF—Radio Frequency
RMS—Root Mean Square
SAW—Surface Acoustic Wave
QAM modulation is commonly used, for example, in many existing cable television network headends, as well as many other possible uses.
FIG. 1
illustrates a conventional double-conversion QAM modulator
100
, which includes a modulator portion
102
and an up-converter portion
172
. With current systems, a digital modulator
105
in a device termed a MPS generates a QAM signal at an IF, such as a 44 MHz or 36 MHz international standard.
The digital signal output from the modulator
105
passes through a D/A converter
110
, a low-pass filter
115
, an IF amplifier
120
, a SAW filter
125
, an IF amplifier
130
, and an, IF output driver
140
.
An external up-converter
172
receives the IF signal from the IF output driver
140
and translates the IF signal to a RF signal. The up-converter
172
includes a 1.3 GHz mixer
145
, a 1.3 GHz band-pass filter
150
, a 1.3-2.16 GHz mixer
155
, a band-pass filter
160
, a RF driver
165
and a 50 to 880 MHz coupler
170
which provides an output signal on line
180
. A monitor
175
monitors an output of the coupler
170
.
This IF-RF approach requires a two-stage RF converter (the so-called “double-conversion” technology). Thus, in such current modulators, one digital modulator
105
and a two-stage analog converter are used. That is, the up-converter 172 has two stages—mixers
145
and
155
.
However, the conventional modulation approaches have a number of disadvantages, including:
(1) This technology requires two high frequency local oscillators
145
and
155
. There are several disadvantages associated with these local oscillators (mixers)
145
and
155
. First, they require more materials, board space and power supply. In addition to cost issues, high frequency signals are a source of EMI and EMC problems. Additional L-band conversion introduces phase noise to the modulated signal. Double-conversion increases manufacturing difficulty and reduces product reliability. In this configuration, the SAW filter has—20 dB attenuation. More amplifiers are required to compensate the filter loss.
To control the phase noise, each LO
145
,
155
has to be phase locked to a reference source using a PLL. Active filters are required for the PLL to provide a good frequency resolution and to reduce RMS phase errors.
(2) The IF output requires the SAW filter
125
to filter out spurious images and harmonics. However, the SAW filter
125
introduces a significant attenuation, such as—20 dB. Accordingly, more amplifiers (e.g., amplifiers
120
,
130
) are required to compensate for the amplitude loss due to the SAW filter
125
. Moreover, these amplifiers
120
,
130
have to be adjusted to balance harmonic distortion and signal-to-noise ratio. In the conventional QAM design tested by the inventor, a one-stage amplifier
120
was used to drive the SAW filter
125
and three more stages of amplifiers (in amplifier function
130
) on the output to increase the output level to the required level. The second IF amplifier (1.3 GHz)
130
also required multi-stage filtering (in the band-pass filter
150
) to remove the image and harmonics. Buffers and amplifiers were also required for these filters.
(3) EMI and EMC are very important issues with high-frequency LOs and mixers. Specifically, very good EM shields are required for all oscillators, mixers and filters. However, these shields not only require circuit board space, but also add difficulties and expense to manufacturing and trouble-shooting.
(4) Double-conversion introduces additional phase noise into the QAM signal.
(5) Double-conversion reduces product reliability.
(6) The cost for modulating each channel is very high—approximately 1,200 —for conventional modulation and upconversion technology.
(7) Separate boxes (e.g., packaging) for the modulator
102
and upconverter
172
make status monitoring, operation control and redundancy control more complicated.
Accordingly, it would be desirable to provide a digital modulation technology that addresses the above problems.
The system should provide improved performance over conventional modulators, and should be implementable in a more compact design and at a lower cost.
The system should provide combined modulation and upconversion in a single package for applications such as digital cable television transmission.
The system should be implementable using off-the-shelf DSP and ASIC devices.
The present invention provides a system having the above and other advantages.
SUMMARY OF THE INVENTION
The present invention relates to a direct QAM modulator.
A direct quadrature amplitude modulation (QAM) modulator in accordance with the invention provides amplitude and phase pre-equalization to reduce complexity and cost. In-phase (I) and quadrature-phase (Q) QAM signal components are provided from an analog modulator. A radio-frequency (RF) driver provides a RF signal with the QAM components. A monitoring device monitors phase and/or amplitude errors in the RF signal, and provides a corresponding signal to a digital complex phase and amplitude equalizer embedded in a digital Nyquist filter. The equalizer uses the feedback signal to provide a pre-equalizing signal to a digital-to-analog converter, which provides a corresponding analog equalizing signal. This signal is, in turn, low-pass filtered and fed back to the analog I/Q modulator to equalize the phase and/or and amplitude of the I and Q components there.
The invention is particularly suitable for generating 64/256-QAM signals in the 50 MHz-880 MHz range without an Intermediate Frequency (IF) stage and double-conversion, and may be used, e.g., for communicating digital television data via a cable network.
REFERENCES:
patent: 5293406 (1994-03-01), Suzuki
patent: 5852389 (1998-12-01), Kumar et al.
patent: 6246286 (2001-06-01), Persson
General Instrument Corporation
Lipsitz Barry R.
Lugo David B.
McAllister Douglas M.
Tse Young T.
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