Pulse or digital communications – Receivers – Angle modulation
Reexamination Certificate
1999-11-09
2001-10-09
Ghebretinsae, Temesghen (Department: 2631)
Pulse or digital communications
Receivers
Angle modulation
C375S321000, C375S316000, C348S726000
Reexamination Certificate
active
06301312
ABSTRACT:
The invention relates to bandpass phase trackers used in the detection of digital signals transmitted using amplitude-modulated radio waves—e.g., of vestigial sideband (VSB) or quadrature-amplitude-modulation (QAM) type—which bandpass phase trackers are useful in digital television (DTV) receivers, for example.
BACKGROUND OF THE INVENTION
A Digital Television Standard published Sep. 16, 1995 by the Advanced Television Systems Committee (ATSC) specifies vestigial sideband (VSB) signals for transmitting digital television (DTV) signals in 6-MHz-bandwidth television channels such as those currently used in over-the-air broadcasting of National Television System Committee (NTSC) analog television signals within the United States. The radio receiver portions of the HDTV receiver used by the Advanced Television Systems Committee (ATSC) for field testing of the standard were designed by Zenith Electronics Corporation. In the Zenith receiver, phase tracking is done at baseband after synchronous detection is done. Digitization is done after synchronous detection. The digital transmission scheme authorized by the ATSC is unusual because it uses vestigial-sideband amplitude modulation (VSB AM).
In U.S. Pat. No. 5,479,449 entitled “DIGITAL VSB DETECTOR WITH BANDPASS PHASE TRACKER, AS FOR INCLUSION IN AN HDTV RECEIVER”, which issued Dec. 16, 1995 to C. B. Patel and A. L. R. Limberg, digitization is done before synchronous detection; and phase tracking is done at intermediate frequencies before generating complex-number digital samples for synchronous detection. U.S. Pat. No. 5,479,449 teaches that, despite lack of symmetry of VSB AM upper and lower sidebands, phase tracking can be done at intermediate frequencies before generating complex-number digital samples for synchronous detection in VSB AM receivers. Narrow bandpass filtering is done to achieve symmetry of upper and lower sidebands before extracting carrier to be synchrodyned to baseband to develop control signal for the bandpass tracker.
Alternatively, the carrier is extracted from the asymmetrical upper and lower sidebands, synchrodyned to baseband and lowpass filtered to develop control signal for the bandpass tracker, the cut-off frequency of the lowpass filter being so low in frequency that there is little response to the asymmetrical portion of the carrier sideband structure.
Bandpass phase trackers are also useful for detecting digital television signals transmitted by QAM of a center-channel carrier as described in U.S. Pat. No. 5,506,636 entitled “HDTV SIGNAL RECEIVER WITH IMAGINARY-SAMPLE-PRESENCE DETECTOR FOR QAM/VSB MODE SELECTION”, which issued Apr. 9, 1996 to C. B. Patel and A. L. R. Limberg, and in allowed U.S. Pat. No. 6,104,442 entitled “RADIO RECEIVER FOR RECEIVING BOTH VSB AND QAM DIGITAL HDTV SIGNALS”, which was filed Jun. 28, 1994 for C. B. Patel and A. L. R. Limberg.
U.S. Pat. No. 5,479,449 digitizes the sidebands of the in-phase synchronous detection result after converting the real samples to complex samples using a digital filter with Hilbert transform system function for generating the imaginary samples. This Hilbert transformation is done by digital filtering of intermediate-frequency (IF signals with system functions between one and ten MHz in frequency, which is considerably simpler to do than performing the Hilbert transformation at baseband. The delay required to achieve a 90° phase shift at a megahertz is considerably less than that required to approximate 90° phase shift at close to zero frequency. Nonetheless, the Hilbert transformation filter circuitry involves a substantial amount of digital hardware one would prefer to avoid having to use.
C. B. Patel and A. L. R. Limberg considered replacing the Hilbert transformation filter circuitry with differential 90° phase shift networks using FIR or IIR digital filters. U.S. Pat. No. 5,548,617 issued Aug. 20, 1996 and entitled “DIGITAL VSB DETECTOR WITH BANDPASS PHASE TRACKER USING RADER FILTERS, AS FOR USE IN AN HDTV RECEIVER” describes differential 90° phase shift networks using IIR digital filters based on a type described by C. M. Rader in his article “A Simple Method for Sampling In-Phase and Quadrature Components”, IEEE TRANSACTIONS ON AEROSPACE AND ELECTRONIC SYSTEMS, Vol. AES-20, No. 6 (November 1984), pp. 821-824. In U.S. Pat. No. 5,731,848 issued Mar. 24, 1998 and entitled “DIGITAL VSB DETECTOR WITH BANDPASS PHASE TRACKER USING NG FILTERS, AS FOR USE IN AN HDTV RECEIVER” C. B. Patel and A. L. R. Limberg describe differential 90° phase shift networks using FIR digital filters based on types generally described by T. F. S. Ng in United Kingdom patent application 2 244 410 A published Nov. 27, 1991 and entitled “QUADRATURE DEMODULATOR”.
The Hilbert transformation filter circuitry is implemented as a digital filter in the bandpass trackers described above after analog-to-digital conversion is performed by a single analog-to-digital converter (ADC) operative on the penultimate intermediate-frequency signal used in the receiver. This penultimate IF signal is located in the very high frequency (VHF) band somewhat below television broadcast channel two. A DTV receiver using a bandpass tracker usually will be designed as a triple-conversion receiver, converting radio-frequency (RF) signals as received from an antenna or cable connection to a first intermediate-frequency signal located in the ultra-high frequency (UHF) band somewhat above television broadcast channel eighty-three, converting amplified UHF first IF signal to the VHF penultimate IF signal, and finally converting amplified VHF penultimate IF signal to an ultimate IF signal somewhere within about a 1-10 MHz frequency range, for synchrodyning to baseband. Using a single ADC in a digital communications receiver avoids any problem of matching separate ADCs respectively used for converting a real component and an imaginary component of analog ultimate IF signal, as well as any problem of matching the gains of the real and imaginary components respectively supplied to these ADCs. Also, the problem of developing real and imaginary components of the ultimate IF signal that are in accurate 90° phasing is largely avoided.
Further, the practice when digitizing signals in a digital communications receiver has been to use a flash analog-to-digital converter, and the high 10.76 million symbols per second symbol rate and eight- or sixteen-level symbols used in DTV signals impose very difficult operating demands on a flash converter. A flash converter has a considerable amount of circuitry for incorporation within a monolithic integrated circuit (IC) die, employing a (2n−1)-resistor ladder voltage divider and (2n−1) comparators to achieve n-bit digital resolution, n being a positive integer. Considerable area is taken up on the die, so ADC cost is quite high, in the several dollar range. A flash converter consumes considerable power for operating at at least 21.52 million samples per second rate as required in the receiver for digitizing VSB AM DTV signals with 10.76 million symbols per second, when a bandpass phase tracker is used. The desire to use as few expensive, power-consuming IC devices as possible directs one skilled in the art away from considering the use of plural-phase analog-to-digital conversion.
In order to get digital resolution of ten to twelve bits at 21.52 million samples per second rate, in order better to facilitate equalization filtering, the inventor has considered the use of analog-to-digital conversion methods other than flash conversion. The inventor discerns that a single flash converter can be replaced by twenty-four ADCs of successive binary approximation type arranged for staggered sampling to provide 24-phase analog-to-digital conversion with up to eleven or twelve bits resolution without need for successive binary approximation rates above DTV symbol rate. Each ADC digitizes a sample of one-half symbol period duration. Conversion rate of each ADC is one-twenty-fourth that of the flash converter, which tends to reduce power consumption by the square of twenty-fou
Ghebretinsae Temesghen
Samsung Electronics Co,. Ltd.
Sughrue Mion Zinn Macpeak & Seas, PLLC
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