Pulse or digital communications – Receivers – Particular pulse demodulator or detector
Reexamination Certificate
1998-11-03
2003-12-30
Chin, Stephen (Department: 2634)
Pulse or digital communications
Receivers
Particular pulse demodulator or detector
C375S349000
Reexamination Certificate
active
06671334
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates generally to demodulators and more particularly to a demodulator for measurement receivers.
Measurement receivers, such as spectrum analyzers, vector signal analyzers, and the like, receive an RF input signal and down convert the RF signal to an intermediate frequency (IF) signal in a superheterodyne process to allow measurements to be made on the signal. The RF carrier signal being measured generally contain modulation information which is measured by the measurement receiver. Some measurement receiver, such as the 2715 Spectrum Analyzer, manufactured by Tektronix, Inc., include specialized circuitry for making measurements on particular types of signals. The 2715 Spectrum Analyzer includes circuitry for making automated cable television measurements on NTSC signals, such as in-service measurements for in-channel carrier-to-noise ratio, composite triple beats, and the like.
Receivers designed for receiving broadcast signal, such as radio and television signals, include front end down converter circuitry and a demodulator for extracting the modulated information from the broadcast signal. Such demodulators are designed to remove or suppress signal distortions, such as noise and the like, that are produced during the generation, transmission, and reception of the broadcast. However, signal distortions affect the quality of the broadcast signal and measuring these distortions are of interest to broadcast engineers. This is especially true for the new digital television standard adapted by the Federal Communications Commission.
The Digital Television Standard was developed by the Advanced Television Systems Committee (ATSC) to transmit high quality video, audio and ancillary data over a 6 MHZ channel. The Standard describes the channel coding and modulation RF/transmission subsystems for terrestrial and cable applications. The modulation subsystem uses a digital data stream to modulate the transmitted signal and may be implemented in two modes: a terrestrial broadcast mode (8-VSB) delivering about 19 Mbps, and a higher data rate mode (16-VSB) delivering about 38 Mbps for cable television systems where higher signal to noise is ensured.
The modulation technique implemented in the Digital Television Standard was developed by Zenith Corp. and employs vestigial sideband modulation. The overall system response of the transmitter and receiver filtering corresponds to a raised cosine filter to avoid system generated intersymbol interference. The system response is implemented with serially coupled, nominally identical root raised cosine filters in the transmitter and in the receiver.
The incoming digital data stream is randomized, forward-error-correction (FEC) encoded and interleaved. The randomized, FEC coded and interleaved data is trellis encoded as an 8-level (3-bit) one dimensional constellation. The outputs of the trellis coder are mapped into symbols that are one of eight symmetric odd-valued integer levels from −7 to +7 units. To aid synchronization in low signal to noise and/or high multipath situations, segment and field syncs are inserted in the 10.76 Msymbols/sec symbol stream. A small pilot tone is added as well at the carrier frequency generated by offsetting the real or I channel of the complex signal containing the data and the sync pulses by 1.25 units. The offset causes the pilot tone to be in-phase with the I channel signal component. At the transmitter, the composite signal passes through a root raised cosine filter and modulates an intermediate frequency carrier signal which is up-converted to an RF frequency for transmission at the desired channel frequency. Alternately, the composite signal may be used to directly modulate the RF carrier.
The Hewlett-Packard HP 89441A Vector Signal Analyzer is a general purpose measurement instrument having specialized filters and processes for making measurements on a number of RF modulated signals, such as 8-VSB signals, IS-95 wireless communication signals and the like. The HP 89441A includes a superheterodyne receiver having a first LO and mixer for up-converting the incoming signal to a first IF frequency. Second and third LOs and mixers respectively generate second and third IF frequencies of 40 MHz and 10 MHz. The 10 MHz IF is digitized by an analog-to-digital converter with the digitized data being down converted to baseband data. The baseband data values are passed to a digital signal processor for FFT conversion and additional signal processing. The 89441A includes a user interface for selecting the appropriate filter for the signal being measured including no filter at all. For an 8-VSB signal, a root-raise cosine transmission system receiver filter is applied to the 8-VSB signal. One of the measurements displayed by the 89441A is a constellation display of the 8-VSB symbols in I and Q space. It has been observed that removing the root-raised cosine filter before the 8-VSB constellation measurement results in no constellation output display. From this observation and the fact that the instrument does not provide nonlinearity measurement outputs, is it assumed that only filtered 8-VSB measurements can be made with this instrument.
The received signal in transmission systems, such as the 8-VSB digital television system, generally contain distortions caused by the system's transmitter and receiver and the medium which through the signal travels. The existence of these distortions can severely degrade the signal quality of a digitally transmitted signal. Often the distortions present in the received signal are a mixture of linear and nonlinear magnitude errors, linear and nonlinear phase error, additive noise, and phase noise. To monitor the quality of the transmitted signal and to trouble-shoot a degraded transmission system, accurate measurements of these distortions are very useful. However, traditional demodulators suppress some of these distortions, in part, by applying the transmission system receiver filter to the incoming signal. Since linear and nonlinear transfer functions are not usually commutative, meaning that the nonlinear functions observed from the demodulated baseband signal are different from the original nonlinear functions produced by the power amplifier at the transmitter, it is difficult to derive the original nonlinear transfer function from what is observed from the demodulated baseband signal for performing, for example, transmitter nonlinearity measurements, especially with a randomized digital signal. Also, strong nonlinearity causes signal spectrum spreading. The transmission system's receiver filter could significantly attenuate the out-of-band portion of the spread spectrum signal with the loss of spectral information characterized by the nonlinear distortions.
What is needed is a demodulator for a measurement receiver that measures various type of distortions generated in a digital transmission system. Such a demodulator needs to produce multiple types of outputs for measuring linear and nonlinear type of distortions, as well as carrier phase jitter. The demodulator needs to be efficiently implemented and flexible in design for easy modification for demodulating different types of signal.
SUMMARY OF THE INVENTION
Accordingly, an object of the present invention is a demodulator that produces filtered and unfiltered signal samples where the filtered signal samples are processed through a transmission system receiver filter and the unfiltered signal samples are not.
An additional object of the present invention is a demodulator that produces synchronization parameters, a scaling factor and filter coefficients using transmission receiver filter signal samples in a first processing channel for use in producing unfiltered signal samples in a second processing channel.
A measurement receiver receives a radio frequency signal modulated with digital symbols at a symbol rate and generated by a transmission system having a transmitter filter and a receiver filter. The receiver down converts the modulated radio frequency sig
Bernard Kyle L.
Chen Xiaofen
Deshpande Nikhil M.
Kuntz Thomas L.
Bucher William K.
Chin Stephen
Lugo David B.
Tektronix Inc.
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