Pulse or digital communications – Systems using alternating or pulsating current – Plural channels for transmission of a single pulse train
Reexamination Certificate
2000-03-20
2002-03-05
Chin, Stephen (Department: 2734)
Pulse or digital communications
Systems using alternating or pulsating current
Plural channels for transmission of a single pulse train
C375S340000, C329S323000
Reexamination Certificate
active
06353639
ABSTRACT:
BACKGROUND OF THE INVENTION
FIELD OF THE INVENTION
The present invention relates to a filter configuration for quadrature amplitude modulated (QAM) signals. The filter configuration can be used in particular but not exclusively in a frequency range equalizer for such signals. The invention also relates to a slope detector for detecting a slope of the power spectrum of the QAM signal.
Quadrature amplitude modulation is employed particularly in the color television industry for modulation of two mutually independent signals, such as a luminance signal and a color difference signal, to a common carrier oscillation by modulation of its amplitude and phase. For a demodulation of the QAM signal back to the base band range, the QAM signal is multiplied by a (cosine) oscillation in phase with the original carrier and, respectively, by a (sine) oscillation phase-shifted by 90° and is low-pass filtered, as a result of which two signal components are obtained. These signal components will be distinguished hereinafter by the designations I and Q.
Filter configurations for QAM signals modulated in this way generally include two largely identically configured channels for the two components of the signal. For instance from the article “Architecture and Circuit Design of a 6 GOPS Signal Processor for QAM Demodulator Applications” by De Man et al., IEEE JSSC, Vol. 30, No. 3, March 1995, a filter configuration is known that has a first channel for a cosine demodulated component of the QAM signal and a second channel for a sine demodulated component of the QAM signal. The filter configuration also has a filter circuit, which receives the two signal components and for each signal component has one transfer function that is composed of terms that are in phase with this signal component and terms that are phase-shifted by &pgr;/2 and/or −&pgr;/2 with respect to it. The filter configuration further includes a cross branch for picking up signal components from the respectively other channel, wherein the signal components correspond to the phase-shifted terms of the transfer function. The cross branch is provided twice and each respective cross branch is fixedly assigned to one of the two channels.
Another filter configuration for demodulated QAM signals, also known from the above-cited article by de Man et al., is a slope detector which receives the two components of a QAM signal. The slope detector has a differential stage, which forms the difference between a received data value and the most recently received previous data value of the same component. The slope detector further has a multiplication stage, which forms the product of a difference value, derived from the first component, with a difference value derived from the second component. Separate channels for the two components are provided, and each channel includes one difference stage and one multiplication stage.
This doubling of circuit elements is complicated and expensive, and for an integrated embodiment of the filter configuration it requires a considerable substrate surface area.
SUMMARY OF THE INVENTION
It is accordingly an object of the invention to provide a filter configuration, such as a frequency range equalizer or a slope detector, for demodulated QAM signals which overcomes the above-mentioned disadvantages of the heretofore-known configurations of this general type and which substantially avoids the above-mentioned double expense.
With the foregoing and other objects in view there is provided, in accordance with the invention, a filter configuration for a demodulated QAM signal, including:
a first channel for a cosine demodulated QAM signal component;
a second channel for a sine demodulated QAM signal component;
a filter circuit for receiving the signal components, the filter circuit having transfer functions for the signal components, the transfer functions including terms in phase with the signal component of a respective one of the first an second channels and including terms phase-shifted by at least one of &pgr;/2 and −&pgr;/2 with respect to the signal component of the respective one of the first an second channels;
the filter circuit including a cross branch having an output and an input for picking up signal portions from a respective other one of the first and second channels, the signal portions corresponding to the terms of the transfer functions phase-shifted by at least one of &pgr;/2 and −&pgr;/2; and a switch configuration having a first state and a second state, the switch configuration, when being in the first state, connecting the input of the cross branch to the first channel and the output of the cross branch to the second channel, the switch configuration, when being in the second state, connecting the input of the cross branch to the second channel and the output of the cross branch to the first channel.
In the filter configuration described initially above, a first way of attaining the object of the invention is to provide a circuit configuration which in a first state connects the input of the cross branch to the first channel and the output of the cross branch to the second channel, and in a second state connects the input of the cross branch to the second channel and the output of the cross branch to the first channel. This provision makes it possible to assign the cross branch to the first and, respectively, to the second channel in alternation, so that only one cross branch is now required.
Preferably, the circuit configuration includes in each channel a switch, which passes arriving data to the channel or to the cross branch in alternation. As a result, data values of one component, which are needed only for generating phase-shifted terms in the respectively other channel, are suppressed in the channel at which they arrive. Thus the affected channel is relieved of processing tasks whose results are not needed later anyway.
The cross branch preferably includes a multiplier for multiplication by a weighting factor a. Particularly if the filter configuration of the invention is meant for use as a frequency range equalizer, transfer functions of the form
S
I
(z)=ia+z
−T/2
−iaz
−T
and
S
Q
(z)=−ia+z
−T/2
+iaz
−T
for the channels I and Q, respectively, can be implemented in it. T designates the clock period of the data of the demodulated signal components. Here, one multiplier suffices. The value of the factor a is expediently adjustable as a function of the distortion of the signal spectrum, and this distortion is measured by a slope detector. The cross branch also preferably includes a register for delaying a data value by the time T.
Each channel can be assigned one adder, and the circuit configuration delivers the output signal of the cross branch in alternation to one of the two adders for addition to the signal transmitted over the applicable channel.
A further simplification is obtained if instead of the two adders, each assigned to one channel, only one adder is provided, having a first input that is connected in alternation by a switch to the first and second channel, respectively, and having a second input that is connected to the output of the cross branch.
In accordance with another feature of the invention, the cross branch includes at least one delay register.
In accordance with yet another feature of the invention, the phase-shifted terms of one of the transfer functions are of equal magnitude and opposite to the phase-shifted terms of another one of the transfer functions.
In accordance with a further feature of the invention, a first adder assigned to the first channel and a second adder assigned to the second channel are provided. The first adder connects the output of the cross branch to the first channel, the second adder connects the output of the cross branch to the second channel.
In accordance with another feature of the invention, the first and second adders add signal portions transmitted via the cross branch with respectively different signs to respective ones of the signal components transmit
De Man Erik
Geib Heribert
Prange Stefan
Al-Beshrawi Tony
Chin Stephen
Infineon - Technologies AG
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