Pulse or digital communications – Receivers
Reexamination Certificate
2000-06-30
2004-05-18
Bocure, Tesfaldet (Department: 2631)
Pulse or digital communications
Receivers
C375S324000, C329S300000
Reexamination Certificate
active
06738433
ABSTRACT:
BACKGROUND OF THE INVENTION
Field of the Invention
The invention lies in the electronics field. More specifically the invention relates to a quadricorrelator for a demodulator for frequency-modulated signals. The system is of the following type:
a first input is provided for receiving an in-phase component of a signal to be demodulated, and a second input for receiving a quadrature component of the signal to be demodulated;
the in-phase component and the quadrature component of the signal to be demodulated can be produced in a mixing stage that is connected upstream of the quadricorrelator;
the first input of the quadricorrelator is followed by a first multiplier unit, and the second input is followed by a second multiplier unit, and the output of the first multiplier unit is connected with a positive sign to a first input of an output adder unit, and the output of the second multiplier unit is connected with a negative sign to a second input of the output adder unit; and
the in-phase component which is supplied to the first input of the quadricorrelator is supplied with a first transfer function applied to it to the second multiplier unit, and the quadrature component is supplied with a second transfer function applied to it to the first multiplier unit, in each case as an input signal.
When frequency-modulation methods are used in transmission systems, such as DECT (Digital European Cordless Telecommunications standard), the signal which is modulated onto the carrier must be demodulated in the receiver chain after amplification and channel selection have been carried out. When a so-called low-IF structure (low intermediate-frequency structure) is used, the demodulation is normally carried out by means of a quadricorrelator whose demodulator characteristic is symmetrical about the frequency f=0. Such a quadricorrelator and its use in a demodulator are described, for example, in IEEE Trans. Comm. Volume COM-33, February 1985, pages 131-138. A frequency-modulated input signal is demodulated, in the structure used according to the published article, in such a way that the input signal is first of all split into two paths, with the signal in the one path being mixed with a signal in the form of a cosine wave at a specific frequency, and the signal in the other path being mixed with a signal in the form of a sine wave at the same frequency. The components of the spectrum of the mixed signal above the difference frequency are suppressed by means of a low-pass filter provided in each path. That signal path in which the input signal has been mixed with the signal in the form of a cosine wave then carries the in-phase component of the input signal, which is supplied to the first input of the actual quadricorrelator. In a corresponding way, the other signal path, in which the input signal has been mixed with the signal in the form of a sine wave, carries the quadrature component of the input signal, which is then supplied to the respective second input of the actual quadricorrelator.
The actual quadricorrelator then comprises a signal structure in which the in-phase component is differentiated in a straight-through path and is supplied to a multiplier. The quadrature component is likewise differentiated and is supplied to a further multiplier. The in-phase component is likewise supplied as an input signal to the further multiplier and the quadrature component is supplied as an input signal to the first multiplier. The output signal from the further multiplier, finally, is subtracted from the output signal from the first multiplier in order to obtain the output signal from the quadricorrelator, which corresponds to the demodulated signal.
The disadvantages of that implementation of the quadri-correlator include, firstly, the fact that the in-phase component and the quadrature component of the intermediate frequency obtained after the mixing of the input signal are not equal to zero and, thus, since the demodulator characteristic is symmetrical about the frequency f=0, the output signal has a constant offset superimposed on it. The offset is then normally removed in subsequent offset correction in order to compensate the influence of the intermediate frequency on such a demodulator characteristic during the demodulation process. This involves additional complexity.
Furthermore, the prior art configuration of a quadricorrelator has the disadvantage that, as a result of the differentiation of the in-phase component and the quadrature component in the straight-through paths through the quadricorrelator, which corresponds to multiplication of the relevant component by the transfer function of a high-pass filter in the frequency domain, the quadricorrelator has a wide noise bandwidth. With such a structure, the noise is filtered out only at very high frequencies, by virtue of the parasitic capacitances which are always present. Overall, this results in an adverse effect on the sensitivity of the quadricorrelator.
Finally, the property of the demodulator characteristic being symmetrical about the frequency f=0 results in the disadvantage that the linearity of the symmetrical demodulator characteristic must be maintained over a very wide bandwidth region about the mid-frequency f=0. This necessarily means that the demodulator characteristic must have a relatively low gradient, and thus low demodulation sensitivity as well.
The article “Equalization Problems on a Digital FM Receiver” by K. D. Kammeyer in Signal Processing 9(1985), pages 263-76 describes a digital receiver for frequency-modulated signals, which has digital equalizer networks and is suitable for use in FM sound radio receivers.
The article “Phaselock Techniques” by F. M. Gardner, John Wiley & Sons, New York 1979, pages 84-87 describes a quadri-correlator which operates as a difference-frequency detector.
SUMMARY OF THE INVENTION
The object of the invention is to provide a quadricorrelator for a demodulator for frequency-modulated signals which overcomes the above-noted deficiencies and disadvantages of the prior art devices and methods of this general kind, and which provides for a demodulator with better sensitivity.
With the above and other objects in view there is provided, in accordance with the invention, a quadricorrelator for a demodulator for frequency-modulated signals, comprising:
a first input for receiving an in-phase component of a signal to be demodulated, and a second input for receiving a quadrature component of the signal to be demodulated;
a first multiplier unit connected to the first input and having an output, and a second multiplier unit connected to the second input and having an output, an output adder unit having a first input connected with a positive sign to the output of the first multiplier unit, a second input connected with a negative sign to the output of the second multiplier unit, and an output forming an output of the quadricorrelator;
the second multiplier unit receiving as an input signal the in-phase component from the first input via a first transfer function, and the first multiplier unit receiving as an input signal the quadrature component from the second input via a second transfer function;
the first and second transfer functions being chosen such that a demodulator characteristic formed between the input of the quadricorrelator and the output of the output adder unit cuts a frequency axis at a point where the frequency is not zero and runs essentially linearly in a sufficiently wide frequency band surrounding or adjacent to the intersection point; and
the first transfer function comprising a first transfer function element applied to the in-phase component, and a second transfer function element applied to an input signal to the first multiplier unit, wherein the signals weighted by the first transfer function element and the second transfer function element are added to generate an added signal forming the input signal to the second multiplier unit; and
the second transfer function comprising a first transfer function element applied to the quadrature compo
Puma Guiseppe Li
Van Waasen Stefan
Bocure Tesfaldet
Ghulamali Qutbuddin
Greenberg Laurence A.
Infineon - Technologies AG
Locher Ralph E.
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