Pulse or digital communications – Receivers – Particular pulse demodulator or detector
Reissue Patent
1993-08-06
2001-04-17
Ton, Dang (Department: 2732)
Pulse or digital communications
Receivers
Particular pulse demodulator or detector
C327S351000, C375S349000
Reissue Patent
active
RE037138
ABSTRACT:
TECHNICAL FIELD
The invention relates to an improved method and arrangement of apparatus for digitalizing and subsequently processing numerically radio signals in those instances when the levels of said signal can vary over a wide dynamic range and where the level values cannot be readily determined beforehand with the aid of sampling techniques.
BACKGROUND ART
It is always possible to represent an arbitrary radio signal as a sequence of composite (complex) vectors. The real and imaginary parts of the vector sequence correspond to bipolar amplitude modulation (double side band suppressed carrier AM) of a cosinusoidal and sinuosidal carrier wave respectively (quadrature carriers). When wishing to process a radio signal numerically using digital arithmetic implemented in either specific hardware logic or in software on a computer, microprocessor or some other programmable apparatus, it is first necessary to convert the signal in to numerical form with the aid of a A/D-converter (Analogue to Digital converter).
One common method of achieving this is to first resolve the radio signal into its real and imaginary complex vector part, by correlation with locally generated cosine and sine waves in two balanced mixers, and then to digitalize the two results by means of A/D-conversion. Sometimes there is used a variation of this technique, in which the radio signal is sampled pairwise, separated by one quarter period of its centre frequency. This so-called quadrature sampling technique combines the functions of sampling and A/D-conversion with resolution in real and imaginary parts.
DISCLOSURE OF INVENTION
The aforesaid, known solutions have practical limitations with respect to the possibilities of handling the dynamic ranges of the signals. Despite the absence of an input signal, the arrangement used in accordance with the first method, in which balanced mixers are used as correlators, does not necessarily produce a zero (0) volts, output signal. The output signal will typically have a D.C. off-set of some few millivolts or some tens of millivolts. At the same time, the acceptable, maximum signal level of the available supply voltage is limited to, for instance, +2.5 volts or, in the case of diode-ring mixers, perhaps to a still lower level of, for instance, +250 mV. The dynamic range for which the signal is, on one hand, much higher than the D.C. offset (mixer imbalance) and, on the other hand, lower than the saturation level, may be as small as 20 dB (decibel). This then requires the introduction of some form of automatic amplification control, in order to maintain the signal level of the mixer in the optimum range. In the case of a receiver which must necessarily accept random transmission of data in the form of bursts from different transmitters, it is not possible, however, to predict the level of amplification required, when applying this method.
A further drawback, applicable to both of the aforesaid methods, resides in limited resolution during the A/D-conversion process. Assume that an A/D-converter is able to represent the whole of the signal level range. Further assume that the highest signal level may be equal to the supply voltage, e.g. 5 volts. An LSB-bit (Least Significant Bit) then corresponds to 5/256 volts, i.e. approximately 20 millivolt. Consequently, a signal beneath 20 mV will remain totally undiscovered, while a signal of 320 mV will only be digitalized to a resolution of 4 bits, which is perhaps insufficient for subsequent signal processing. If a 4 bit resolution is nevertheless acceptable, the range in which the signals can be processed will be 16:1 or 24 dB, which is a very poor dynamic range in the case of radio applications.
Radar receivers are typical examples of systems in which it is impractical to use automatic amplification control for the purpose of maintaining the receiver output within narrow limits, this impracticability being due to a number of unknown parameters, for instance such parameters as the distance to the reflecting object, the size of said object and the duration of the pulse. Because of this a radar receiver will normally operate with a chain of intermediate frequency amplifiers known as “logarithmic amplifiers”. Such an arrangement comprises a plurality of sequentially saturating, cascade-connected amplifiers each being provided with an amplitude detector rectifier) whose output signals are intended to be added together. The arrangement functions in the following manner: In the case of the weakest input signal levels, it is solely the detector which is located at the end of the chain which will receive a signal whose level of amplification is sufficient for the detector itself to produce an output signal. This ability increases with increasing input signal levels, until the amplifying stage concerned is saturated. At this stage, and with correct selection of amplification for each amplifying stage, the preceding amplifying stage in the chain will begin to receive a signal which is sufficiently strong for detection purposes and therewith takes over the contribution to the output signal. For each X dB increase in input signal level, where X is 20 log 10 of the voltage amplification in each stage, the saturation point is moved rearwardly one stage in the chain, the net detected output signal therewith increasing by one unit. The net detected output signal is thus followed by an approximately rectilinear relationship with the logarithm on the input signal level. The dynamic range for which this coincides is limited solely by the number of amplifying stages and the thermal noise. The method of digitalizing the detected output signal for subsequent numeric processing of the signal in an arrangement according to the aforegoing is insufficient when handling arbitrary radio signals, since the complex vector nature of the arbitrary radio signal will be lost in such a sequential detecting process.
The method and arrangement solving said problems are characterized by the patent claims and involve the introduction of a further digitalizing process which operates on the saturated output of the last amplifying stage in an amplifier chain in accordance with the aforegoing, extracting the vector information which otherwise would be lost. This procedure is followed by a multiple of numeric operations on the two digital quantities, in order to restore the complete vector characteristic of the signal. This can be effected with the aid of hardware logic or with programmable digital signal processors (microprocessors). The inventive digitalizing arrangement, intended for processing composite signals having a large dynamic range, thus includes a logarithmic amplifying chain similar to the kind used in radar receivers and in which the detected output signal from the amplifier is digitalized in a first A/D-converter, whereafter a second A/D-converter digitalizes the angle or phase information of the signal. The phase information is retained by utilizing a carefully configured chain of saturating amplifiers, and is available on the saturated output of the last amplifier stage, at which point the signal has obtained a constant level and all variations in amplitude have therewith been eliminated. The exact method in which phase information is extracted in the form of a numeric quantity is not an objective of the present invention and
will therefore not be
only a representative example will be
described in this document.
The advantages afforded by the inventive method and inventive arrangement reside in the solution of a troublesome problem within the field of radio communications, in a technically uncomplicated manner, therewith achieving high precision at low costs.
REFERENCES:
patent: 3496298 (1970-02-01), Crookshanks et al.
patent: 3668535 (1972-06-01), Lansdowne
patent: 3849595 (1974-11-01), Ishiguro
patent: 4492930 (1985-01-01), Knowles et al.
patent: 5001776 (1991-03-01), Clark
patent: 07178146 (1985-07-01), None
The article by W. L. Barber et al., “A True Logarithmic Amplifier for Radar IF Applications”,IEEE Journal of Solid-State Circuit
Burns Doane Swecker & Mathis L.L.P.
Telefonaktiebolaget LM Ericsson
Ton Dang
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