Coded data generation or conversion – Analog to or from digital conversion – Analog to digital conversion
Reexamination Certificate
2000-06-13
2001-09-04
Young, Brian (Department: 2819)
Coded data generation or conversion
Analog to or from digital conversion
Analog to digital conversion
C341S111000
Reexamination Certificate
active
06285307
ABSTRACT:
FIELD OF THE INVENTION
The present invention generally relates to signal processing, in particular signal sampling, signal discretization and analog-to-digital conversion in communication devices, such as radio communication devices.
BACKGROUND OF THE INVENTION
Sampling of an analog electric signal consists of both time discretization and amplitude discretization of a time-invariant signal, i.e. a voltage or current.
A simple representation of a sampling operation is a switch, which at a certain rate, i.e. the sampling rate, permits transfer of the analog signal during a short time relative to the sampling period. The thus sampled analog signal than consists of short segments or sample values of the analog signal having an amplitude which corresponds to the amplitude of the analog signal at the time of sampling.
The sampling operation can be represented as a multiplication of the analog signal and a periodic pulse train, the period of which is the sampling rate.
It can be shown that such multiplication operation, in the frequency domain, results in a spectrum of signals comprising the spectrum of the sampled analog signal and a periodic repetition of the spectrum with a spacing determined by the sampling rate and having a decreasing spectral power. The attenuation is determined by the sampling process, i.e. the duration and the amplitude of the pulses of the periodic pulse train and the sampling rate.
The minimum sampling rate at which no overlap of the repeated analog signal is encountered, is called the nominal Nyquist rate. That is, the minimum sampling frequency has to be at least twice the upper frequency of a frequency band limited signal.
A more elaborated discussion on sampling can be found in many textbooks dealing with electric communication such as “Communication Systems”, by A. Bruce Carlson, McGraw-Hill 1975, second edition Chapter 8.
In practice, real analog signals are not strictly band limited, such that whenever frequencies occur in the analog signal which are undersampled, that is the Nyquist criterion is not met for such frequencies, a phenomenon occurs which is called aliasing. With aliasing spectral overlap of repeated spectra occurs, such that after reconstruction of the sampled signal, frequencies which were originally outside the frequency band of the sampled signal will now appear at the reconstructed output in the form of lower frequencies, distorting the reconstructed sampled analog signal.
Aliasing is combatted by removing or filtering as much as possible frequency components above the frequency band of the signal to be sampled before the actual sampling thereof and, if possible, sampling at a much higher rate than the nominal Nyquist rate. Such type of filtering is also called anti-aliasing filtering. Reference is made to European patent application 0,335,037.
In practice, there is a compromise between the order of the filter, i.e. a Low Pass (LP) or a Band Pass (BP) filter and the sample rate which may relate to the conversion speed of an Analog-to-Digital Converter (ADC) used, and thus its power consumption.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a novel method of sampling an analog signal having an improved rejection of spurious frequencies in case of non-strictly bandlimited analog signals.
It is a further object of the present invention to relieve the requirements put on aliasing filters used in the sampling of non-bandlimited analog signals.
It is in particular an object of the present invention to provide a method of sampling for use with image rejection signal processing circuitry, such as used in modern transceiver design.
It is also an object of the present invention to provide a device wherein the method according to the present invention is put into operation.
According to the present invention there is provided a method of sampling an analog signal, comprising the steps of:
a) obtaining In-phase I and Quadrature-phase Q components of the analog signal, the method being characterized by the steps of:
b) obtaining first sequences of discrete sample values of the I and Q components and second sequences of discrete sample values of the and Q components, these first and second sequences being obtained with equal sampling rate and a mutual phase shift of 90° relative to their sampling rate period;
c) summing sample values of the I component of the first sequence and sample values of the Q component of the second sequence, forming a third sequence of sample values;
d) effectively subtracting sample values of the I component of the second sequence from sample values of the Q component of the first sequence, forming a fourth sequence of sample values, and
e) repeating steps c) and d) for each sampling rate period.
In the method according to the invention, sequences of discrete sample values of the I and Q components of an analog input signal are obtained, either a baseband signal or a carrier modulated signal. The requirement that both the I and Q components of an analog signal have to be available matches with the image rejection techniques used in circuitry for frequency conversion in transceiver devices.
According to the invention, two sequences of discrete sample values of the I and Q components are obtained with a mutual phase shift of 90°, which may be a phase lead or a phase lag of one sequence with respect to the other.
Selective summation and subtraction of sample values of the I and Q components of both sequences leads to the cancellation of frequency components external to the frequency band of the desired analog signal.
Due to the cancellation or suppression effect according to the present invention, the anti-aliasing filter requirements are considerably relieved, leading to a less higher filter order or, with the same filter as used in conventional AD conversion, to a much less distorted reconstructed output signal. Those skilled in the art will appreciate that relieving the filter requirements is very advantageously from a cost point of view and enhances the possibility of integration of sampling devices on a semiconductor substrate.
The sampled I and Q components obtained according to the method of the present invention may be further processed, among others, by subjecting both the I and Q output components to an ADC operation, such to provide a digital equivalent, for example an 8 or 12 bit representation of the sampled analog signal for further processing thereof in the digital domain.
An analog signal sampling device according to the present invention comprises means for converting an analog signal into In-phase I and Quadrature-phase Q components, characterized by means for converting the I and Q components at a sampling rate into first and second sequences of discrete sample values such that the first and second sequences comprise a mutual phase-shift of 90° relative to their sampling rate period, means for summation of sample values of the I component of the first sequence and sample values of the Q component of the second sequence, outputting a third sequence of sample values, means for summation of negative sample values of the I component of the second sequence and sample values of the Q component of the first sequence, outputting a fourth sequence of sample values, wherein the summation means are arranged to operate at each sampling rate period.
In a preferred embodiment of the device according to the present invention, designed as an ADC, the processing means comprise analog-to-digital converter means, such to provide digital I and Q output signals of the analog input signal, for example.
Instead of subjecting the sampled output components to an ADC operation, each sample obtained may be first AD converted after which the summation and further processing can be accomplished in the digital domain.
The above-mentioned and other features, advantages and applications of the invention are illustrated in the following detailed description with reference to the enclosed drawings.
REFERENCES:
patent: 5099243 (1992-03-01), Tsui et al.
patent: 5631646 (1997-05-01), Erhage
patent: 5877713
Burns Doane Swecker & Mathis L.L.P.
Telefonaktiebolaget LM Ericsson (publ)
Young Brian
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