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Circuit and method for detecting ac voltage pulsesRelated Patent Categories: Telephonic Communications, Subscriber Line Or Transmission Line InterfaceCircuit and method for detecting ac voltage pulses description/claimsThe Patent Description & Claims data below is from USPTO Patent Application 20060210055, Circuit and method for detecting ac voltage pulses. Brief Patent Description - Full Patent Description - Patent Application Claims
PRIORITY INFORMATION [0001] This application is a Continuation of U.S. Application Ser. No. 10/431,901, filed May 8, 2003, now published as U.S. Patent Application Publication No. 2003/0207677 A1 on Nov. 6, 2003, which application and publication are incorporated herein by reference. BACKGROUND Field of the Invention [0002] The invention relates to a circuit for detecting AC voltage pulses at a defined frequency, to a telephone having such a circuit, and to a method for identifying AC voltage pulses at a defined frequency. [0003] In the European analog telephone network, charge unit pulses are transmitted to the subscriber terminals, in order to allow the accumulating charges to be detected at the subscriber end. The charge unit pulses are short AC voltage pulses at a frequency of 16 kHz, or in some countries at about 12 kHz. These charge pulses must be identified and counted in the subscriber device. [0004] Originally, the charge unit signals were output from the reception path by a bandpass filter, and were supplied to a mechanical meter, which was incremented by one step for each pulse. More recent solutions provide for the charge unit signals to be output from the reception path which is provided for audio signal processing, and to be filtered by a bandpass filter. The charge unit signals are then converted by a comparator to square-wave signals, whose period duration is determined digitally by a counter. If the digitally detected length of the charge pulse is within a predefined tolerance band, a charge pulse is registered. [0005] This solution has the disadvantage of requiring a relatively large number of discrete components for outputting and bandpass filtering the charge unit signals. Discrete components are expensive and impede further miniaturization of the subscriber terminals. SUMMARY OF THE INVENTION [0006] It is accordingly an object of the invention to provide a circuit for detecting AC voltage pulses at a defined frequency, a telephone having such a circuit, and a method for identifying AC voltage pulses at a defined frequency, which overcome the above-mentioned disadvantages of the prior art apparatus and methods of this general type. [0007] In particular, it is an object of the invention to further reduce the number of discrete components required to detect AC voltage pulses at a defined frequency. [0008] With the foregoing and other objects in view there is provided, in accordance with the invention, a circuit for detecting AC voltage pulses at a defined frequency. The circuit includes: an analog/digital converter unit for converting an input signal to a sequence of sample values; at least one mixing stage for multiplying the sequence of sample values by a mixing frequency signal; at least one filter stage for defining a permissible intermediate frequency band and for obtaining converted sample values by filtering the sequence of sample values having been multiplied by the mixing signal; and an evaluation unit for determining whether an AC voltage pulse is present by using the converted sample values. [0009] The detection circuit has an analog/digital converter unit for converting the input signal to a sequence of sample values. The sequence of sample values is multiplied by a mixing frequency signal using at least one mixing stage. Furthermore, the detection circuit has at least one filter stage for defining the permissible intermediate frequency band. The converted sample values are used by an evaluation unit to determine whether an AC voltage pulse is present. [0010] In contrast to the situation in the prior art, the incoming AC voltage pulse is first of all digitized; the rest of the processing is then carried out in digital form. Both the mixing stage or the mixing stages and the filter stage or the filter stages are in the form of digital circuits or a digital signal processor. [0011] The superheterodyne principle is used in order to allow frequency-specific detection of AC voltage pulses. This is done by multiplying the sample values of the input signal by a mixing frequency signal at the frequency .omega..sub.M. An AC voltage pulse at the frequency .omega..sub.0 is thus on the one hand up-mixed to the frequency .omega..sub.0+.omega..sub.M, and on the other hand is down-mixed to the frequency .omega..sub.0-.omega..sub.M. A downstream filter stage defines the permissible intermediate frequency band and thus the tolerance band for the frequency of the AC voltage pulse. For example, the up-mixed component .omega..sub.0+.omega..sub.M can be suppressed by a low-pass filter. [0012] A low-pass filter can also be used to define the permissible intermediate frequency band for the down-mixed component .omega..sub.0-.omega..sub.M, because |.omega..sub.0-.omega..sub.M|.ltoreq..omega..sub.G and thus .omega..sub.M-.omega..sub.G.ltoreq..omega..sub.0.ltoreq..omega..sub.M+.om- ega..sub.G at a cut-off frequency .omega..sub.G. Instead of a low-pass filter, a filter with a different characteristic, for example, a bandpass filter, may also be used to define the permissible intermediate frequency band. [0013] The AC voltage signal to be detected is thus first of all down-mixed to the intermediate frequency band, and the frequency band within which AC voltage pulses are intended to be detected is then defined by limiting the intermediate frequency band. The evaluation unit then uses the converted sample values to determine whether an AC voltage pulse has occurred in the frequency window defined in this way. [0014] The use of the superheterodyne principle for detecting AC voltage pulses at a defined frequency means that there is no need for filter arrangements with discrete components. The signal processing is carried out completely digitally and can be carried out by digital signal processors, thus allowing the evaluation circuit to be implemented in a space-saving and cost-effective manner. [0015] According to one advantageous embodiment of the invention, the circuit for detecting AC voltage pulses has at least one decimation stage, which reduces the signal rate by omitting sample values. Decimation stages such as these allow the sampling rate for the sample values to be reduced in steps; the actual signal processing can then be carried out at a lower sampling frequency. More processing time is thus available for signal processing for each sample value, and the requirements for the performance of the digital signal processor or of the digital evaluation circuit can therefore be made less stringent. A further advantage of a low processing frequency is that the power consumption of the DSP or of the evaluation circuit is reduced. [0016] A further advantageous embodiment of the invention provides for the mixing stage or the mixing stages each to have a quadrature signal path and an in-phase signal path. The sequence of sample values is multiplied by the sine signal at the mixing frequency in the quadrature signal path, and the sequence of sample values is multiplied by the cosine signal at the mixing frequency in the in-phase signal path. [0017] The cosine signal of the mixing frequency, which is used in the in-phase signal path, is phase-shifted through 90.degree. relative to the sine signal which is used in the quadrature signal path. An AC voltage signal which is applied to the input of the evaluation circuit can thus be processed irrespective of its phase angle relative to the mixing frequency signal. Either the quadrature signal path or the in-phase signal path produces the stronger signal, depending on the phase angle of the input signal. [0018] In this case, it is particularly advantageous for the multiplication by the sine signal at the mixing frequency to be carried out by multiplying the sequence of sample values by the sequence 0, 1, 0, -1, 0. . . . This value sequence corresponds to the profile of the sine signal at the mixing frequency in the situation where four times the mixing frequency is chosen as the sampling frequency. If the mixing frequency is 16 kHz, the sampling frequency must therefore be chosen to be 64 kHz. The multiplication of the sequence of sample values by the sequence 0, 1, 0, -1, 0 . . . can be carried out in a simple manner by every alternate sample value being set to zero and every fourth sample value being inverted. The multiplication of the sequence of sample values by the sequence 0, 1, 0, -1, 0 . . . can be linked in a simple manner to a decimation step, by first of all omitting every alternate sample value. Every alternate sample value of the remaining sample values is then inverted. [0019] In a corresponding manner the multiplication by the cosine signal at the mixing frequency can be carried out by multiplying the sequence of sample values by the sequence 1, 0, -1, 0, 1, . . . , if the sampling frequency is four times as great as the mixing frequency. This takes account of the 90.degree. phase shift between the sine signal and the cosine signal. [0020] It is advantageous for the evaluation unit to in each case square, add and compare with a threshold value, the converted sample values in the quadrature signal path and in the in-phase signal path. Once the AC voltage pulse has passed through the various mixing and filter stages, the square of the magnitude of the signal can be obtained by squaring and adding the converted sample values from the quadrature signal path and from the in-phase signal path. The splitting of the signal path into a quadrature signal path and an in-phase signal path always results in the same square of the magnitude irrespective of the phase angle of the input signal relative to the mixing signal. Continue reading about Circuit and method for detecting ac voltage pulses... Full patent description for Circuit and method for detecting ac voltage pulses Brief Patent Description - Full Patent Description - Patent Application Claims Click on the above for other options relating to this Circuit and method for detecting ac voltage pulses patent application. ###  How KEYWORD MONITOR works... a FREE service from FreshPatents1. Sign up (takes 30 seconds). 2. Fill in the keywords to be monitored. 3. Each week you receive an email with patent applications related to your keywords. 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