Circuit and method for detecting AC voltage pulses

1. A circuit for detecting AC voltage pulses at a defined frequency, comprising:

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, said mixing stage having a quadrature signal path and an in-phase signal path;

at least one low pass 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 frequency signal for suppressing an up-mixed component; and

an evaluation unit for determining whether an AC voltage pulse is present by using the converted sample values, said evaluation unit squaring, adding and comparing the converted sample values in said quadrature signal path and in said in-phase signal path with a threshold value.

2. The circuit according to claim 1 , further comprising at least one decimation stage for reducing a signal rate by omitting sample values.

3. The circuit according to claim 2 , wherein:

in said quadrature signal path the sequence of sample values is multiplied by a sine signal at the mixing frequency signal; and

in said in-phase signal path the sequence of sample values is multiplied by a cosine signal at the mixing frequency signal.

4. The circuit according to claim 3 , wherein in said quadrature signal path, the sequence of sample values is multiplied by the sine signal by multiplying the sequence of sample values by a sequence 0, 1, 0, −1, 0, . . . .

5. The circuit according to claim 3 , wherein in said in-phase signal path, the sequence of sample values is multiplied by the cosine signal by multiplying the sequence of sample values by a sequence 1, 0, −1, 0, 1, . . . .

6. The circuit according to claim 1 , wherein the mixing frequency signal of said mixing stage has a frequency corresponding to the frequency of the AC voltage pulses that will be detected.

7. The circuit according to claim 1 , further comprising:

a first mixing stage defining said mixing stage, said first mixing stage for multiplying the sequence of sample values by a first mixing frequency signal having a frequency corresponding to a frequency of a first type of AC voltage pulses; and

a second mixing stage configured between said first mixing stage and said evaluation unit;

said second mixing stage for multiplying the sequence of sample values by a second mixing frequency signal having a frequency corresponding to a difference frequency formed by a difference between the frequency of the first type of AC voltage pulses and a frequency of a second type of AC voltage pulses.

8. The circuit according to claim 7 , further comprising: a lowpass filter stage having a cut-off frequency above the difference frequency; said low-pass filter stage configured between said first mixing stage and said second mixing stage.

9. The circuit according to claim 1 , wherein the AC voltage pulse is a charge unit signal in a telephone network.

10. A telephone portion including a circuit for detecting charge unit signals at a defined frequency, the circuit comprising:

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 low pass 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 frequency signal for suppressing an up-mixed component;

an evaluation unit for determining whether a charge unit signal is present by using the converted sample values; and

a speech path for performing audio signal processing, the charge unit signal being detected via said speech path.

11. The telephone portion according to claim 10 , wherein:

said analog/digital converter unit is configured in the speech path; and

the charge unit signals are digitized by said analog/digital converter unit.

12. The telephone portion according to claim 10 , further comprising at least one digital signal processor for implementing at least said mixing stage and said filter stage.

13. A method for identifying different types of AC voltage pulses each having a defined frequency, the method which comprises:

digitizing an input signal to form a sequence of digitized sample values;

obtaining a sequence of converted sample values by converting the sequence of sample values using any desired sequence of steps including multiplying the sequence of sample values by predetermined mixing frequency signals and low pass filtering the sequence of sample values to define a permissible intermediate frequency band and for suppressing an up-mixed component; and

using the sequence of converted sample values to determine whether a specific type of AC voltage pulse is present by squaring, adding and comparing the sequence of converted sample values with a threshold value.

14. The method according to claim 13 , which further comprises, while performing the step of converting the sequence of sample values, reducing a signal rate by reducing a number of the sample values in the sequence of sample values.

15. The method according to claim 13 , wherein the step of multiplying the sequence of sample values by predetermined mixing frequency signals includes multiplying the sequence of sample values by a mixing frequency signal having a frequency corresponding to a frequency of the AC voltage pulses being identified.

16. The method according to claim 13 , wherein the step of multiplying the sequence of sample values by predetermined mixing frequency signals includes first multiplying the sequence of sample values by a first mixing frequency signal having a frequency corresponding to a frequency of a first type of AC voltage pulses, and subsequently multiplying the sequence of sample values by a second mixing frequency signal having a frequency corresponding to a difference between the frequency of the first type of AC voltage pulses and a frequency of a second type of AC voltage pulses.

17. The method according to claim 13 , wherein the AC voltage pulses are charge unit signals in a telephone network.