Method to demodulate a signal component from a sampled input signal and field bus device

1. A method to determine the magnitude M A of a signal component with frequency ω A from a set of N digital samples of an input signal acquired at a sampling rate R, the input signal having a discrete spectral representation containing n bins with frequencies ω 1 , . . . , ω n and corresponding magnitudes M 1 , . . . , M n , the spectral representation being derivable from the input signal using a transform, the method comprising:

choosing an extraction bin with index kε[1, . . . , n] and frequency ω k ≠ω A from the discrete spectral representation;

determining a magnitude M k of this extraction bin;

determining an allocation factor, the allocation factor indicating a portion M ks resulting from a reference sinusoidal signal with frequency ω A and unity magnitude, that is allocated to the extraction bin when the transform is applied to the reference sinusoidal signal, thereby generating a spectral representation out of the sinusoidal signal; and

determining the magnitude M A of the signal component from the magnitude M k of the extraction bin in combination with the allocation factor, the magnitude M A being equal to the magnitude of the extraction bin M k divided by the portion M ks of the sinusoidal signal with frequency ω A (M A =M k /M ks ).

2. The method of claim 1 , wherein the input signal is a frequency shift keyed signal with a first carrier frequency equal to ω A and at least a second carrier frequency equal to ω B .

3. The method of claim 2 , wherein the second carrier frequency ω B is one of the frequencies ω 1 , . . . , ω n in the discrete spectral representation, with corresponding magnitude M B .

4. The method of claim 2 , wherein the second carrier frequency ω B is not an integer multiple of the first carrier frequency ω A .

5. The method of claim 1 , wherein the input signal additionally includes an analog signal representing a value measured by at least one sensor.

6. The method of claim 1 , wherein the input signal is chosen to be a field bus signal that conforms to a wired or wireless Highway Addressable Remote Transducer standard.

7. The method of claim 1 , wherein the transform is chosen to produce a discrete spectral representation wherein the first bin frequency ω 1 is zero and a second non-zero bin frequency and following non-zero bin frequencies ω 3 , . . . , ω n are integer multiples of the first non-zero bin frequency ω 2 .

8. The method of claim 7 , wherein the a second carrier frequency ω B is one of the frequencies ω 1 , . . . , ω n in the discrete spectral representation, with corresponding magnitude M B , and

wherein the first non-zero bin frequency ω 2 corresponds to the second carrier frequency ω B .

9. The method of claim 3 , wherein the magnitude M k of the extraction bin, and/or the magnitude M B corresponding to the second carrier frequency ωB, is determined individually from a set of input signal samples.

10. The method of claim 9 , wherein the magnitude M B corresponding to the second carrier frequency ω B is determined individually from the set of input signal samples.

11. The method of claim 9 , wherein the magnitude M k of the extraction bin is determined individually from the set of input signal samples.

12. The method of claim 9 , wherein the magnitude M k of the extraction bin, and the magnitude M B corresponding to the second carrier frequency ω B , are determined individually from the set of input signal samples.

13. The method of claim 9 , wherein the magnitude M k , and/or the magnitude M B , is determined from the set of input signal samples using discrete Fourier transform and/or using the Goertzel algorithm.

14. The method of claim 1 , wherein the magnitude M k of the extraction bin is augmented by a magnitude M M corresponding to a frequency ω M that is a mirror frequency of the frequency ω k of the extraction bin, about a frequency R/2.

15. A device for use on a field bus that is controlled and/or managed using a frequency shift key-encoded digital data stream sent over the field bus, wherein in said data stream, one bit value is encoded by a first carrier frequency ω A and the other bit value is encoded by a second carrier frequency ω B , the device comprising:

a first unit configured to acquire a set of N digital samples of an input signal from the field bus at a sampling rate R, said device being configured to determine the magnitude M B that a frequency component with frequency ω B has in a discrete spectral representation of an input signal including n bins with frequencies ω 1 , . . . , ω n and corresponding magnitudes M 1 , . . . , M n , wherein one of the bin frequencies ω 1 , . . . , ω n corresponds to the second carrier frequency ω B and the discrete spectral representation is derivable from the input signal using a transform,

wherein the device is further configured to determine the magnitude M A that a frequency component with frequency ω A has in the discrete spectral representation of the input signal,

wherein the device is further configured to determine a magnitude M k of a bin frequency component ω k ≠ω A with index kε[1, . . . , n], and

wherein the device is further configured to determine M A from M k in combination with an allocation factor that indicates a portion M ks generated from a reference sinusoidal signal with frequency ω A and unity magnitude that is allocated to the bin with index k, when the transform is applied to the reference sinusoidal signal to generate a spectral representation out of the reference sinusoidal signal, the magnitude M A being equal to the magnitude of the extraction bin M k divided by the portion M ks of the sinusoidal signal with frequency ω A (M A =M k /M ks ).

16. The device of claim 15 , further comprising:

a second unit, configured to perform the transform on the reference sinusoidal signal with frequency ω A in order to obtain M ks , and/or to individually obtain M ks from a set of N digital samples of the sinusoidal signal.

17. The device of claim 15 , further comprising:

a memory including a pre-stored value of M ks .

18. The device of claim 17 , wherein the memory further includes a lookup table with a plurality of pre-stored values of M ks for one or more different combinations of sampling rate R and number N of acquired digital samples.