Cable detection apparatus with a comb filter
View Patent ↗An apparatus that uses harmonics of the power line frequency at which electrical power is distributed is presented. The apparatus includes a sensor antenna to detect an electromagnetic field produced by a cable. The output of the sensor is fed to a mixer together with a signal from a sine wave generator at less than the fundamental power frequency to frequency shift the signals from the sensor. The mixed output is then amplified to provide an audible output. A single comb filter receives signals from the mixer and outputs a filtered signal removing non-harmonic frequencies but preferably passing both odd and even harmonics. The output of the comb filter is then integrated at and displayed to indicate the detection of a cable based on the filtered signal.
1. A cable detection apparatus comprising:
a sensor to detect an electromagnetic field produced by a cable and provide signals, the electromagnetic field expected to have a component at a first frequency;
a frequency shifter to frequency shift the signals from the sensor by a second frequency of half the first frequency;
an amplifier that receives the frequency shifted signals;
a first filter to receive signals from the amplifier and output a filtered signal, the first filter having a higher transmittance at the second frequency and at the second frequency shifted by whole multiples of the first frequency than at least one frequency between two of said shifted frequencies; and
an indicator to indicate the detection of the cable based on the filtered signal.
2. The cable detection apparatus of claim 1 , wherein the whole multiples include both odd and even multiples.
3. The cable detection apparatus of claim 1 , wherein there is only a single comb filter between the sensor and the indicator.
4. The cable detection apparatus of claim 1 , wherein the first filter is a comb filter.
5. The cable detection apparatus of claim 1 , wherein the frequency shifter includes a mixer and a sine wave generator operating at the second frequency.
6. The cable detection apparatus of claim 1 , further comprising a second filter between the amplifier and the first filter, the second filter being a high-pass filter to remove a B-field of the earth from the signals received from the amplifier.
7. A method of cable detection, the method comprising:
sensing an electromagnetic field produced by a cable to provide signals, the electromagnetic field expected to have a component at a first frequency;
frequency shifting the signals by a second frequency of half the first frequency;
amplifying the frequency shifted signals;
filtering, using a first filter, the amplified signals, the first filter having a higher transmittance at the second frequency and at the second frequency shifted by whole multiples of the first frequency than at least one frequency between two of said shifted frequencies; and
indicating detection of the cable based on the filtering.
8. The method of claim 7 , wherein the whole multiples include both odd and even multiples.
9. The method of claim 7 , further comprising filtering using only a single comb filter.
10. The method of claim 7 , wherein the first filter is a comb filter.
11. The method of claim 7 , wherein the frequency shifting includes mixing in a sine wave having the second frequency.
12. The method of claim 7 , further comprising filtering, using a high-pass filter, the amplified signals to remove a B-field of the earth from the amplified signals.