Patent
Fast and noise-insensitive load status detection
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TLDR
In this article, a stimulus waveform having multiple frequency components is applied to the line and current and voltage at the near end of the line are coherently sampled and transformed to the frequency domain.Abstract:
Method and apparatus for detecting whether load coils are attached to a telephone line. A stimulus waveform having multiple frequency components is applied to the line. The current and voltage at the near end of the line are coherently sampled and transformed to the frequency domain. The frequency spectra are used to compute auto and cross power spectra of the current and voltage. These power spectra are then used to compute the impedance on the line as well as a coherence function that indicates the extent to which the computed impedance was influenced by noise. If the coherence values indicate that the computed impedance is sufficiently reliable, load coils are detected by finding peaks in the magnitude of the impedance function or sign changes in the phase of the impedance function. Calibration, offset adjustments and ensemble smoothing are used to increase the accuracy of the results. The computation is fast because computing the spectra avoids the needs for individual measurements at multiple frequencies. The computation is accurate because it is not sensitive to noise.read more
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References
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Patent
Digital measurement of amplitude and phase of a sinusoidal signal and detection of load coil based on said measurement
TL;DR: In this article, the amplitude and phase of a sinusoidal signal of known frequency by the least square error method are carefully selected to satisfy a certain relationship, under these conditions, the measurements are simplified and accuracy improved.
Patent
Subscriber line impedance measurement device and method
TL;DR: In this article, a multi-level rectangular wave signal is used as the test signal and analyzed at the dominant frequency derived from the rectangular signal, or other harmonics, which constitute line impedance.
Patent
Method and apparatus to determine whether a subscriber line is loaded or non-loaded
TL;DR: In this article, the authors proposed a method and apparatus to determine if a telephone subscriber line is loaded or non-loaded including circuitry to provide a signal having at least one given frequency which is coupled to a telephone line under test having a subscriber subset on-hook either directly or through the switching system such that this signal can be received from the subscriber line under-test and analyzed to determine the value of at least the real part of the onhook impedance of the line.
Patent
Testing loaded transmission lines
TL;DR: In this article, the authors approximate a ladder network of series inductors and shunt capacitors equivalent to the input impedance of the transmission line from the pole and zero frequencies of the differential phase versus frequency characteristic thereof.
Patent
Method of predicting voltages in telephone line measurement
TL;DR: In this paper, the steady state voltage of a telephone line is predicted from nine samples of the voltage at the terminal using a closed form second order solution of a circuit model of the telephone line derived from a transformation of the circuit module into the Z domain and using the matched pole zero technique.