Method and apparatus for pinpointing common path distortion

A method of pinpointing a CPD source in a CATV network that carries a plurality of TV program signals in a forward path frequency band and return path signals in a return path frequency band, said method comprising the steps of:

• (a) receiving the plurality of TV program signals from the CATV network;
• (b) generating a plurality of intermodulation products from the plurality of TV program signals, for use as a reference signal;
• (c) receiving the return path signals from the CATV network, the return path signals including intermodulation products of the plurality of TV program signals created by the CPD source;
• (d) delaying the reference signal to create a time delayed reference signal;
• (e) cross-correlating the time delayed reference signal with the intermodulation products created by the CPD source to produce a cross-correlation function;
• and
• (f) determining a time delay associated with the CPD source based on the cross-correlation function, whereby the time delay associated with the CPD source is used in pinpointing the CPD source.

A method of calibrating a CATV network between a reference point and a calibration point, the CATV network having a forward path frequency band and a return path frequency band and containing a source of CPD at or near the calibration point, said method comprising the steps of:

• (a) transmitting from the reference point a probing signal in the forward path frequency band of the CATV network, the source of CPD producing an intermodulation product of the probing signal that represents a distortion signal having a spectrum within the return path frequency band and characterized by a first sequence of pulses;
• (b) receiving the probing signal at the calibration point;
• (c) generating at the calibration point an intermodulation product of the received probing signal, the intermodulation product representing a reference signal characterized by a second sequence of pulses that corresponds with the first sequence of pulses;
• (d) creating a calibration signal from the reference signal, including suppressing every other pulse of the second sequence of pulses to produce a calibration signal sequence of pulses, the calibration signal having a spectrum within the return path frequency band;
• (e) transmitting from the calibration point the calibration signal in the return path frequency band of the CATV network;
• (f) receiving the calibration signal and the distortion signal at the reference point;
• (g) inverting the phase of every other pulse of the first sequence of pulses of the distortion signal, received in step (f), to produce a distortion signal sequence of pulses;
• and
• (h) accumulating the pulses of the distortion signal sequence of pulses and the pulses of the calibration signal sequence of pulses, such that distortion signal pulses substantially cancel each other out due to the phase inversion of every other pulse and the calibration signal pulses substantially accumulate because they are in-phase, whereby the calibration signal is detected without substantial interference from the distortion signal.

An apparatus for pinpointing a CPD source in a CATV network that carries a plurality of TV program signals in a forward path frequency band and return path signals in a return path frequency band, said apparatus comprising:

• (a) a first input for receiving the plurality of TV program signals from the CATV network;
• (b) a CPD generator coupled to said first input and configured to generate a plurality of intermodulation products from the plurality of TV program signals received from said first input, for use as a reference signal;
• (c) a second input for receiving the return path signals from the CATV network, the return path signals including intermodulation products of the plurality of TV program signals created by the CPD source and a plurality of service signals defined by an energy spectrum;
• (d) a filter, coupled to said second input, for filtering the return path signals received from said second input, said filter having a frequency response that substantially matches and is substantially complementary to the energy spectrum of the plurality of service signals, such that the plurality of service signals are substantially suppressed by said filter;
• (e) a correlator coupled to said CPD generator to receive the reference signal and coupled to said filter to receive the intermodulation products created by the CPD source, said correlator including a first time delay unit being adapted to delay the reference signal by a predetermined amount, said correlator being adapted to perform a cross-correlation of the delayed reference signal with the intermodulation products created by the CPD source and produce a cross-correlation function;
• and
• (f) a processor coupled to said correlator to receive the cross-correlation function, said processor being adapted to determine a time delay associated with the CPD source based on the cross-correlation function, whereby the time delay associated with the CPD source is used in pinpointing the CPD source.