Apparatus for detecting leakage in digitally modulated systems

Abstract

In a method for detecting leakage in a digital cable system, at least one first signal is inserted on the cable system. The at least one first signal has an amplitude multiple tens of dB below the digital channel power of the digital channels carried on the cable system. A second signal containing the first signal is received. The second signal is converted to an intermediate frequency (IF) signal. The IF signal is digitized and samples of the digitized IF signal are obtained. Digitized samples of a third signal at the nominal frequency of the first signal at maximum amplitude converted to the IF are provided. The digitized IF signal and the digitized samples of a third signal at the nominal frequency of the first signal at maximum amplitude converted to the IF are correlated. The presence of the inserted first signal is detected based upon the result of the correlation. In another method, a pair of first signals are inserted on the cable system. The pair of first signals are spaced apart a fixed frequency and with amplitudes multiple tens of dB below the digital channel power of the digital channels carried on the cable system. A second signal containing the first signal is received and converted to an intermediate frequency (IF) signal. The IF signal is digitized, samples of the digitized IF signal are obtained, and a large scale Fast Fourier Transform (FFT) is applied to the samples to generate an FFT output. The FFT output is examined for generally equally sized signals separated from each other by the fixed frequency in the FFT output. If generally equally sized signals separated from each other by the fixed frequency are detected in the FFT output, a decision is made that the second signal represents detected leakage from the digital cable system.