Television – Monitoring – testing – or measuring
Reexamination Certificate
1997-11-17
2001-11-06
Eisenzopf, Reinhard J. (Department: 2614)
Television
Monitoring, testing, or measuring
C348S180000, C348S192000, C348S726000, C348S731000, C455S067700, C455S324000, C324S095000
Reexamination Certificate
active
06313874
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to communication system testing, and more particularly, to the measurement of leakage signals in a communication system.
BACKGROUND OF THE INVENTION
A Community Antenna Television (“CATV”) system, or cable television system, includes at its basic elements a centralized transmitter and a distribution network. The centralized transmitter typically includes an aerial antenna for receiving radio frequency (“RF”) television signals. The centralized transmitter is further connected to communicate television signals to CATV subscribers through the distribution network. The distribution network includes an array of coaxial cable, repeater amplifiers, and other devices that often connect as many as hundreds of thousands of subscribers to the centralized transmitter. The complexity and size of the distribution network require that network operation and performance be periodically tested and/or monitored.
One test often performed by CATV service providers is leakage detection. Leakage detection is an operation which measures the leakage of CATV signals to the exterior of the distribution network. CATV signal leakage occurs in faults or anomalies within the distribution network. For example, if a portion of the coaxial cable is insufficiently shielded, then significant levels of CATV signals may leak to the environment surrounding the cable. Government regulations limit the amount of permissible CATV signal leakage. Accordingly, service providers employ leakage detection to determine compliance with government regulations and otherwise obtain information as to the performance of the distribution network.
A common method of leakage detection consists of physically scanning the distribution network with a portable leakage detector. Specifically, a technician drives or walks along the route of the distribution network with an activated portable leakage detector. The leakage detector, which includes an RF receiver, measures the level of RF signals within the CATV spectrum and provides an indication of that level to the technician. Accordingly, if a technician passes by a portion of the distribution network in which a leak is present, a relatively high signal level will be measured.
A number of leakage detection devices are presently known. In connection with industry and regulatory standards, such devices typically are tuned to detect and/or measure leakage signals in the frequency range that overlaps with frequencies allocated for aeronautical use. Specifically, a typical leakage detector is tuned to receive a select television signal having a CATV channel frequency of between 115 MHz and 140 MHz. In order to distinguish leaked CATV signals from other sources of RF energy, the leakage detection device measures energy in an extremely narrow band around the CATV channel frequency. Such a measurement has a higher probability of being representative of actual CATV leakage signals, as opposed to RF signals from other sources.
To achieve a signal strength measurement in such a narrow signal band, the leakage detectors typically down-convert the received RF signal to an intermediate frequency substantially below the 115 MHz range. Such frequency conversion allows for the use of conventional filters to produce the required narrow bandwidth signal. The Searcher Plus leakage detector, available from Trilithic, Inc., operates in this manner.
The one problem associated with presently available leakage detection devices, and in particular, those that use the frequency conversion step referenced above, is power consumption. The various electronic circuitry within the leakage detectors, including the frequency conversion circuitry, consumes a significant amount of power. High power consumption is problematic in part because leakage detectors are typically battery-powered. Specifically, it has been observed that batteries within portable leakage detectors frequently require recharging or replacement. Frequent battery recharging and/or replacement can be inconvenient from the perspective of the field technician.
While advances in battery technology may improve battery charge-life to partially overcome the need to frequently recharge or replace the batteries in leakage detectors, there is nevertheless a need for leakage detection equipment that consumes less power while providing the sensitivity and capabilities that are currently available.
SUMMARY OF THE INVENTION
The present invention fulfills the above needs, as well as others, by providing a leakage detection receiver circuit that directly demodulates the detected RF signals. In other words, the leakage detector does not frequency-convert or mix down the detected RF signals before demodulation. The elimination of the circuit components required for frequency conversion, for example, an oscillator and a mixer, significantly reduces the power consumed by the leakage detector. The reduced power consumption has several benefits, including increasing the charge-life of the batteries used to provide power to the detector.
An exemplary receiver circuit according to the present invention, which may be used in a leakage detector, includes an RF input, a narrow band filter, and a direct demodulator. The leakage detector in which the receiver circuit may be used is operable to measure leakage in a communication system, the communication system operable to transmit broadband communication signals comprising one or more radio frequency carrier signals modulated by one or more baseband communication signals, wherein each radio frequency carrier signal has a channel frequency. Essentially, the exemplary receiver circuit according to the present invention is operable to generate a baseband leakage signal.
To this end, the RF input is operable to receive a broadband communication signal, the broadband communication signal including a first signal associated with a first channel frequency. The narrow band filter is operably coupled to the RF input to receive at least a portion of the broadband communication signal and producing a filtered first signal therefrom, the narrow band filter having a Q value at least on the order of 1000 and further having a pass band that includes the first channel frequency. The direct demodulator is coupled to the narrow band filter, and is operable to generate a baseband leakage signal from the filtered first signal.
In a preferred embodiment, the first signal is a television signal, and the baseband leakage signal is a narrow band version of the baseband television signal. Well known methods of signal level measurement may then be used on the baseband leakage signal to determine leakage levels.
The narrow band filter allows for direct demodulation of high frequency signals, for example, those between 115 MHz and 140 MHz. The use of a direct demodulator, in other words, a demodulator that demodulates an unconverted RF signal, eliminates the need for frequency conversion circuitry. The elimination of the need for frequency conversion circuitry reduces power consumption by the detector.
The above described features and advantages, as well as others, will become more readily apparent to those of ordinary skill in the art by reference to the following detailed description and accompanying drawings.
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Bowyer Andrew E.
Chappell Daniel K.
Eisenzopf Reinhard J.
Maginot Addison & Moore
Natrael Paula
Wavetek Corporation
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