Remote estimation of amplifier functionality

A method of remotely estimating the functionality of a preamplifier mounted adjacent an antenna at the top of a mast or tower and coupled to a base station via a transmission medium, such as a coaxial cable, includes taking noise and/or signal power measurements at the base station via the coaxial cable for the amplifier when both powered and unpowered. The ratio of the respective noise and/or signal powers, after the receiver noise figure at the base station and the cable loss (estimated or measured) including any connection device loss are accounted for, provides an estimate of the functionality of the amplifier.

BACKGROUND OF THE INVENTION

The present invention relates to component subsystem measurements, and more particularly to the remote estimation of amplifier functionality using a measurement instrument coupled to the amplifier via a transmission medium.

A traditional method of measuring the frequency response of an amplifier is to couple the amplifier's input and output to a network analyzer or similar device, such as a spectrum analyzer with a tracking generator or the like, and to apply a swept frequency signal to the amplifier from the analyzer. The network analyzer measures the output of the amplifier over the swept frequency range and provides a frequency versus amplitude display of the output. This type of measurement is usually performed in a laboratory or shop where the amplifier is easily accessible.

Measuring the response of an amplifier becomes much more difficult when the amplifier is positioned adjacent to an antenna that is not easily accessible. Such configurations are used with telecommunication systems, such as wireless communications systems and the like, deep space satellite antennas, radio astronomy antennas and the like. In a wireless communications system the antenna is generally positioned on a tower, side of a building or the like to provide the maximum coverage area for the antenna. A preamplifier is mounted in a weather-proof box close to the antenna and is coupled to receive the signal from the antenna. The preamplifier amplifies the signal and couples the signal out of the box via a transmission cable, such as a coaxial cable, to a base station remote from the antenna. The cable enters the base station and may be connected via a jumper cable directly to an equipment rack that generally has a tap for connecting test equipment to the transmission cable. Power for the preamplifier is provided from the equipment rack. Alternatively, a jumper cable may connect the transmission line to a junction box that receives an alternating current (AC) voltage input. The AC voltage is converted to direct current (DC) voltages for powering the preamplifier. A second jumper cable connects the junction box to the equipment rack.

Measuring the parameters of the preamplifier in situ at the top of an antenna tower currently requires a technician to climb the tower with a portable network analyzer or the like. The technician opens up the weather-proof box containing the preamplifier and disconnects it from the antenna and transmission cable, connects the network analyzer to its input and output, and measures its frequency response. After completing the test the technician reconnects the amplifier to the antenna and transmission cable, reseals the weather-proof box and descends the tower.

There are a number of drawbacks to the above method of determining the functionality of a preamplifier mounted adjacent to a remote antenna. The first is that the weather-proof box has to be opened when testing the preamplifier. Generally, once the box is opened the weather-proof integrity of the box is compromised. It is very difficult to completely reseal the box to prevent ingress of moisture into the box. Second, the technician authorized to make the measurement may not be the same technician that supports the base station. Thus, it requires more than one individual to maintain the antenna and base station. This results in added overhead. Further, company and/or government safety requirements may dictate that special equipment, emergency support, special training and the like be used and provided for anyone climbing the tower. Again, this adds costly overhead to the operation of the tower and base station.

What is needed is a method of estimating the functionality of an amplifier that is remotely located from a measurement instrument.

BRIEF SUMMARY OF THE INVENTION

Accordingly, the present invention provides a method of remotely providing an estimate of the functionality of an amplifier situated adjacent to a receiver antenna in a telecommunications system or the like where the antenna is located at a distance from a base station where measurements are taken. The amplifier is coupled to the base station via a transmission medium, such as a coaxial cable, and a measurement instrument is coupled to the transmission medium at the base station. The measurement instrument may be used to control power to the amplifier. The measurement instrument obtains measures of output power for the amplifier both when power is applied to the amplifier and when power is not applied to the amplifier. The measures may be based on frequency spectra data within a measurement frequency spectrum range, and may be taken for either or both a constant signal channel within the measurement frequency range and/or a region within the measurement frequency range where signals are absent. Taking into consideration a receiver noise figure and a value for the transmission medium loss, including any connection devices, between the measurement instrument and the amplifier, the estimate is determined from the measures for display in a usable format.

DETAILED DESCRIPTION OF THE INVENTION

Referring toFIG. 1a wireless communications cell site10is shown having an antenna-mounted amplifier, or preamplifier11, that requires testing to estimate if the amplifier is functioning properly. The cell site includes a mast or tower12on which are mounted transmitting and receiving antennas14and16. Generally, each cell site10has three sets of transmitting and receiving antennas14,16positioned in a triangular array. Each side of the triangular array has a transmitting and receiving antenna set with two receiving antennas16mounted toward the ends of the side and a transmitter antenna14mounted between the receiver antennas toward the middle. The cell site10may be set-up with amplifiers11connected to each of the receiver antennas16at the top of the mast or tower12. The amplifiers11are secured in weather-proof housings to protect them from the elements. The outputs of the amplifiers11are coupled to a base station18via coaxial cables20. Inside the base station18are equipment racks containing transmitting, receiving, processing and switching equipment that control the transmitting and receiving of cell phone traffic. The coaxial cables20are connected to the receiver equipment, which provides power to the amplifiers11at the top of the mast or tower12. Generally, power is provided to each of the amplifiers11via the center conductor of the coaxial cable20.

FIG. 2shows generically a measurement instrument30usable for remotely estimating the functionality of the preamplifier11at the top of the mast or tower12. In the preferred embodiment, the measurement instrument30is a modular system having an RF measurement module32and a control and display module34, such as the YBT250*Y350C NetTek™ BTS Field Tool, manufactured and sold by Tektronix, Inc., Beaverton, Oreg. The RF measurement module32, such as a spectrum analyzer module, may be a two stage down converter that is tunable over a range of frequencies covering the various analog and digital wireless communications, cellular (800-1000 MHz) and PCS (Personal Communications Service—1700-2000 MHz) standards. The RF measurement module32has a coaxial input connector36that is coupled to the coaxial cable20coming from the amplifiers11mounted adjacent to the receiver antenna16on the tower12. This connection is preferably made within the base station18. Typically dual signal paths within the RF measurement module32couple the coaxial input connector36to a power insertion unit42. One signal path is a straight through connection that bypasses the power insertion unit42, while the other input path provides power to the amplifier11and amplifies the incoming signal. The powered signal path provides a voltage to the center conductor of the coaxial input connector36. The output of the power insertion unit42is coupled to an RF selective measurement unit44which in turn provides measurement results to the control and display module34. In the preferred implementation of the RF measurement instrument30, the control and display module34has a touch screen display with front panel controls incorporated into the display as display knobs, scroll bars, touch pads and the like. The RF measurement instrument30in the preferred embodiment of the invention may be a personal computer (PC) based system functioning under the overall control of a WINDOWS® CE operating system, manufactured and sold by Microsoft, Corp., Redmond, Wash. The RF measurement instrument30may also be configured as a single instrument having separate DSP and system controllers and separate DSP and system memories. Additionally, the RF measurement instrument30may also be configured with the DSP controller and the system controller being a single device and the DSP and system memory being combined into a single memory.

To estimate the functionality of the amplifier11at the top of the tower12from the base station18, the RF measurement instrument30is coupled to the coax cable20that is coupled to the amplifier, as indicated by the process flow in FIG.3. The process involves turning on and off the amplifier11from the base station18while measuring the noise power across a frequency spectrum with the RF measurement instrument30. Knowing the noise figure of the base station receiver and/or the RF measurement instrument30, which is generally provided by the manufacturer, and the loss in the length of cable20between the amplifier11and the RF measurement instrument, which loss may be determined during installation by using a time domain reflectometer to measure the ratio between applied and reflected energy when the remote end of the cable is either shorted or opened or which loss may be estimated from manufacturer's data concerning loss per 100′ of cable, the functionality of the amplifier may be estimated. If the noise from the amplifier11, amplified by its gain, is sufficient to overcome the noise of the receiver including the transmission line20and associated connection device loss, then the amplifier is likely functional. This method incorporated into the RF measurement instrument30ofFIG. 2may be automated to provide the functionality estimate of the remotely mounted amplifier11. The internal power insertion unit42replaces the one that is normally part of the base station18.

The method is even better if a signal is present, assuming that the amplifier11does not have a bypass relay associated with it. Then the difference in signal strength as measured by the RF measurement instrument30when the amplifier11is powered on and off is significant if the amplifier is functioning properly. The signal may be provided from a transmitting antenna proximate the receiver antennas16, such as the transmitter antennas14for the base station18. When the preamplifier11does incorporate a bypass relay, then an actual measurement of S/N ratio with preamplifier may be compared to an actual measurement of S/N ratio in bypass mode (power removed). This may be used to provide the noise figure for the preamplifier11. The signal power ratio ON/OFF provides gain.

The RF measurement instrument30may be used to sweep across a range of frequencies to determine which channels of the system are receiving signals and which are not. Then when the results for the powered and unpowered states of the amplifier11are obtained, the results for the channels where there is an absence of signal and for the channels where there is a signal constantly as determined by repetitive measurements may be compared to appropriate threshold values based on the parameters mentioned above. The estimate of the functionality of the amplifier11may be displayed as an average signal to noise ratio of the amplifier, or as an ON/OFF ratio of signal power and/or noise power.

Additional confidence in the functionality of the amplifier11may be attained by also measuring the current that the amplifier draws from the power supply. It the amount of current is correct, then further confidence of proper operation of the amplifier11is reinforced.

Thus the present invention provides a method of remotely estimating the functionality of an amplifier by measuring the noise and/or constant signal power outputs when the amplifier is powered on and powered off, taking into consideration the noise figure of the measuring instrument or other receiver and the transmission medium including any connection device loss between the amplifier and the measuring instrument.