1. Field of the Invention
The present invention relates to a device for the detection of faults in a communications network. In particular although not exclusively the present invention relates to a device for the location of passive intermodulation faults in a coaxial cable network.
2. Discussion of the Background Art
Passive intermodulation distortion (PIM) is a form of electromagnetic interference that is often encountered in radio communications systems such as cellular mobile telephone networks. It can arise wherever there are devices or components with nonlinear transfer characteristics. Examples include oxidised metal-on-metal junctions, components containing ferrites (such as RF circulators), components with sharp metallic edges, loose connectors, and a myriad of other imperfections such as defective plating, dirt and other forms of contamination.
PIM is especially prevalent in full-duplex systems, where the transmitted and received radio signals are diplexed onto the same feeder cable. In such environments, the high-power transmit signals generate PIM as they pass through defective components, after which the PIM signals then propagate in two directions:                Towards the antenna in the same direction as the transmit signals (so-called “forward PIM” or “through PIM”); and        Back towards the base station (“reflected PIM”)        
Of the two cases, reflected PIM is usually the greater concern. This is because in many situations the frequency of the PIM signal falls within the passband of the base station's receiver, resulting in loss of sensitivity and an increased dropped call rate.
PIM faults in cell sites can be difficult and time-consuming to diagnose and repair. This is because the interconnection between BTS and antenna can consist of multiple components, including feeder cables, diplexers, filters, combiners, jumper cables, masthead amplifiers and bias tees. Furthermore, the radio transceiver and antenna are themselves potential sources of PIM.
The conventional approach to measuring PIM is the so-called two-tone test. This is an industry-standard test that entails applying a pair of high-power carriers (typically +43 dBm each) of different frequencies to the input of the cable network, and measuring the reflected PIM product(s) that land in the cell site's receive band. In theory the two-tone test creates PIM products at an infinite number of discrete frequencies. However, in many (but by no means all) cell sites the only PIM products of interest are the odd-ordered products below the carrier frequencies F1 and F2, as it is these products that are usually responsible for the majority of PIM problems encountered in practice. A simple equation for calculating the frequencies of these products is as follows:FIMx=F1−m(F2−F1) m=1,2,3 . . .                where                    FIMx=frequency of xth order PIM product located below F1 and F2             x=2 m+1            F1, F2=frequencies of high-power carriers                        
It must be emphasised that other PIM products the besides the odd-ordered products below carrier frequencies F1 and F2 are capable of causing problems in a cell site, depending on the particular spectrum allocation used in the network. For example, some cellular networks are vulnerable to the odd-ordered PIM products located above the transmit band rather than below it.
Similarly, even-ordered PIM products can be problematic in some situations. For example cell sites with co-sited GSM900 and PCS systems can potentially be affected by second-order PIM products from the 900 MHz transmitter landing in the PCS receive band.
For the sake of clarity of explanation, the following discussion will focus on a test apparatus that has been designed to measure odd-ordered PIM products below carrier frequencies F1 and F2. However, it shall be understood that the same technique can be used with any measurable PIM product of either odd or even order, at frequencies either above or below the high-power carrier frequencies.
At present the only commercially available instrument for troubleshooting PIM faults in cell sites is the scalar PIM analyser. This is simply a portable two-tone test instrument containing all of the necessary hardware in one box, including frequency synthesisers, high-power amplifiers, triplexer, low-noise receiver and a results display. Two such scalar PIM analysers are discussed in the Applicants co-pending U.S. application Ser. No. 11/936,968 and U.S. Ser. No. 11/941,712 entitled Passive Intermodulation Test Apparatus the disclosures of which are herein incorporated by reference.
Unfortunately, scalar PIM analysers as discussed in the applicant's earlier applications are only capable of measuring the overall PIM level in a cable network. They do not provide any information about the locations of individual PIM faults. With a scalar PIM analyser there are only two ways of isolating PIM faults:                Percussive testing: This involves tapping every component and cable joint in the network with a small rubberised mallet or screwdriver handle, while continuously monitoring the PIM level. Defective components will usually (but not always) cause the PIM level to fluctuate wildly when perturbed in this manner; or        Progressive assembly/disassembly of cable network, with PIM testing being performed on the partially assembled network at every stage.        
Both of the above approaches are less than optimal. The first method is usually a two-person job, and may require one of the testing personnel to climb the antenna mast in order to locate the source of the PIM fault. Similarly, the second method is potentially very labour-intensive and time-consuming, especially in crowded cell sites with large bundles of feeder cables strapped together on the same mast.
Clearly it would be advantageous to provide a system and method for the detection of the location and magnitude of sources of PIM in a communications network with a high degree of reliability and accuracy.