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RF & Wireless Coverstory

RF & Wireless Coverstory Cable and Antenna Measurements Using Tektronix USB Spectrum http://info.tek.com/de-free-RF-trial-em.html Tektronix www.tek.com This application note looks at the basics of line sweeping measurements on cable and antenna systems using a spectrum analyzer and a tracking generator, including a look at why they are important and how to perform them. Specific measurements covered include return loss/voltage standing wave ratio, cable loss, antenna isolation, and distance to fault measurements. Tracking generators play a key role in allowing spectrum analyzers to perform transmission loss, transmission gain, and return loss measurements. As will be discussed later, tracking generators are simply variable, or swept, RF generators that track with the spectrum analyzer sweep frequency. In other words, the tracking generator produces signals as the analyzer sweeps, measuring power across a frequency range. This allows the user to provide a known stimulus to a circuit and view the response. There are a number of portable spectrum analyzers on the market today available with tracking generators. However, the majority of this equipment incorporates slow, low-power processors and offer limited to no real-time capabilities. The Tektronix family of USB spectrum analyzers, on the other hand, work in conjunction with a laptop or tablet PC to offer desktoplevel real-time performance in an easily portable package, including models that are battery powered and IP52 rated. Cable and Antenna Testing – What Can Go Wrong? It’s estimated that about 60 percent of cellular base station problems result from faulty cables, connectors and antennas. Some problems occur during installation and are immediately apparent. But over time, connecting cables, adapters and antennas may become damaged or gradually degrade. Component failures often result in poor coverage and unnecessary handovers in the case of cellular systems. But cellular is simply the most obvious and pervasive example – any communication system is bound to degrade without ongoing testing to verify performance and isolate the source of problems. Cabling and antennas are expected to cope with a diverse range of environments including outdoor and indoor installations, each posing different challenges. Typical outdoor installations involve mounting antennas on the tops of tall buildings or on towers, often in remote locations where the antenna and portions of the coaxial cabling can be exposed to extreme weather conditions including wide temperature swings, rain, snow, ice, wind and lightning. Such conditions can exact a major toll on the integrity of the system, resulting in physical damage such as failed waterproofing at connector joints, failed cable splice seals, and cracks in insulating materials. Indoor installations range from stationary set-ups like equipment shelters and office buildings to more mobile (and therefore more vulnerable) applications on ships, airplanes and trains, as well as cars and trucks. Even sheltered installations face a range of hazards including mishandling, stress, heat, vibration, chemicals and contamination. Problems are especially prevalent where solder joints and cable crimps weaken over time and break or degrade. Running cables up and down towers, through walls or underground can be a messy job. It’s not hard to tear, stretch, dent, crush or poorly route a cable during installation – problems that can sometimes manifest themselves long after the initial installers have moved on. Another problem occurs when the minimum bend radius is exceeded such as in the case of lowloss coaxial cables, which can significantly degrade electrical 72 hf-praxis 1/2017

RF & Wireless performance. Fortunately, it is not necessary to use highly specialized tools for cable and antenna test and troubleshooting. Portable spectrum analyzers are already used in the installation and maintenance of RF transmission systems. They can test many different aspects of an RF transmission system, from overall performance to analysis of individual components. There fore, adding a tracking generator to a spectrum analyzer is a cost-effective solution to the problem. Tracking Generator Basics Since spectrum analyzers receive and measure a signal, they can be considered passive instruments. As such, spectrum analyzers, by themselves, are not able to make cable and antenna measurements that require known signals to be applied to a particular device or network under test in order to measure the output or response. There are two main types of test equipment used for making these stimulus-response measurements. The traditional type of test equipment is an RF or scalar network analyzer. The other option is a spectrum analyzer with a tracking generator. A vector network analyzer is typically required if exceptional accuracy is needed, but in most other cases a spectrum analyzer and tracking generator arrangement is an excellent solution. This is particularly true with the advent of low-cost high-performance USB-based spectrum analyzers. The tracking generator operates by providing a sinusoidal output to the input of the spectrum analyzer. By linking the sweep of the tracking generator to the spectrum analyzer, the output of the tracking generator is on the same frequency as the spectrum analyzer, and the two units track the same frequency. As shown in Figure 1, the return loss bridge is the subsystem that allows reflections of the generated signal to be detected by the spectrum analyzer. Connecting the output of the tracking generator to the input of the spectrum analyzer, such as during normalization, results in a single flat line, with the level representing the reference loss of the direct connection. For measurements, an unknown device is placed between the output of the tracking generator and the input of the spectrum analyzer. The response of the device under test alters the signal and this change is then measured by the spectrum analyzer. Normalization, Calibration and Measurements Tracking generators in spectrum analyzers may either make purely scalar measurements, or they may measure vector parameters. For transmission gain measurements, the RSA500 and RSA600 use a scalar normalization of the measured power to create a normalized display of frequency vs. amplitude. However, for measurements such as return loss, VSWR, cable loss, and distance to fault, a vector calibration is required. The RSA500 and RSA600 are shipped with a factory vector calibration that is useful for many troubleshooting applications and can be fully user-calibrated using an Open, Short, Load (OSL) method for greater accuracy. More information on calibration techniques can be found in the user help files for SignalVu- PC. All of the vector measurements made in this application note were performed using the factory calibration provided in the instrument. Return Loss and VSWR Return loss and VSWR measurement are at the core of cable and antenna measurements. These measurements allow the user to determine if the system in question is working the way it should. If problems show up during this test, chances are that the system’s overall performance is being impacted. These measurements are based on the principle that some parts of a signal are reflected due to mismatches in impedance between cables, antennas, or connectors. The ratio of the input signal to the reflected signal is called the voltage standing wave ratio or VSWR. This ratio can also be measured in dB, and expressed as return loss. Return loss and VSWR can reveal significant problems. For instance, a poorly matched antenna will reflect costly RF energy which will not be available for transmission and will instead end up in the transmitter. This extra energy returned to the transmitter can distort the signal and affect the efficiency of the transmitted power, reducing coverage area. Return loss and VSWR show the same information expressed in different ways. To convert from VSWR to return loss: The return loss is the ratio of reflected power to reference power in dB. The return loss view is usually preferred because of the benefits with logarithmic displays – it’s easier to compare a small and large number on a logarithmic scale. The default return loss scale for Tektronix USB spectrum analyzers is +10 dB to -40 dB since this falls Figure 1: RSA500/600 Series spectrum analyzer with optional tracking generator hf-praxis 1/2017 73

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