Spectrum Analyzer Apps

Modern spectrum analyzers play an important role in maintaining today's communications networks. The surge in usage is due to the instrument's flexibility, which makes it an ideal tool for fulfilling basic wireless-system commissioning and troubleshooting functions such as identifying and locating RF signal interference, path loss, transmitter power and receiver sensitivity.

Interference from external sources is a common cause of poor quality in voice communications and can corrupt fax and data transmissions or, in extreme cases, disable wireless channel operations entirely. The tricky part in identifying interference is that any transmitter has the potential to bring down a network. The spectrum analyzer offers several methods to identify the origin of interfering signals.

In tracking down signal problems, begin by examining the interfering signal on the spectrum analyzer. Adjusting the instru-ment's scan width (scan widths of less than 10kHz are preferred) allows clear visibility of audio frequencies as they move around the carrier. If the signal is modulated, the spectrum analyzer's FM or AM demodulator can be used to listen to the audio content. This simple technique may reveal the type or origin of the interference -- even digital signals exhibit unique sounds that can be readily identified to the trained ear.

Add Directional Antenna If looking and listening provide no definitive insights, you can use a spectrum analyzer coupled to a directional antenna to find the source. For example, a Midwestern cellular carrier experienced a sudden surge in dropped calls at an area cell site. A field engineer was dispatched immediately to locate the problem's source. Armed with a spectrum analyzer and directional antenna, the engineer drove a service vehicle around the affected cell, monitoring small changes in signal strength relative to antenna orientation. Using increases in signal amplitude to indicate signal proximity, he was able to pinpoint the source of interference: A power transformer at the top of a utility pole. Perplexed by the unusual finding, the engineer carefully inspected the transformer and discovered that it was fake. Its purpose was to hide a covert surveillance video camera and transmitter installed by a local law-enforcement agency. The malfunctioning transmitter was operating in the receive band of the cell site instead of the intended receiver. The problem was solved quickly by cycling the power on the transmitter, which then began to operate at its assigned frequency.

Malfunctioning mobile phones are another common source of cell-site performance problems. These cases are more difficult to locate because they are neither stationary nor constant. One case involved a mobile that was not in service but was mounted in a vehicle and powered up. The mobile would transmit for 90 seconds every 5 minutes. The transmitter would transmit a dead carrier on channel 1 and disrupt channel 1 calls in the location of the vehicle. As in the previous case, an engineer using a spectrum analyzer and directional antenna located the offending mobile.

Basic Functions When combined with a tracking generator, the spectrum analyzer provides complete resources for making swept measurements to characterize active and passive RF devices. For example, determining transmitter power or receiver sensitivity, the losses through filters, attenuators and transmission lines must be taken into account. These system elements must be characterized with respect to the frequencies at which they are used. Advanced features allow easy evaluation and characterization of device frequency response, insertion loss or gain.

To measure power accurately, it is important to compensate the indicated power level by adding the insertion loss of the test cable. The measurement requires the use of a spectrum analyzer with tracking generator and a reference cable. First, tune the analyzer to the operating frequency band of interest with the tracking generator set to sufficient level for easy viewing on the analyzer display. The reference cable is placed between the analyzer's RF input and its tracking generator output. Select a display reference (A-B) mode that provides a zero reference indication, and then connect the cable under test in-line with the reference cable, using an appropriate adapter.

The resulting display indicates the amount of test cable signal loss at the frequency of interest. You can avoid significant measurement and alignment errors by adjusting power-meter readings or generator levels to compensate for the loss of the interconnecting test cables. The example in Figure 1 illustrates a typical insertion-loss plot that was captured with an IFR software utility.

Antennas and transmission lines are a frequent source of problems because they are exposed to a variety of elements. Storms with high winds and rain can cause the transmit or receive antenna to malfunction. These problems can disable a site and must be identified quickly. A simple return-loss measurement can identify a bad antenna system quickly. Return-loss values approaching 0dB indicate that most of the power is being reflected back to the source, resulting in poor performance.

Measuring Return Loss Return loss can be measured in the same manner as insertion loss, except the reference trace is established through the return-loss bridge, with the bridge being open-ended at the device test port. Typically, a return-loss bridge has three ports. Connect the source port of the return-loss bridge to the spectrum analyzer's tracking generator output. Connect the reflected port to the spectrum analyzer's input port. Establish a reference trace by setting the analyzer's start and stop frequencies to the band of interest and adjust the tracking generator for a near top-of-screen indication on the spectrum analyzer. Select a display reference (A-B) mode to provide a zero reference indication. With the reference trace established, connect the antenna or transmission line to the device test port of the return-loss bridge. The resulting return loss is displayed directly on the spectrum analyzer. The example in Figure 2 on page 50 shows the resonant points of an antenna connected to the return-loss bridge.

Most operators like to see a return loss of greater than 17dB when measuring antennas and transmission lines at the operating band in a cell site. A value of less than 17dB indicates an impedance mismatch in the line that can cause excessive power reflections to the transmitter or poor reception on a receive antenna.

The Bottom Line The modern spectrum analyzer has not changed much over the years. Today it is digital rather than analog, it is easier to use, and it is much more portable than its older cousins. But in the high-stakes wireless market, all well-equipped maintenance programs begin with a spectrum analyzer.