Tessco Wireless Journal February March 2015 Page 5 TESSCO Wireless Journal February-March 2015

(continued from page 1) Multifunction RF Power Meters Completely Transformed See TESSCO's entire product offering and your pricing on www.tessco.com or call TESSCO at 800.472.7373. 5 February/March 2015 average power ratio versus the percent of time that the waveform is at or exceeds a specific peak/ average power ratio. Conventional Mode The Bird Model 7022 provides highly accurate average power measurements from 350 MHz to 6 GHz across a power range from 250 mW to 500 W (33 dB dynamic range). This average power mode also provides for measurements of antenna and transmission system perfor- mance such as VSWR and return loss. All measurements are conveniently displayed using Bird's VPM3 Windows- based Virtual Power Meter software, which is supplied with the sensor. In all measurement modes, directional coupler frequency response characteristics are corrected through automatic frequency measurement and correction techniques. Time Domain Mode As the Model 7022 can measure the power-versus-time characteristics of waveforms, parameters such as aver- age burst power and peak power may be determined and displayed either automatically or by placing markers on the waveform displayed on the screen. Internal and external trigger options are also available, with trigger hold-off, level and delay settings available as well. The time domain mode is extremely useful for time-varying waveforms such as TDMA-based channel access methods or Time Division Duplex (TDD) formats. A good example of a system employing this technology is a Digital Mobile Radio (DMR) subscriber unit. DMR is an open-standard communica- tions format used primarily in land mo- bile radio systems with Mototrbo being its most prolific implementation. Other formats well suited to this mode are TETRA and TETRAPOL, APCO Project 25 Phase 1 and Phase 2, GSM- GSM-R and LTE-TDD. The analysis of time- domain-based waveforms such as TDMA may require measurement of the peak or burst average power of a single time slot within the TDMA frame. This measurement is easily performed on the VPM3 software by placing mov- able cursors on either side of the specific time slot to be measured and simply reading the power char- acteristics from the table. More advanced pulse power parameters such as top-level power, minimum power and other mea- surements are available by enabling the pulse measure- ment mode, available on the right side of the display. Several video filter settings (4.5 kHz, 400 kHz, 5 MHz, 20 MHz) are available in the Model 7022 in order to tailor the instrument response to the signal being measured. Several video smoothing settings are available as well. Statistical Mode Before the transition from analog to digital modulation schemes, information was en- coded via amplitude, frequency or phase modulation using linear modulators. Measuring the envelope power of these signals is straightforward and produces repeatable and predictable results. In contrast, most modern wire- less communications systems employ complex modulation and channel access methods like Orthogonal Frequency Division Multiplexing (OFDM) or Code Division Multiple Access (CDMA). These methods use a combination of ampli- tude and phase modulation to create symbol-based multichannel or multicar- rier systems that result in pseudorandom or noise-like power envelopes. As a result, modulation parameters, such as AM depth or FM modulation index, are not useful because the peak-to-average power ratio of the modulated carrier is a complex function of the data stream content rather than just amplitude and is not constant with time. While the average power of the above waveforms can be easily measured, this measurement yields only limited informa- tion about the performance of the trans- mitter system such as parameters that are ratiometric in nature (antenna VSWR or return loss). The development of these new waveforms required the use of more advanced instruments. A more universal power measure- ment approach uses statistics to provide a display of the percentage of time that a particular waveform exists at a specific value of peak-to-average power. Figure 4 illustrates this concept as applied to an LTE-TDD waveform. The horizontal axis in the figure represents the peak-to- average power ratio of the waveform being measured, and the vertical axis represents time as a percentage. Reading a specific point on the graph provides information about the percent- age of time that the signal being mea- sured exhibits a specific peak-to-average power ratio characteristic. The maximum peak-to-average power ratio of the waveform being mea- sured in Figure 4 is at the point where the curve intersects the horizontal axis. This corresponds to a value of 11.5 dB. Two movable cursors are available within the VPM3 display that can be placed at any point on the curve in order to determine specific values of the waveform peak-to -average ratio and corresponding time. Interpreting Statistical Data There are many factors that influence the performance of modern communications systems, including: the operating bandwidth of the system. resulting in multiple reflection within the transmission system. amplifier compression, resulting in nonlinear distortion and poor fidelity of the transmitted waveforms. resulting in high transmission system reflection (high VSWR and return loss.) performance resulting in high error vector magnitude (EVM). Many of these issues may be identified through the use of the statistical tech- niques described previously. For exam- ple, if an LTE radio system is known to be dropping calls at a higher rate than ex- pected, a service technician must know whether the problem is within the radio itself, with some element of the transmis- sion system, or with the air interface. Measuring the waveform statistics of the base station radio while terminated with a high-quality, 50-ohm termination and then again with the radio connected to the transmission and antenna system will provide clues about where the issues may originate. Summary Wireless telecommunications is rap- idly approaching the day when digital modulation schemes will be universally employed. Over the last 15 years or so, this has transformed not just the trans- mission systems themselves, but the instruments employed to commission and maintain them. Measurement of RF power is a classic example of the trans- formation, as conventional RF power measurements are no longer useful for characterizing time-based signals char- acteristic of higher-order modulation schemes. The Bird Model 7022 RF Power Sensor with statistical analysis func- tions is designed to provide the ability to measure digital waveforms, as well as to make all conventional measurements, in a rugged, compact package well suited for use in the field. Visit www.tessco.com/go/bird. Time Domain Mode (Figure 3) Conventional Mode (Figure 2) Statistical Power Display (Figure 4) Statistical Power Sensor TESSCO No. 511055

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