Digitally modulated signals are ubiquitous in modern wireless-communications systems, enabling multimedia services over limited bandwidths.The number of tests required to evaluate digitally modulated signals can be daunting.Fortunately,the Agilent MXA signal analyzer provides the many test capabilities needed to evaluate the modulation quality of modern signals.By using the MXA signal analyzer with a simple three-step procedure,it is possible to speed and simplify the measurement of troubleshooting of even the most complex signals, such as multicarrier orthogonal-frequency-division-multiplex (OFDM) signals.

The Agilent MXA signal analyzer (Fig. 1) combines the capabilities of a traditional swept-tuned spectrum analyzer and a vector signal analyzer. The midrange family of signal analyzers currently offers four versions spanning 20 Hz to 3.6 GHz, 8.4 GHz, 13.6 GHz, and 26.5 GHz. Its fast tuning speed and standards-based measurement applications might land it on the production line, but it is also a powerful research and design tool for general purpose, aerospace, and defense applications.

Since instantaneous bandwidth is important when analyzing wideband modulation, the MXA signal analyzer combines traditional spectrum analysis with vector signal analysis and a generous analysis bandwidth: 10 MHz in standard units and 25 MHz with Option B25. The MXA analyzers also feature the attributes needed for successful signal-modulation analysis,-including high-frequency and amplitude accuracy, extremely low noise levels, and wide dynamic range.

The MXA signal analyzers are available with a wide range of standard-specific software applications, pre-programmed according to the requirements of specific wireless-communications standards, including IEEE 802.16e WiMAX, WCDMA, HSDPA/HSUPA, and automated phase-noise measurements. The MXA analyzers can perform signal-modulation analysis on everything from digital-video (DV) and cellular systems through WiMAX wireless local-area network (WLAN), and private-mobile-radio (PMR) systems and their components, with test applications for all leading wireless-communications standards, such as Bluetooth, GSM, EDGE, TETRA 1 and 2, TD-SCDMA, and ZigBee.

The MXA signal analyzer provides extensive spectrum-analysis functions and complete vector signal analysis in a single instrument, with the capability of switching between modes in a fraction of a second. Modulation analysis on wireless signals can be challenging, often prompting test-equipment operators to commence with vector signal analysis. But a measurement sequence that begins with spectrum analysis may yield a finished design and more meaningful test results faster than trying to immediately make demodulation measurements on complex modulated waveforms. The MXA signal analyzer can be used as part of a three-step process (Fig. 2) proven effective for measuring and troubleshooting digitally modulated signals:

• Begin with basic spectrum measurements.
• Continue with vector measurements that combine frequency and time measurements.
• Conclude with digital demodulation and modulation quality analysis for isolating and identifying specific problems

This process is well suited to the design phase of research & development (R&D), although it can also be applied to design verification and other tasks typically found in prototype and pilot-scale manufacturing.This sequence improves the chances of finding signal problems at the earliest stages of a design.

Traditional spectrum measurements are fundamental and familiar, and can provide information about essential signal traits such as power, distortion, noise or signal to noise, and phase noise. Traditional spectrum measurements are also used to verify frequency-conversion operations for proper frequency and amplitude (Fig. 3). The MXA signal analyzers employ all digital resolution bandwidth filters as part of their digital intermediate-frequency (IF) section, for sharp and accurate signal differentiation during spectrum analysis.

Most digitally modulated signals vary with time, including on/off RF bursts and changes in signal composition due to equalizer training and synchronization sequences. Time-specific measurements can provide a great deal of insight into these signals. Fortunately, the MXA signal analyzer blends three tools that can simplify the analysis of time-related signal characteristics: a traditional spectrum analyzer, a VSA with the capabilities of Agilent's powerful 89601A analyzer built into an application program, and a host of standard-specific measurements that provide pre-programmed test setups for quickly getting information about a signal. Using Fast Fourier Transform (FFT) and digital-signal-processing (DSP) operations, the MXA's 89601A VSA application offers important triggering capabilities for capturing time-specific signal characteristics. These capabilities include pulse-triggering measurements with adjustable levels, holdoff, and positive/negative trigger delays to precisely select the start of the measurement interval; flexible time gates to select the desired portion of the signal for the measurement; adjustable frequency resolution and resolution-bandwidth filter shapes to optimize amplitude accuracy and time versus frequency resolution; powerful gating for all measurements, including spectrum, power occupied bandwidth, complementary cumulative distribution function (CCDF), power spectral density; time capture and replay functions for single-shot measurements-(as many as 42 million measurement points); and adjustable FFT time record size with records as large as 409,600 points, enabling single-trace measurements of even the longest and widest RF bursts.